Snow, A. A., D. A. Andow, P. Gepts, E. M. Hallerman, A. Power., J. M. Tiedje, and L. L. Wolfenbarger. 2004. Genetically engineered organisms and the environment: Current status and recommendations. Ecological Applications 15 (2):377-404.
The Ecological Society of America has evaluated the ecological effects of current and potential uses of field-released genetically engineered organisms (GEOs), as described in this Position Paper. Some GEOs could play a positive role in sustainable agriculture, forestry, aquaculture, bioremediation, and environmental management, both in developed and developing countries. However, deliberate or inadvertent releases of GEOs into the environment could have negative ecological effects under certain circumstances. Possible risks of GEOs could include: (1) creating new or more vigorous pests and pathogens; (2) exacerbating the effects of existing pests through hybridization with related transgenic organisms; (3) harm to nontarget species, such as soil organisms, non-pest insects, birds, and other animals; (4) disruption of biotic communities, including agroecosystems; and (5) irreparable loss or changes in species diversity or genetic diversity within species. Many potential applications of genetic engineering extend beyond traditional breeding, encompassing viruses, bacteria, algae, fungi, grasses, trees, insects, fish, and shellfish. GEOs that present novel traits will need special scrutiny with regard to their environmental effects. The Ecological Society of America supports the following recommendations. (1) GEOs should be designed to reduce environmental risks. (2) More extensive studies of the environmental benefits and risks associated with GEOs are needed. (3) These effects should be evaluated relative to appropriate baseline scenarios. (4) Environmental release of GEOs should be prevented if scientific knowledge about possible risks is clearly inadequate. (5) In some cases, post-release monitoring will be needed to identify, manage, and mitigate environmental risks. (6) Science-based regulation should subject all transgenic organisms to a similar risk assessment framework and should incorporate a cautious approach, recognizing that many environmental effects are GEO- and site-specific. (7) Ecologists, agricultural scientists, molecular biologists, and others need broader training and wider collaboration to address these recommendations. In summary, GEOs should be evaluated and used within the context of a scientifically based regulatory policy that encourages innovation without compromising sound environmental management. The Ecological Society of America is committed to providing scientific expertise for evaluating and predicting the ecological effects of field-released transgenic organisms.
Ortiz-García,S., E. Ezcurra, B. Schoel, F. Acevedo, J. Soberón, and A. A. Snow. 2005. Absence of detectable transgenes in local landraces of maize in Oaxaca, Mexico (2003-2004). Proceedings of the National Academies of Science, USA.
In 2000, transgenes were detected in local maize varieties (landraces) in the mountains of Oaxaca, Mexico [Quist, D. & Chapela, I. H. (2001) Nature 414, 541–543]. This region is part of the Mesoamerican center of origin for maize (Zea mays L.), and the genetic diversity that is maintained in open-pollinated landraces is recognized as an important genetic resource of great cultural value. The presence of transgenes in landraces was significant because transgenic maize has never been approved for cultivation in Mexico. Here we provide a systematic survey of the frequency of transgenes in currently grown landraces. We sampled maize seeds from 870 plants in 125 fields and 18 localities in the state of Oaxaca during 2003 and 2004. We then screened 153,746 sampled seeds for the presence of two transgene elements from the 35S promoter of the cauliflower mosaic virus and the nopaline synthase gene (nopaline synthase terminator) from Agrobacterium tumefaciens. One or both of these transgene elements are present in all transgenic commercial varieties of maize. No transgenic sequences were detected with highly sensitive PCR-based markers, appropriate positive and negative controls, and duplicate samples for DNA extraction. We conclude that transgenic maize seeds were absent or extremely rare in the sampled fields. This study provides a much-needed preliminary baseline for understanding the biological, socioeconomic, and ethical implications of the inadvertent dispersal of transgenes from the United States and elsewhere to local landraces of maize in Mexico.
Mercer, L. Kristin, Allison Snow, and Helen M. Alexander. 2009. Effects of early life history on fitness in wild sunflower. Ecological Society of America, Annual Meeting.
Background/Methods/Questions. Gene flow between crops and their wild relatives has long influenced the dynamics of wild populations. With the introduction of transgenic crops, concerns emerged regarding the possibility for increased invasiveness in wild populations. Studies of hybridization and estimates of the fitness of crop-wild hybrids relative to their wild counterparts have allowed us to understand which crop-wild complexes might be most vulnerable to such changes. Further, studies of important life history stages and selective responses have improved our understanding of how population dynamics and population genetics could jointly affect the outcome. Through these studies we have developed an understanding that ecological interactions within populations may well affect the process of introgression of crop alleles in wild populations. Previous studies identified alterations in life history characteristics of crop-wild sunflower hybrids including greater emergence, larger seedlings, and a greater likelihood of survival to reproduction, along with lower seed production. The relative fitness of crop-wild hybrids has also been shown to increase considerably under stressful conditions, such as competition.
Results/Conclusions. In the work reported here, we present data on the wild plants in the study, in particular the relationship of early life history characteristics to survival and reproduction in sunflowers grown in realistic, high-density populations with ratios of hybrids and wilds expected in high and low gene flow scenarios. Wild plants in the high gene flow population produced fewer seeds and had reduced survival to reproduction compared to plants in low gene flow populations, possibly indicating that greater numbers of hybrids create a more challenging or competitive environment for the wilds. The actual values of these fitness components varied across maternal families, with survival ranging from 0-29% and seed production ranging from 3-58 seeds per plant. Early life history characteristics showed interesting variation as well. Most seedlings emerged over a 25 day period, but the proportion of seedlings that emerged from each population ranged from 30-78%. Days to emergence and seedling size appear to be related to fitness components indicating selection on these early stages. These results can help us understand how competitive interactions and possible alterations of life history with hybridization may ultimately affect the potential for crop gene introgression into wild populations in crop-wild hybrid zones.
Snow, Allison, Patricia Sweeney, Michael Reagon, Su Su, Calvin Pearson, and Donna Rath. 2008. Volunteers Vary: Fecundity, Seed Shattering, and Dormancy of Volunteers from Open- and Hand-Pollinated Sunflowers (Helianthus annus) Tenth International Symposium on the Biosafety of Genetically Modified Organisms.
As researchers continue to introduce genetically modified traits into crop species, the importance of developing methods to confine GM traits within crops and prevent their spread into wild relatives increases. To date, most bio-confinement research has focused on developing methods to prevent the flow of GM germplasm via pollen. However, seed mediated gene flow can occur and, in species such as sunflower (Helianthus annus, Asteraceae), could be facilitated by volunteer plants. In our previous research, flowering times of volunteer progeny of both normal and off-type sunflower plants overlapped with wild sunflower (Reagon and Snow, 2006). This suggests that volunteer sunflower can serve as a bridge between crop and wild sunflower even when crop and wild flowering do not overlap. Here we evaluated the fecundity of volunteers and crop-wild hybrids, the potential for volunteer seed to persist in the seed bank, and evidence that crops have contributed to adjacent wild populations via seed. For two of three regions, off-type volunteers exhibited the greater fecundity than volunteers of normal crop plants and lower fecundity than crop-wild hybrids from the same geographic region. Off-type volunteers also exhibited seed shattering intermediate to the crop-wild hybrids and normal crop volunteers. In addition, our results showed that while crop-wild hybrids did not survive in the seed bank for more than a year, off-type volunteers consistently exhibited small amounts of dormancy and persisted in the seed bank. Finally, the presence of the cytoplasmic male sterility (CMS) allele in a portion of the progeny of roadside volunteers confirmed that crop seed had previously contributed to these populations. The consistence, albeit small, persistence in the seed band, increased fecundity compared to normal volunteers, and increased seed dispersal suggest that off-type progeny, in particular, could play a major role in the movement of genes from crop to wild sunflower.
Reagon, Michael, and Allison A. Snow. 2006. Cultivated Helianthus annuus (Asteraceae) volunteers as a genetic "bridge" to weedy sunflower populations in North America. Journal of Botany 93(1) 127-133.
In many crops, unharvested seeds can create populations of volunteer plants that increase opportunities for crop-to-wild gene flow. Pollen-mediated gene flow between cultivated and wild sunflower (both Helianthus annuus, Asteraceae) is well documented, but the role of seed dispersal and volunteers has not been investigated. We compared flowering times and other phenotypic traits of volunteers from both "normal" and "off-type" (multi-headed) crop plants with those of wild sunflowers. Normal and off-type volunteers typically had a maternally inherited, crop-specific DNA marker. Seedlings of wild plants, normal volunteers, and off-type volunteers from Colorado were cultivated in a greenhouse and at a field site in Ohio. We used a classification tree approach to differentiate the three plant types and identify phenotypic traits that can be used to recognize volunteers in the field in future surveys. In greenhouse and field experiments, we observed sufficient overlap in flowering times to allow gene flow among the three plant types. Volunteers from off-type crop plants were more likely to cross-pollinate with wild plants than volunteers from normal crop plants. Our results suggest that both types of crop volunteers have the potential to act as conduits for gene exchange between cultivated and wild sunflowers.
Cummings, C. L., H. M. Alexander, A. A. Snow, L. H. Rieseberg, M. J. Kim, and T. M. Culley. 2002. Fecundity selection in an experimental sunflower crop-wild system: how well do ecological data predict crop allele persistence. Ecological Applications, 12(6) 1661-1671.
Genes that spread from transgenic crops to populations of weedy relatives can be a cause of concern if fitness-related, transgenic traits persist and enhance weed invasiveness. Studies of the prevalence of crop-specific genetic markers in wild populations can provide data on such introgression. Ideally, it would also be useful to develop selection models that use ecological data to predict levels of crop allele persistence. Using both of these approaches, we conducted a field experiment in eastern Kansas to measure changes in frequencies of crop-specific genetic markers in wild sunflower (Helianthus annuus). Three allozyme markers were monitored in three artificial populations that each initially consisted of 100 wild and 100 F1 hybrid plants. Survival, flowering time, and average seed production per plant were quantified during the first year of the study (1997). Hybrid plants produced only 1-2 % as many seeds per plant as wild seeds, but did not differ in survivorship. Simple selection models incorporating fecundity differences between wild and F1 hybrids accurately predicted crop allele frequencies in the 1998 seedlings. We predicted that frequencies of crop alleles in 1998 would average ~0.03 for the three populations. Crop-specific allele frequencies dropped from the initial level of 0.25 to a mean of 0.03 in the 1998 seedlings, and averaged 0.05 in the next generation of seeds. Accounting for differences in flowering phenology and predispersal seed predation did not improve the accuracy of our predictions for 1998 seedlings. Our results suggest that ecological data can be useful for estimating the frequencies of crop genes following episodes of crop-wild hybridization in sunflowers. This approach can be applied to other study systems in which data on survivorship and fecundity are used to estimate a genotype’s evolutionary fitness.
Snow, A. A., D. Pilson, L. H. Rieseberg, M. Paulsen, N. Pleskac, M. Reagon, and D. Wolfe. 2002. A Bt transgene reduces herbivory and enhances fecundity in field populations of BC1 common sunflower (Helianthus annuus). 42nd Annual Meeting of the Weed Science Society of America, Reno, NV, February 11, 2002.
Gene flow from transgenic crops is expected to introduce novel traits from unrelated species into sexually compatible wild relatives. Transgenes that confer resistance to insects, diseases, herbicides, or harsh growing conditions may enhance the fitness of recipient species, but the effects of this process on wild populations have not been studied. Here, we report the first empirical evidence that wild plants can benefit from a bacterial transgene under uncaged, natural conditions. Cultivated sunflower (Helianthus annuus) is known to hybridize frequently with common sunflower (also H. annuus) where they co-occur in the western and mid-western USA. We studied a crop-developed Bt transgene (Cry1Ac) in backcrossed (BC1) common sunflower populations that segregated for the presence or absence of the transgene. These plants were selected to be male-sterile to prevent the dispersal of Bt pollen. The transgene exhibited dominant Mendelian inheritance and was very effective in BC1 plants. Lepidopteran damage on transgenic plants was strongly reduced relative to nontransgenic control plants, while damage by several weevil and fly species was unaffected. Transgenic plants produced 55% more seeds per plant than controls at a field site in Nebraska and 14% more seeds at a site in Colorado. Much of this fecundity advantage appeared to be due to protection against lepidopteran insects that fed within stems, roots, and developing heads; the amount of insect damage to seeds was quite low (<5%). In a greenhouse experiment involving drought-stressed, nutrient-stressed, and control BC1 plants, the transgene had no effect on fecundity. This suggests that no inherent cost or benefit was associated with the transgene in the absence of herbivores. If Bt sunflowers are released commercially, we expect that Bt genes will spread to wild and weedy populations, kill susceptible herbivores on these plants, and increase seed production when these herbivores are common. We are conducting further studies to examine whether a strong fitness benefit due to a novel transgene could affect the abundance and/or distribution of this common agricultural weed.
M. R. Reagon and A. A. Snow. 2002. The potential for volunteer sunflowers to start feral populations and exchange genes with weedy Helianthus annuus. 42nd Annual Meeting of the Weed Science Society of America, Reno, NV, February 11, 2002.
The wild or common sunflower (Helianthus annuus L.) and the crop sunflower (also H. annuus) frequently co-occur throughout the central and northern Great Plains of North America. Pollen-mediated gene flow between these populations is common during the short flowering period of cultivated plants, but little is known about seed-mediated gene flow from the crop, i.e., via volunteers. To better understand the role that seeds from open-pollinated crop plants may play in this process, we compared phenological and morphological characteristics of wild plants from Colorado with progeny from crop and “off-type” crop plants. Off-types presumably originate from crop seeds, but they are multi-headed and may have a greater chance of receiving pollen from wild plants. Progeny from these three seed sources were grown in a greenhouse (2000) and in a common garden (2001) in Ohio. Analyses from the 2001 garden experiment are in progress. In the greenhouse, crop volunteers had more heads per plant as compared to their crop parents, and a few volunteers had the same numbers of heads (e.g., 6-9 per plant) as wild plants. Off-types were intermediate between the crop and wild progeny for 7 out of 10 measured traits. In particular, off-types were intermediate for days to first flower and the total number of heads per plant, allowing a larger window for gene exchange with wild plants. These data suggest that crop volunteers, especially off-types, can act as conduits for gene exchange between cultivated and wild sunflowers. Some crop volunteers may also be capable of starting feral populations de novo, although our experimental evidence for this is preliminary. We will discuss further research on these questions and the relevance of our findings to predictions about how quickly novel transgenes could diffuse through wild populations.
Snow, A., L. Rieseberg, H. Alexander, C. Cummings, and L. Spencer. 2001. Crop- to-weed gene flow and transgenic crops. Gene Flow Symposium conducted as part of 41st Annual Meeting of the Weed Science Society of America, Greensboro, NC, USA, February 12, 2001.
Many crops have been genetically engineered to possess single-gene traits that can potentially spread to weedy relatives. Research involving nontransgenic genetic markers can provide useful information about the extent of crop-weed hybridization prior to commercialization. In sunflowers, for example, we used allozymes and RAPD markers to show that crop genes spread to weedy populations and persist for many generations. Similar studies have been carried out by others with oilseed rape, sugar beet, sorghum, rice, squash, radish, and other species. Genes that are selectively neutral or beneficial (e.g., glyphosate resistance) are likely to persist in weed populations unless they are linked to genes that are deleterious to the weed. Introgression can proceed even if F1 crop-weed hybrids have lower fitness than wild/weedy genotypes, as we documented in sunflower (Helianthus annuusradish (style="font-size: 12.0pt; mso-bidi-font-size: 10.0pt; mso-fareast-font-family: Times New Roman; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA; layout-grid-mode: line">Raphanus raphanistrum), and squash (Cucurbita pepo). Gene flow via the dispersal of crop and hybrid seeds can further contribute to the spread of crop genes in wild populations. Weed scientists need to understand these processes in order to determine the extent to which herbicide resistance transgenes are able to spread and persist in weed populations. Other fitness-related transgenes, such as those conferring tolerance to environmental stresses, herbivores, and disease, could also enhance the abundance and distribution of weed species, depending on how each of these factors affects the population dynamics of the weed. In cases where crops hybridize with weeds, it is important to ask whether this process has implications for weed management. This is a difficult and challenging area of research, especially given the constraints imposed by regulatory agencies and biotech companies, which often deal with confidential business information.
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Snow, Allison A., Lesley G. Campbell, Theresa M. Culley, and Caroline E. Ridley. 2007. Long-term field studies of the evolutions of crop-weed hybrids in radish: implications for invasiveness. Gene Flow among Transgenic Plants and their Wild Relatives: Implications for Risk Assessment. Proceedings of the Gene Flow Symposium of the North Central Weed Science Society Annual Meeting. pp. 23-24.
Many cultivated plants hybridize naturally with wild and weedy relatives, but little is known about the evolutionary effects of this process on recipient populations. To examine the dynamics of introgression in a natural setting, we monitored crop-specific genetic markers in replicated field populations of weedy Raphanus raphanistrum in Michigan, USA, for ten years. Four isolated hybrid populations were established in 1996 using a 1:1 ratio of R. raphanistrum and F1 crop-wild hybrids (R. raphanistrum x R. sativus). The sites were tilled and fertilized annually to mimic agricultural fields, and plants were exposed to local biotic and abiotic selective pressures. Initially, F1 hybrids had reduced fitness relative to wild genotypes, but the populations quickly regained wild-type pollen fertility, presumably by losing a crop-specific reciprocal translocation. Recombination and natural selection allowed the populations to absorb two crop-specific allozyme markers at relatively high frequencies in all populations, even exceeding their initial frequency of 0.25 in a few cases. Frequencies of a crop-specific white petal color allele were much lower, but this allele also persisted in all populations. Overall, frequencies of the three crop-specific alleles varied considerably among locations, years, and loci. In the tenth year, plants from each hybrid population were grown in a common garden experiment along with wild genotypes. The lifetime fecundity of these advanced-generation hybrids was similar to that of the wild genotypes. This long-term study provides a unique example of how easily certain crop alleles can become established in weed populations while others remain rare or disappear.
In a second study, we tested the hypothesis that crop-wild hybridization can allow weeds to become more successful. A third study was carried out simultaneously to determine whether cultivated radish could generate feral populations. We established replicated populations of wild, hybrid, and “volunteer” cultivated radishes in Michigan and let them evolve for three growing seasons, starting in 2002. Three of the five volunteer populations died out. The two remaining populations became contaminated with wild genes and evolved traits that were similar to crop-wild hybrids (Campbell and Snow, in prep.). Although we did not find evidence for ferality in the absence of hybridization with R. raphanistrum, further studies involving more populations and locations might detect ferality.
Results from the wild and hybrid populations were reported in Campbell et al. (2006) and are summarized briefly here. The initial frequency of crop alleles in these hybrid populations was 0.50 (twice the level in our introgression study above) because all plants were F1 hybrids. Frequencies of white-flowered plants declined slightly, unlike our previous study, and then remained relatively constant. We suspect that the sharp drop in white-flowered plants in our previous study was due to the fact that many hybrid plants flowered very late or not at all, and the white petal allele is linked to delayed reproduction (Campbell, 2007). In 2005, advanced-generation hybrid and wild seedlings were grown in common garden experiments in Michigan and California. Hybrid-derived plants had slightly lower fecundity than wild plants in Michigan, but exhibited ~270% greater lifetime fecundity and ~22% greater survival than wild plants in California. These results support that hypothesis that crop-wild hybridization may create genotypes with the potential to displace parental taxa in new environments, which is consistent with other studies of hybrid-derived wild radish populations in California (C. E. Ridley et al., in prep.). In summary, our combined field studies of evolving crop-wild hybrids show that conventional crop alleles can persist in wild populations and may increase the fitness of wild relatives in some cases. Further research is needed to confirm the common assumption that enhanced fitness results in more abundant weed populations.
Campbell, Lesley G., Allison A. Snow, and Patricia M. Sweeney. 2007. Crop–wild hybridization and the rate of evolution in weeds. Gene Flow among Transgenic Plants and their Wild Relatives: Implications for Risk Assessment. Symposium of the North Central Weed Science Society Annual Meeting. pp. 10-11.
When species hybridize their offspring routinely suffer from reduced fertility and poorly adapted phenotypes. Consequently, it seems unlikely that these plants could be successful weeds. Reflecting this belief, risk assessments of crop-wild hybrids often dismiss the potential for crop gene flow to produce ‘superweeds’. However, in the absence of empirical evidence, the evolutionary potential of early-generation hybrids remains hypothetical. Here, we explore the potential for rapid evolution in crop-weed hybrids and its consequences for crop allele introgression. Using hybrids of a cosmopolitan weed (Raphanus raphanistrum) and its cultivated relative (R. sativus), we compared the ability of hybrid and wild lineages to respond to artificial selection for early flowering and large size at reproduction, two life-history strategies which characterize weedy species.Raphanus raphanistrum grows a rosette with a thin, fibrous taproot, bolts within a few weeks after germination and produces yellow flowers soon after. On the other hand, crop breeding has emphasized delayed bolting with white flowers in R. sativus in order to produce the edible, enlarged roots (Snow and Campbell, 2005). Early flowering may be adaptive for weedy radishes because growing seasons for weeds are often curtailed by tilling schedules, herbivores, frost, and other causes of mortality or severe stress. The evolutionary potential to evolve earlier flowering may be more important for hybrid radishes given that hybrid fecundity, relative to wilds, may be limited by delayed flowering, a trait inherited from their cultivated parent and by low pollen fertility due to a reciprocal translocation that affects chromosome pairing (Snow et al. 2001; Campbell and Snow, in prep.). When hybridization occurs between species with such diverse life histories, the individual offspring will be phenotypically variable. Populations created with this initial diversity should have the opportunity to evolve along diverse trajectories with respect to life history. If crop-wild hybrids can evolve quickly from maladaptive intermediates to adaptive phenotypes, they may more difficult to control. Large size in annual weeds is often correlated with rapid growth rates. In weedy radish, leaf length is correlated with high flower and seed production, suggesting that plants with rapid growth rates would also be highly fecund. If large size is adaptive, this may facilitate the introgression of additional adaptive quantitative traits into weed populations.
In wild and hybrid lineages, four generations of selection were performed to determine whether these traits exhibited a response to selection (i.e., were heritable) and the relative magnitude of their response across wild and hybrid lineages. Hybrid lineages exhibited a greater response to selection for early flowering suggesting its heritability is greater in hybrid lineages versus wild lineages. Early-generation hybrids had longer leaves than wild plants and they maintained this length difference after selection for longer leaves. This suggests that polygenic traits, such as size, inherited from domestic relatives may easily introgress into weed populations.Four generations of selection also resulted in the correlated evolution of hybrid flower petal color and hybrid pollen fertility. Large hybrid lineages exhibited higher than expected frequencies of plants with white petals, a crop-specific, simply inherited trait. Therefore, selection for a polygenic crop-specific trait accelerated the introgression of an additional crop-specific trait. Further, pollen fertility of early-flowering hybrid lineages was similar to that of wild lineages, and at least 12% higher than hybrid control lineages. Therefore, selection for earlier flowering in hybrid lineages led to rapid evolution of fertility, a key component limiting hybrid fitness. Despite selection for the early-flowering, wild phenotype, hybrid lineages maintained high frequencies of the crop-derived trait, white flower color, confirming persistent introgression. The persistence of white flower color and increase in pollen fertility after experimental manipulation of the selection environment may explain some results from our long-term studies in crop allele introgression (Snow et al., in prep; Campbell et al., 2006)
Both wild and hybrid lineages apparently possess substantial additive genetic variation for size at reproduction.Nevertheless, hybrid lineages evolved more rapidly under selection for age at reproduction and exhibited more extreme phenotypes under selection for large size at reproduction than their weedy parents. We suggest that hybrids have the potential to rapidly respond to newly invaded environments and may become more invasive weeds than their wild progenitors.
Campbell, L. G., A. A. Snow, and C. E. Ridley. 2006. Weed evolution after crop gene introgression: greater survival and fecundity of hybrids in a new environment. Ecology Letters 9: 1198-1209.
Crop-wild hybridization may produce offspring with lower fitness than their wild parents due to deleterious crop traits and outbreeding depression. Over time, though, selection for improved fitness could lead to greater invasiveness of hybrid taxa. To examine evolutionary change in crop-wild hybrids, we established four wild (Raphanus raphanistrum) and four hybrid radish populations (R. raphanistrum × R. sativus) in Michigan, USA. Hybrid populations exhibited increased pollen fertility and similar population growth rates to wild populations across four generations. We then measured hybrid and wild fitness components in two common garden sites within the geographic range of wild radish (Michigan and California). Advanced-generation hybrids had slightly lower lifetime fecundity than wild plants in Michigan, but exhibited ~270% greater lifetime fecundity and ~22% greater survival than wild plants in California. Our results support the hypothesis that crop-wild hybridization may create genotypes with the potential to displace parental taxa in new environments.
Campbell, L. G., and A. A. Snow. 2006. Competition alters life-history traits and increases the relative fecundity of crop-wild hybrids (Raphanus spp.). New Phytologist: 173:648-660.
The evolutionary impact of crop-to-wild gene flow depends on the fitness of hybrids under natural, competitive conditions. We measured the performance of third-generation (F3) hybrids (Raphanus raphanistrum x R. sativus) and weedy R. raphanistrum to understand how competitive interactions affect life history and relative fecundity. Three wild and three F1 crop-wild hybrid radish populations were established in semi-natural, agricultural conditions in Michigan, USA. Two years later, we measured the effects of competition on life-history traits and fecundity of F3 progeny in a common garden experiment. Third-generation hybrid plants generally produced fewer seeds per fruit and set fewer fruits per flower than wild plants, resulting in lower lifetime fecundity. With increasing competition, age at reproduction was delayed and relative number of seeds per fruit was reduced in wild plants, and differences between hybrid and wild fecundity diminished. Competition may enhance the fecundity of advanced-generation hybrids relative to wild plants by reducing differences in life history, potentially promoting the introgression of crop alleles into weed populations.
Snow, A. A., K. L. Uthus, and T. M. Culley. 2001. Fitness of hybrids between cultivated radish and weedy Raphanus raphanistrum: implications for rapid evolution in weeds. Ecological Applications 11:934-943.
Weed species are known to evolve rapidly with their associated crops. A better understanding of the mechanisms and rates of weed evolution could aid in limiting or at least anticipating this process. Spontaneous hybridization between crops and related weed species can transfer crop genes coding for fitness-enhancing traits to wild populations, but little is known about how easily this takes place in various weed-crop complexes. We studied interspecific hybrids between wild and cultivated radishes (Raphanus raphanistrum x R. sativus ), which often co-occur and share pollinators. To determine whether the F1 generation represents a strong barrier to subsequent introgression, we compared the fitness of wild and wild-crop hybrids. Two experiments were carried out in Michigan, USA, one with potted plants and the other involving four artificially established populations. In the artificial populations, we used white flower color, a dominant, crop-specific allele, to document the persistence of crop genes over time. Wild plants had yellow flowers, which is a recessive trait. F1 hybrids had lower fitness than wild plants due to lower pollen fertility (x = 63% vs. 92-96%), fewer seeds per plant (x = 193 vs. 396 when grown in pots), and delayed flowering (22-40% of the hybrids never produced fruits in the field vs. 3-8% of wild plants). Despite these disadvantages, hybrids contributed substantially to each population’s gene pool. After three years, frequencies of white-flowered plants in the artificial populations ranged from 8-22%, demonstrating that crop genes persisted. Other studies of flower color variation in wild populations of R. raphanistrum provide circumstantial evidence for frequent crop-to-wild gene flow. We predict that if cultivated radish is engineered to possess transgenes coding for traits such as resistance to insect herbivores, disease, herbicides, or environmental stress, these fitness-related crop genes will easily spread to R. raphanistrum.
Uthus, K. L. 2001. The effect of competition on fitness of hybrid progeny of wild and cultivated radish, Raphanus. Annual Meeting of the Ecological Society of America.
Wild and cultivated radish have been demonstrated to cross in field situations. Because most studies were conducted with potted plants, however, it was unclear how hybrid offspring and their progeny would fare under natural conditions. To test this, in 1999 we planted wild, F and BC1 radish seeds in a fallow field and subjected each cross type to two growing conditions: with and without competition. Germination ranged from 77-89% and was significantly different among cross types (BC>F1). Mortality differed significantly among cross types and was greatest in BC1 plants. Plant maturation time was significantly different among cross types, and slowest in F1’s. Among plants that produced seeds, there were no significant differences in flower, fruit, or seed production among cross types or between competition treatments. Among all surviving plants, however, F1’s produced significantly fewer flowers and fruits than wild or BC1 plants, and all cross types differed significantly in seed production. Although measures of performance were generally lower with competition, it did not significantly impact flower, fruit, or seed production. These results indicate that while hybrid plants suffer greater mortality and produce fewer seeds than wild plants, even early generation hybrids are capable of ecologically significant levels of reproduction despite competition from other naturally-occurring weeds. This study suggests that hybridization between crop and wild radish could lead to long-term introgression of crop alleles into wild radish populations.
Allison Snow, P. M. Sweeney, Nguyen Lang, Bui Buu. 2006. Hybrids between weedy and cultivated rice (Oryza sativa) in the Mekong Delta of Vietnam exhibit heterosis: implications for rapid evolution in rice. Annual Meeting of the Botanical Society of America.
Weedy rice (Oryza sativa) hybridizes with cultivated rice (also O. sativa) in many regions of the world, but little is known about variation in the fitness and vigor of early and advanced-generation crop-weed hybrids. We hypothesized that crop-weed hybrids would perform better than their weedy parents due to heterosis. The relative fecundity of crop-weed hybrids is expected to influence rates of crop allele introgression, and certain fitness-enhancing crop alleles could accelerate this process. We compared the fecundity of F1, F2 and BC1 progeny of three weedy rice accessions (W1, W2, W3) crossed with two local rice cultivars (IR-64, OM-1490) with that of their respective weedy parents in the Mekong Delta of Vietnam. Fecundity, measured as seed per plant, was evaluated in two fields, one flooded and one non-flooded. In all cases, F1, F2, and BC1 progeny performed as well or better than their weedy parent. Non-flooded plants had higher fecundity than flooded plants, and numerous interactive effects were significant in ANOVAs. The F1 and F2 progeny of both W1 and W2 had higher fecundity than their respective weedy parents. In addition, the BC1 progeny of W2 out-performed the F1 and F2 progeny and produced nearly twice as many seeds as the weedy parent. In contrast, the fecundity of W3 progeny was not significantly different from their weedy parent. Therefore, F1, F2, and BC1 progeny can have enhanced fecundity relative to their weedy parents, depending on the weedy accession tested, suggesting that heterosis can boost rates of introgression after hybridization has occurred. If this effect is common in rice fields with weedy rice populations, rates of crop allele introgression may be greater than one would expect based solely on the small proportion of F1 hybrid seeds that are produced on weedy rice plants
Chen, Liang-Yan, Allison A. Snow, Feng Wang, Bao-Rong Lu . 2006. Effects of insect-reistance transgenes on fecundity in rice (Oryza sativa, Poaceae: A test for underlyihg cost. American Journal of Botany 93(1) 94-101.
Understanding the balance between yield benefits and possible underlying yield costs that are associated with transgenic cultivars is useful for evaluating crop performance and the fitness of crop-wild hybrid progeny, but few researchers have tested for such costs under rigorous experimental conditions. We examined shifts in net costs and benefits of in sect-resistance transgenes in cultivated rice (Oryza sativa) using two levels of insect pressure (low vs. moderate) and two types of competition (pure vs. mixed lines). We compared the growth and fecundity of potted rice plants from three transgenic lines, Bt, CpTI, and Bt/CpTI, relative to isogenic control plants at outdoor locations in Fuzhou, China. Net yield costs were detected, but only in Bt/CpTI plants in mixed-line competition with low insect pressure. These plants produced 16% fewer tillers, 6% smaller seeds, and 30% fewer seeds than competing control plants. Under moderate insect pressure, Bt and Bt/CpTI plants produced 36-65% more seeds than controls, but the net benefit for Bt/CpTI plants disappeared in mixed-line competition pots. To our knowledge, this is the first report of yield costs in cultivars with transgenic insect resistance. Our results suggest that these costs may be negligible in monotypic rice fields, especially when target insects are abundant.
Lu, Bao-Rong and Allison A. Snow. 2005. Gene flow from genetically modified rice and its environmental consequences Bioscience 55(8) 669-678.
Within the next few years, many types of transgenic rice (Oryza sativa) will be ready for commercialization, including varieties with higher yields, greater tolerance of biotic and abiotic stresses, resistance to herbicides, improved nutritional quality, and novel pharmaceutical proteins. Although rice is primarily self-pollinating, its transgenes are expected to disperse to nearby weedy and wild relatives through pollen-mediated gene flow. Sexually compatible Oryza species often co-occur with the crop, especially in tropical countries, but little is known about how quickly fitness-enhancing transgenes will accumulate in these populations and whether this process will have any unwanted environmental consequences. For example, weedy rice could become much more difficult to manage if it acquires herbicide resistance, produces more seeds, or occurs in a wider range of habitats because of the spread of certain transgenes. Rice-growing countries urgently need publicly available ecological assessments of the risks and benefits of transgenic rice before new varieties are released.
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Snow, Allison A., Patricia Sweeney, Cecile Grenier, Geibisa Ejeta, Tesfay Tesso, Issofrou Kapran, Gurling Bothma, and Jeffery F. Pedersen. 2008 Lifetime fecundity of F1 crop-wild sorghum hybrids: Implications for gene flow from transgenic sorghum in Africa. Tenth International Symposium on the Biosafety of Genetically Modified Organisms
Researchers are developing transgenic crops with enhanced nutrition and higher yields for Africa, but few studies have assessed environmental risks of growing these crops. Since wild relatives of sorghum (Sorghum bicolor) are often weedy and represent valuable germplasm, plans to release transgenic sorghum should consider consequences of gene flow. Our previous studies in Ethiopia and Niger showed that wild and cultivated sorghum often co-occur and flower simultaneously. Here, we tested for spontaneous hybridization between accessions of wild S. bicolor and local cultivars from eastern Africa at times when their flowering periods overlapped. Plants were grown in field plots in Ohio, with a ratio of more than 20 crop plants per wild individual. Microsatellite DNA markers showed that some seeds on wild plants were fertilized by crop pollen. We also studied the fecundity of F1 hybrids between a male-sterile cultivar and three wild accessions. Wild and hybrid progeny were grown in Niger, Ohio, and Indiana. The relative fecundity of hybrids was fairly consistent across locations but differed somewhat among accessions. For two accessions, crop-wild hybrids produced considerably more seeds per plant than the wild parent. For a third accession, hybrids produced fewer seeds per plant in the USA and similar numbers of seeds per plant in Niger. Although one hybrid had poor seedling survival, once established all crop-wild F1 hybrids were vigorous, and fertile, and could easily contribute pollen and seeds to subsequent generations. This study shows that selectively neutral or advantageous crop alleles are likely to persist in wild sorghum populations following hybridization. Before transgenic sorghum varieties are grown near wild relatives in Africa, ecological effects and other consequences of crop-to-wild gene flow should be examined for each transgenic trait.
Snow, Allison A., Patricia Sweeney, Cecile Grenier, Geibisa Ejeta, Tesfay Tesso, Issofrou Kapran, Gurling Bothma, and Jeffery F. Pedersen. 2007 Lifetime fecundity of F1 crop-wild sorghum hybrids: Implications for gene flow from transgenic sorghum in Africa. Gene Flow among Transgeneic Plants and their Wild Reletives: Implications for Risk Assessment. Procedings of the Gene Flow Symposium of the North Central Weed Society Annual Meeting. p. 25.
Researchers are developing transgenic crops with enhanced nutrition and higher yields for Africa, but few studies have assessed environmental risks of growing these crops. Since wild relatives of sorghum (Sorghum bicolor) are often weedy and represent valuable germplasm, plans to release transgenic sorghum should consider consequences of gene flow. Our previous studies in Ethiopia and Niger showed that wild and cultivated sorghum often co-occur and flower simultaneously. Here, we tested for spontaneous hybridization between accessions of wild S. bicolor and local cultivars from eastern Africa at times when their flowering overlapped. Plants were grown in field plots in Ohio, with a ratio of more than 20 crop plants per wild individual. Microsatellite DNA markers showed that some seeds on wild plants were fertilized by crop pollen. We also studied the fecundity of F1 hybrids between a male-sterile cultivar and three wild accessions. Wild and hybrid progeny were grown in Niger, Ohio, and Indiana. The relative fecundity of hybrids was fairly consistent across locations but differed somewhat among accessions. For two accessions, crop-wild hybrids produced more seeds per plant than their wild parent. For a third accession, hybrids produced similar numbers of seeds per plant in Niger, but fewer seeds per plant in the USA. However, this decrease in seed per plant in the USA was not significant. Although one hybrid had poor seedling survival, once established, all crop-wild F1 hybrids were vigorous, and fertile, and could easily contribute pollen and seeds to subsequent generations. This study shows that selectively neutral or advantageous crop alleles are likely to persist in wild sorghum populations following hybridization. Before transgenic sorghum varieties are grown near wild relatives in Africa, ecological effects and other consequences of crop-to-wild gene flow should be examined for each transgenic trait.
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TOP OF PAGE HOMESu Su. 2006. Population genetic structure of shattercane (Sorghum bicolor) in Nebraska: implications for crop-to-weed gene flow. MS Thesis. Ohio State Univ.
Shattercane (Sorghum bicolor) is an annual weed that occurs throughout much of the USA. It is a serious weed pest in both corn and sorghum fields because it geminates and matures at approximately the same time as the crops and also due to the long dormancy of its seed. Shattercane is the same species as sorghum (Sorghum bicolor ). They frequently co-occur throughout North America, and previous research has shown they can hybridize. Sorghum is therefore considered a high-risk crop for genetic engineering, as transgenes are likely to ‘escape’ cultivation via hybridization. Gene flow among shattercane populations will strongly influence introgression of crop alleles, which leads to higher pervasiveness of crop alleles and should be considered when assessing risks. However there is no prior study has addressed the persistence and pervasiveness of crop alleles in shattercane populations. Although determining rates of gene flow among shattercane populations directly is difficult, gene flow can be inferred from population genetic structure. I
In this study we assessed population genetic diversity for 205 individuals sampled from 12 shattercane populations using 6 microsatellite markers. Populations were located along a 160 km North-South transect near Lincoln NE, and were further subdivided into 3 regions based on exposure time to crop sorghum. This design was used to determine the effects of geographic distance and exposure time to sorghum on population differentiation. Mantel tests showed that geographic distance was not significant and distribution of genetic diversity did not fit standard isolation-by-distance model. Although within each region, mantel test showed higher correlation between geographic and genetic distance. My analysis revealed a high degree of structuring among shattercane populations (Fst 0.29) using AMOVA. Exposure time did not have a significant effect on partitioning of genetic variation and explains less than 3% of total variation observed in sample (AMOVA test among regions). Most of the genetic variation occurred within population (69.6%) and 27.8% occurred among populations. Current migration rates were estimated by assignment methods using a Bayesian approach in BayesAss. Only two populations showed migration rates higher than 10%, all others less than 1%. this result suggests that gene flow among shattercane populations is infrequent.
Both traditional non-Bayesian and Bayesian (which has fewer assumptions) methods for analyzing genetic diversity suggest that little gene flow has occurred among shattercane populations and the pervasiveness of fitness-enhancing transgenes could be high within local population, and low among populations, depending greatly on the proximity of these populations to transgenic varieties of sorghum. This data could be used for designing confinement strategies and for modeling process of crop allele introgression.
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TOP OF PAGE HOMETesso, T., I. Kapran, C. Grenier, G. Ejeta, A.Snow, J. Pederson, G. Bothman, D. Marx, and P. Sweeney. 2005. Potential of crop-to-wild gene flow in sorghum in Ethiopia and Niger. Crop Gene Flow & The Occurrences and Consequences of Gene Introgression between Crops and their Sexually Compatible Relatives.
Cultivated sorghum is known to hybridize readily with its wild relatives. Surveys to determine the prevalence of wild and weedy sorghums growing areas were conducted in Niger and Ethiopia. Data collected included date, location name, sorghum cultivar type/name, type of stand, presence of wild or weedy sorghum, incidence of wild or weedy sorghum within or near the field, habitat, synchrony of flowering, and tillering habit. These data demonstrate extensive overlap of distribution and flowering times of wild and cultivated sorghums. Considerable variability in weedy sorghum species was noted. Weedy sorghums were not identified by species, but were grouped into three distinct types based on morphology.
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Spencer, L. J., and A. A. Snow. 2001. Fecundity of transgenic wild-crop hybrids of Cucurbita pepo (Cucurbitaceae): implications for crop-to-wild gene flow. Heredity, 86: 694-702.
Hybridization between crops and their weedy or wild relatives is an area of concern because the widespread use of genetically engineered crops may allow novel, beneficial transgenes to enter nearby populations. We compared fitness components of wild Cucurbita pepo from Arkansas, USA, with wild-crop hybrids derived from yellow squash (a cultivar of C. pepo with transgenic resistance to two viruses). Wild and hybrid progeny were grown in agricultural fields in Arkansas (1996-1998) and Ohio (1996) in six similar experiments. Cross types (wild and hybrid) did not differ significantly in seedling survival, which exceeded 85% in all cases. In Ohio, where more detailed observations were made, hybrid plants produced 41% as many male flowers, 21% as many female flowers, and 28% as many seeds as wild plants. At all sites, flowering periods of the two cross types overlapped extensively. Putative virus symptoms were more common in wild plants than in hybrids. Lifetime fecundity varied considerably among sites and years. The average fecundity of hybrids ranged from 453 – 4,497 seeds per plant and represented 15% - 53% of the numbers of seeds produced by wild plants in the same experiments. These results suggest that the F generation does not represent a strong barrier to the introgression of neutral or beneficial crop genes into free-living populations of C. pepo.
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Snow, A. A., S. E. Travis, R. Wildova, T. Fer, P. M. Sweeney,* J. E. Marburger, S. K. Windels, B. Kubatova, and D. E. Goldberg. 2009. DNA markers for studies of hybridizing cattail populations (Typha latifolia x T. angustifolia) in North America. Society of Wetland Scientist Annual Meeting .
Hybrids between Typha angustifolia (narrow-leaf cattail) and T. latifolia (broad-leaf cattail) are considered more vigorous and invasive than their parents, but efforts to test this hypothesis have been hindered by the availability of molecular markers for identifying backcrossed plants and advanced-generation hybrids. We collected DNA samples from cattail populations in the upper Midwest and eastern USA,and recorded morphological data for a subset of these plants. Using RAPD (random amplified polymorphic DNA) markers known to be species-specific, we identified diagnostic SSR (simple sequence repeat) DNA markers that were consistent with the RAPD markers. SSR markers are codominant and polymorphic, making it easier to detect backcrossed and advanced-generation plants with fewer markers. As expected, F1 hybrids had phenotypes intermediate between the parent taxa. Backcrossed plants were fairly common and their phenotypes overlapped with the hybrids and their parents, illustrating the need for molecular marker data for identifying different categories of hybrids The availability of diagnostic SSR markers should facilitate studies of the population genetics, geographic distribution, and ecological effects of hybrid cattail populations in North America
Campell, Amy. 2007. Sexual Reproduction in the Non-native Common Reed, Phragmites australis (Cav.) Trin. ex Steudal: Seed Viability and Germination. MS Thesis. Ohio State Univeristy
Phragmites australis is a non-native plant species that invades fresh and saltwater wetlands in North America. This aggressive species can rapidly form monotypic populations, decreasing native species diversity. In the U.S., past research suggests Phragmites spreads primarily through vegetative propagation. I investigated seed viability, germinability and dormancy of Phragmites populations growing in four freshwater wetlands adjacent to Lake Erie, Ohio. In June of 2006, I located 24 sampling sites in three hydrologic zones: upland (never inundated), water line (moist soil), and standing water throughout the growing season. In November of 2006, I collected panicles from current-year Phragmites shoots at each site. I tested seed viability using a tetrazoleum test. I then germinated seeds over 12 days to obtain percent germination per site. I found that seeds from all sites were mostly viable. Across hydrologic zones, seeds had low germination percentages (6-25%) when exposed to a constant temperature of 25 degrees C. Germination improved substantially (90-100%) for all sites when the seeds were exposed to a diurnal temperature fluctuation of 10/30 degrees C. These results suggest that seeds may play a larger role in the spread of Phragmites than previously thought.
Snow, Allison A., Patricia M. Sweeney, Radka Wildova, Deborah E. Goldberg, and Jinguo Gao. 2007. Cattail hybrids between Typha angustifolia and Typha latifolia - Is Typha x glauca more invasive than its parents? 2007 Ohio Invasive Plant Research Conference.
Spontaneous crosses between common cattail species in North America has resulted in hybrid populations of Typha x glauca that may be more invasive than their parents. Typha latifolia L. (broad-leaved cattail) is native to North America, while T. angustifolia L. (narrow-leaved cattail) is considered to be invasive and is thought to have been introduced from Europe. Previous studies indicate that T. x glauca are generally first generation (F1) progeny between T. latifolia and T. angustifolia. However, we found evidence for backcrossing in a population in Cheboygan, Michigan. 26 putative hybrids and 40 putative nonhybrids were screened with RAPD markers in 2004. Two plants were genetically distinct, advanced-generation hybrids. The other putative hybrids had a full complement of markers from both parents, as expected for F1 hybrids. In 2006, an F1 hybrid plant was detected at the Olentangy River Wetlands Research Park at Ohio State University. We plan to survey wetlands in Ohio and Michigan to gain a better understanding of the frequency of hybrid populations, including possible advanced-generation “hybrid swarms” with intermediate genotypes. We will also investigate whether hybrids are more invasive than their parent taxa.
Our research is aimed at understanding the ecology of invasions of North American wetlands by both the European cattail, Typha angustifolia, and the hybrid produced by the crossing of that species with the native North American species, T. latifolia. Very little is known about the mechanisms by which either T. angustifolia or the hybrid (known as “T. x glauca”) invade, how fast they disperse, or which characteristics allow particular wetlands to be invaded. The lack of data is at least in part due to the difficulty of distinguishing the hybrid from the parent species in the field. However, we have developed a reliable set of field identification characteristics, confirmed by DNA markers, enabling us to collect rigorous ecological data to compare the cattail taxa. Using these identification criteria, we have been relating the distribution of the different taxa to habitat characteristics of Northern Michigan wetlands, including water level, nutrient and light levels, native vegetation, and anthropogenic disturbance. Going beyond correlative associations, we are separating causes and consequences of invasion by conducting manipulative experiments in wetlands as well as in controlled greenhouse and garden settings. Our long term goal is to combine these different approaches to predict which wetland habitats in Northern Michigan are most susceptible to invasion, enabling managers to develop scientifically-based strategies to deal with this threat.
Selbo, Sarena, Snow, Allison A. 2004. The potential for hybridization between Typha angustifolia and Typha latifolia in a constructed wetland. Aquatic Botany 78(4) April 2004. 361-369.
Three Typha taxa are recognized in the central USA: native Typha latifolia (broad-leaved cattail), the invasive Typha angustifolia (narrow-leaved cattail), and a hybrid between the two species, Typha x glauca. Previous authors have suggested that interspecific hybridization is common in cattails. In a 6-year-old constructed wetland in central Ohio, USA, we found that T angustifolia began flowering about 2 weeks earlier than T. latifolia, with female flowers opening several days earlier than male flowers. T. angustifolia shoots were 15 times more abundant and twice as dense as those of T. latifolia. Male flowers of T. angustifolia far outnumbered the male flowers of T. latifolia when the latter species began flowering, such that interspecific, pollination was likely during the short period of overlap. DNA markers (RAPDs) were used to screen for hybrids. These markers corresponded well to other species-specific traits, such as pollen type (monads versus tetrads) and the presence or absence of a gap in the floral spike. We found no putative hybrids based on surveys involving molecular and/or morphologicaltraits. Thus, we did not detect any gene flow between the cattail species.
Three taxa of cattail are generally recognized in Ohio. Typha latifolia (broad-leaved cattail), a North American native, is common throughout much of the United States and is an integral member of aquatic systems. Typha angustifolia (narrow-leaved cattail), thought to be introduced, is located in the northeastern range of T. latifolia and is considered an invasive species. The hybrid of the two species, Typha x glauca occurs within the range of T. angustifolia and may invade areas not previously inhabited by the parental species. The hybrid has intermediate characteristics of its parental species and thus is difficult to identify morphologically. To identify the species present, I chose 100 individuals at the Olentangy River Wetlands Research Park on the campus of The Ohio State University and took four morphological measurements as well as collected pollen and leaf tissue samples. DNA was extracted from a subset of the individuals and RAPDs were run to identify potential hybrids. The morphological measurements along with the pollen type were analyzed against the DNA evidence and in all but one case were found to correspond. No hybrids were found. In order to understand the potential for hybridization I studied the phenology of T. angustifolia and T. latifolia. Ten plots containing both species were set up and flowering times were recorded tri-weekly by observing which individuals were flowering in each plot. The male flowers of cattail open first and the pollen is released before the female flowers are receptive. I identified each plant to species and recorded if the male or female flowers were receptive. These data indicate that the overlap in flowering time between the two species is minimal. This information may prove to be critical in understanding the potential for hybrid occurrence in the wetlands.
Klips, Robert A.; Sweeney, Patricia M.; Bauman, Elisabeth K. F.; Snow, Allison A. 2005. Temporal and geographic variation in predispersal seed predation on hibiscus moscheutos L. (Malvaceae) in Ohio and Maryland, USA. American Midland Naturalist 154(2) 286-295.
Seed predation has the potential to strongly reduce seed production and thereby act as a selective force on the evolution of flowering traits and other defenses against herbivory. We characterized levels of predispersal seed predation on Hibiscus moscheutos (Malvaceae) during 2001 and 2002 at four sites in Ohio and Maryland, USA. The seed predators were a weevil, Conotrachelus fissinguis (Coleoptera, Curculionidae) and a bruchid beetle, Althaeus hibisci (Coleoptera, Bruchidae). The weevil occurred at three of the four sites and damaged 24% to 94% of fruits in these populations. The bruchid occurred at all four sites, where it destroyed 4% to 27% of the seeds. Seed predation varied between years and among sites for both predators and year-by site interactions were common. Variation in predation levels indicates that seed predators did not influence this species uniformly, but they were often abundant and sometimes destroyed nearly all of the seeds produced. At one of the Ohio sites, we assessed levels of seed predation at 5-d intervals during the 2001 flowering season. At this population, bruchid damage was greatest for seeds produced by flowers that opened in late July, when flowers were scarce, whereas weevil damage was greatest in mid-August and coincided with peak flowering. The timing and greater extent of weevil damage suggest that they may have a greater effect on plant fitness than bruchids.
Bauman, E., A. Snow, R. Klips, and L. Spencer. 2001. Temporal and geographic variation in seed predation in Hibiscus moscheutos. Ecological Society of American Annual Meeting Abstract 872002. 72-73.
Seed predators have the potential to exert strong selective pressure on plant characteristics such as flowering phenology. Predation levels are high in many species of Malvaceae, including the common rose mallow, Hibiscus moscheutos. This self-compatible, herbaceous perennial occurs in fresh and brackish marshes in the eastern USA. During the summers of 2000 and 2001, we characterized the phenology of flowering and seed predation in a central Ohio population during five-day intervals. We also quantified total seed predation at a second site in Ohio and two sites in Maryland, USA, in 2000 and 2001. We quantified damage levels for a weevil, Conotrachelus fissunguis (Coleoptera), by counting weevil exit holes in the fruit capsule, and for a bruchid beetle, Althaeus hibisci (Coleoptera), by calculating the proportion of seeds with beetle exit holes. Predation levels varied between years and among populations, ranging from 0% to 93% of fruits damaged by weevils and 3% to16% of seeds lost to beetles. At the northern Ohio site on Lake Erie, weevils were absent in both years and beetle damage was relatively high. Within seasons at the central Ohio site, damage by both coleopteran species was greatest for flowers that opened before peak flowering and decreased as the season progressed. Given the high levels of fruit and seed damage we observed, we hypothesize that synchronized flowering may be strongly advantageous in this species. Also, damage by seed predators appears to have a greater effect on plant fecundity than pollinator service because previous studies have shown that seed production is not pollinator-limited.
Selbo, S., and A. A. Snow. 2002. Flowering phenology and genetic diversity in native prairie vs. "restored" conservation grasslands in Ohio. Meeting of the Ecological Society of America, 872002 262.
Over the past several decades, prairie conservation and restoration efforts have increased across the Midwestern USA. In Ohio and elsewhere, grassland plantings in the federal Conservation Reserve Program (CRP) have become much more extensive than native prairie remnants. The seed source for CRP grasslands in Ohio often comes from as far away as Texas or Missouri, which may be undesirable from the standpoint of conservation genetics. The goal of this study was to investigate the potential for gene flow and "genetic swamping" of native prairie species with non-local genotypes. Our study focuses on the grass big bluestem (Andropogon gerardii, Poaceae), which is an obligate outcrosser and a common prairie species. We examined the potential for cross-pollination between each of three native and CRP populations of big bluestem by comparing flowering phenologies. Flowering times overlapped extensively, indicating that cross-pollination is possible where native and introduced genotypes occur near each other. To compare genetic variation in native and CRP populations, we collected leaf samples from six populations of each type (total of 296 samples) for DNA analyses. We obtained data for 68 loci from two RAPD primers. An analysis of molecular variance (AMOVA) showed no variation between the two groups (CRP and native), 84% of the genetic variation occurred within populations, and 16% occurred among populations. Native and CRP populations did not cluster together strongly in a neighbor joining tree, and genetic diversity values were similar and relatively high for both population types (68-69% of loci were polymorphic). Taken together, these analyses suggest that genetic differentiation between the two types of Andropogon populations may be minor. Our study represents an initial step in understanding the genetic effects of introducing non-local genotypes of big bluestem for conservation and restoration purposes.
Selbo, S., and J. L. Windus. 2001. Impacts of invasive plants on Ohio's natural areas. Ohio Fish and Wildlife Conference, 2001
Approximately one-fourth of the 3,000+ plant species known to occur in Ohio originate from other parts of the continent or world. These non-native species often lack their natural predators, pests and diseases, which control them in their native habitats. Once a non-native invader is established, the impact may cause tremendous ecological loss, including decreased biodiversity and ecosystem alteration. By replacing diversity with single stands of alien vegetation, invasive plants pose a direct threat to the native fauna as well as causing elimination of native flora. Invasive plants impact all of Ohio's natural plant communities including woodlands, savannas, prairies, and wetlands. The Ohio Division of Natural Areas & Preserves received an Ohio EPA Environmental Educational grant in 1998 to conduct a statewide awareness program concerning the threats of invasive plants. We developed literature including a color brochure and a series of fact sheets highlighting the 26 most invasive plant species in Ohio's natural areas. We presented a series of 13 workshops throughout the state to the general public, public agencies, and organizations. Two current studies in progress by the Division demonstrate species displacement and habitat degradation as a result of invasive plants. The first site at Old Woman Creek State Nature Preserve contains a swamp forest community which has been invaded by reed canary grass (Phalaris arundinacea) and common reed grass (Phragmites australis). Although control methods are not employed at present, this 14-year study documents a decline in native plant diversity. To contrast this study, data will be presented that documents restoration of high-quality fen meadows as a result of removal of glossy buckthorn (Rhamnus frangula) at Myersville Fen State Nature Preserve.
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