Quantifying stochastic introgression processes with hazard rates

Introgression is the permanent incorporation of genes from one population into another through hybridization and backcrossing. It can have large environmental consequences, such as the spread of insecticide or herbicide resistant genes, the escape of transgenes from genetically modified crops, and the invasion of exotic genes into new habitats. Introgression usually involves strong random components, such as rare hybridization and backcrossing events, and demographic variation in reproduction and survival. Most introgression studies ignore these random effects, and consequently fail to accurately assess the risk of introgression. This paper presents a methodology for quantifying stochastic introgression processes, based on multitype branching process models. We derive a quantity called the hazard rate, which can be used to investigate how the risk of introgression depends on crop management and life history.     

Quantifying introgression risk with realistic population genetics

Introgression is the permanent incorporation of genes from the genome of one population into another. This can have severe consequences, such as extinction of endemic species, or the spread of transgenes. Quantification of the risk of introgression is an important component of genetically modified crop regulation. Most theoretical introgression studies aimed at such quantification disregard one or more of the most important factors concerning introgression: realistic genetical mechanisms, repeated invasions and stochasticity. In addition, the use of linkage as a risk mitigation strategy has not been studied properly yet with genetic introgression models. Current genetic introgression studies fail to take repeated invasions and demographic stochasticity into account properly, and use incorrect measures of introgression risk that can be manipulated by arbitrary choices. In this study, we present proper methods for risk quantification that overcome these difficulties. We generalize a probabilistic risk measure, the so-called hazard rate of introgression, for application to introgression models with complex genetics and small natural population sizes. We illustrate the method by studying the effects of linkage and recombination on transgene introgression risk at different population sizes.     

Fine-scale geographical structure of genetic diversity in inland wild beet populations

Introgression arising from crop-to-wild gene flow provides novel sources of genetic variation in plant species complexes. Hybridization within the Beta vulgaris species complex is of immediate concern; crop lineages ( B .  vulgaris ssp. vulgaris ) hybridize easily with their wild relatives ( B .  vulgaris ssp. maritima ) thereby threatening wild beet gene diversity with genetic swamping. Hybridization 'hotspots' occur in European seed production areas because inland ruderal wild beets occur and reproduce in sympatry with cultivated beets. We studied gene flow occurring between seed-producing cultivars and ruderal wild B .  vulgaris in southwestern France to determine whether feral beets, arising from unharvested cultivated seed, represent an opportunity for crop-to-wild gene flow. We surveyed 42 inland ruderal beet populations located near seed production fields for nucleo-cytoplasmic variation and used a cytoplasmic marker diagnostic of cultivated lines. Occurrence of cultivated-type cytoplasm within ruderal populations clearly reflected events of crop seed escape. However, we found no genetic signatures of nuclear cultivated gene introgression, which suggests past introgression of cultivated cytoplasm into a wild nuclear background through seed escape rather than recent direct pollen flow. Overall, patterns of genetic structure suggested that inland ruderal wild beet populations act as a metapopulation, with founding events involving a few sib groups, followed by low rates of seed or pollen gene flow after populations are established. Altogether, our results indicate that a long-lived seed bank plays a key role in maintaining cultivated-type cytoplasm in the wild and highlight the need for careful management of seed production areas where wild and cultivated relatives co-occur.     

Islands and streams: clusters and gene flow in wild barley populations from the Levant

The domestication of plants frequently results in a high level of genetic differentiation between domesticated plants and their wild progenitors. This process is counteracted by gene flow between wild and domesticated plants because they are usually able to inter‐mate and to exchange genes. We investigated the extent of gene flow between wild barley Hordeum spontaneum and cultivated barley Hordeum vulgare, and its effect on population structure in wild barley by analysing a collection of 896 wild barley accessions (Barley1K) from Israel and all available Israeli H. vulgare accessions from the Israeli gene bank. We compared the performance of simple sequence repeats (SSR) and single nucleotide polymorphisms (SNP) marker data genotyped over a core collection in estimating population parameters. Estimates of gene flow rates with SSR markers indicated a high level of introgression from cultivated barley into wild barley. After removing accessions from the wild barley sample that were recently admixed with cultivated barley, the inference of population structure improved significantly. Both SSR and SNP markers showed that the genetic population structure of wild barley in Israel corresponds to the three major ecogeographic regions: the coast, the Mediterranean north and the deserts in the Jordan valley and the South. Gene flow rates were estimated to be higher from north to south than in the opposite direction. As has been observed in other crop species, there is a significant exchange of alleles between the wild species and domesticated varieties that needs to be accounted for in the population genetic analysis of domestication.     

Consequences of recurrent gene flow from crops to wild relatives

Concern about gene flow from crops to wild relatives has become widespread with the increasing cultivation of transgenic crops. Possible consequences of such gene flow include genetic assimilation, wherein crop genes replace wild ones, and demographic swamping, wherein hybrids are less fertile than their wild parents, and wild populations shrink. Using mathematical models of a wild population recurrently receiving pollen from a genetically fixed crop, we find that the conditions for genetic assimilation are not stringent, and progress towards replacement can be fast, even for disfavoured crop genes. Demographic swamping and genetic drift relax the conditions for genetic assimilation and speed progress towards replacement. Genetic assimilation can involve thresholds and hysteresis, such that a small increase in immigration can lead to fixation of a disfavoured crop gene that had been maintained at a moderate frequency, even if the increase in immigration is cancelled before the gene fixes. Demographic swamping can give rise to 'migrational meltdown', such that a small increase in immigration can lead to not only fixation of a disfavoured crop gene but also drastic shrinkage of the wild population. These findings suggest that the spread of crop genes in wild populations should be monitored more closely.     

Selection on domestication traits and quantitative trait loci in crop-wild sunflower hybrids

The strength and extent of gene flow from crops into wild populations depends, in part, on the fitness of the crop alleles, as well as that of alleles at linked loci. Interest in crop-wild gene flow has increased with the advent of transgenic plants, but nontransgenic crop-wild hybrids can provide case studies to understand the factors influencing introgression, provided that the genetic architecture and the fitness effects of loci are known. This study used recombinant inbred lines (RILs) generated from a cross between crop and wild sunflowers to assess selection on domestication traits and quantitative trait loci (QTL) in two contrasting environments, in Indiana and Nebraska, USA. Only a small fraction of plants (9%) produced seed in Nebraska, due to adverse weather conditions, while the majority of plants (79%) in Indiana reproduced. Phenotypic selection analysis found that a mixture of crop and wild traits were favoured in Indiana (i.e. had significant selection gradients), including larger leaves, increased floral longevity, larger disk diameter, reduced ray flower size and smaller achene (seed) mass. Selection favouring early flowering was detected in Nebraska. QTLs for fitness were found at the end of linkage groups six (LG6) and nine (LG9) in both field sites, each explaining 11-12% of the total variation. Crop alleles were favoured on LG9, but wild alleles were favoured on LG6. QTLs for numerous domestication traits overlapped with the fitness QTLs, including flowering date, achene mass, head number, and disk diameter. It remains to be seen if these QTL clusters are the product of multiple linked genes, or individual genes with pleiotropic effects. These results indicate that crop trait values and alleles may sometimes be favoured in a noncrop environment and across broad geographical regions.     

Population genetic structure of Japanese wild soybean (Glycine soja) based on microsatellite variation

The research objectives were to determine aspects of the population dynamics relevant to effective monitoring of gene flow in the soybean crop complex in Japan. Using 20 microsatellite primers, 616 individuals from 77 wild soybean (Glycine soja) populations were analysed. All samples were of small seed size (< 0.03 g), were directly collected in the field and came from all parts of Japan where wild soybeans grow, except Hokkaido. Japanese wild soybean showed significant reduction in observed heterozygosity, low outcrossing rate (mean 3.4%) and strong genetic differentiation among populations. However, the individual assignment test revealed evidence of rare long-distance seed dispersal (> 10 km) events among populations, and spatial autocorrelation analysis revealed that populations within a radius of 100 km showed a close genetic relationship to one another. When analysis of graphical ordination was applied to compare the microsatellite variation of wild soybean with that of 53 widely grown Japanese varieties of cultivated soybean (Glycine max), the primary factor of genetic differentiation was based on differences between wild and cultivated soybeans and the secondary factor was geographical differentiation of wild soybean populations. Admixture analysis revealed that 6.8% of individuals appear to show introgression from cultivated soybeans. These results indicated that population genetic structure of Japanese wild soybean is (i) strongly affected by the founder effect due to seed dispersal and inbreeding strategy, (ii) generally well differentiated from cultivated soybean, but (iii) introgression from cultivated soybean occurs. The implications of the results for the release of transgenic soybeans where wild soybeans grow are discussed.     

A genome-wide analysis of differentiation between wild and domesticated Phaseolus vulgaris from Mesoamerica

Lack of introgression or divergent selection may be responsible for the maintenance of phenotypic differences between sympatric populations of crops and their wild progenitors. To distinguish between these hypotheses, amplified fragment length polymorphism markers were located on a molecular linkage map of Phaseolus vulgaris relative to genes for the domestication syndrome and other traits. Diversity for these same markers was then analyzed in two samples of wild and domesticated populations from Mesoamerica. Differentiation between wild and domesticated populations was significantly higher in parapatric and allopatric populations compared to sympatric populations. It was also significantly higher near genes for domestication compared to those away from these genes. Concurrently, the differences in genetic diversity between wild and domesticated populations were strongest around such genes. These data suggest that selection in the presence of introgression appears to be a major evolutionary factor maintaining the identity of wild and domesticated populations in sympatric situations. Furthermore, alleles from domesticated populations appear to have displaced alleles in sympatric wild populations, thus leading to a reduction in genetic diversity in such populations. These results also provide a possible experimental framework for assessing the long-term risk of transgene escape and the targeting of transgenes inside the genome to minimize the survival of these transgenes into wild populations following introduction by gene flow.This article is dedicated to the memory of Epimaki M. K. Koinange.     

Selection on domestication traits and quantitative trait loci in crop–wild sunflower hybrids

The strength and extent of gene flow from crops into wild populations depends, in part, on the fitness of the crop alleles, as well as that of alleles at linked loci. Interest in crop–wild gene flow has increased with the advent of transgenic plants, but nontransgenic crop–wild hybrids can provide case studies to understand the factors influencing introgression, provided that the genetic architecture and the fitness effects of loci are known. This study used recombinant inbred lines (RILs) generated from a cross between crop and wild sunflowers to assess selection on domestication traits and quantitative trait loci (QTL) in two contrasting environments, in Indiana and Nebraska, USA. Only a small fraction of plants (9%) produced seed in Nebraska, due to adverse weather conditions, while the majority of plants (79%) in Indiana reproduced. Phenotypic selection analysis found that a mixture of crop and wild traits were favoured in Indiana (i.e. had significant selection gradients), including larger leaves, increased floral longevity, larger disk diameter, reduced ray flower size and smaller achene (seed) mass. Selection favouring early flowering was detected in Nebraska. QTLs for fitness were found at the end of linkage groups six (LG6) and nine (LG9) in both field sites, each explaining 11–12% of the total variation. Crop alleles were favoured on LG9, but wild alleles were favoured on LG6. QTLs for numerous domestication traits overlapped with the fitness QTLs, including flowering date, achene mass, head number, and disk diameter. It remains to be seen if these QTL clusters are the product of multiple linked genes, or individual genes with pleiotropic effects. These results indicate that crop trait values and alleles may sometimes be favoured in a noncrop environment and across broad geographical regions.     

Balancing selection,genetic drift,and human‐mediated introgression interplay to shape MHC (functional) diversity in Mediterranean brown trout

The extraordinary polymorphism of major histocompatibility complex (MHC) genes is considered a paradigm of pathogen‐mediated balancing selection, although empirical evidence is still scarce. Furthermore, the relative contribution of balancing selection to shape MHC population structure and diversity, compared to that of neutral forces, as well as its interaction with other evolutionary processes such as hybridization, remains largely unclear. To investigate these issues, we analyzed adaptive (MHC‐DAB gene) and neutral (11 microsatellite loci) variation in 156 brown trout (Salmo trutta complex) from six wild populations in central Italy exposed to introgression from domestic hatchery lineages (assessed with the LDH gene). MHC diversity and structuring correlated with those at microsatellites, indicating the substantial role of neutral forces. However, individuals carrying locally rare MHC alleles/supertypes were in better body condition (a proxy of individual fitness/parasite load) regardless of the zygosity status and degree of sequence dissimilarity of MHC, hence supporting balancing selection under rare allele advantage, but not heterozygote advantage or divergent allele advantage. The association between specific MHC supertypes and body condition confirmed in part this finding. Across populations, MHC allelic richness increased with increasing admixture between native and domestic lineages, indicating introgression as a source of MHC variation. Furthermore, introgression across populations appeared more pronounced for MHC than microsatellites, possibly because initially rare MHC variants are expected to introgress more readily under rare allele advantage. Providing evidence for the complex interplay among neutral evolutionary forces, balancing selection, and human‐mediated introgression in shaping the pattern of MHC (functional) variation, our findings contribute to a deeper understanding of the evolution of MHC genes in wild populations exposed to anthropogenic disturbance.     

Genomewide introgressive hybridization patterns in wild Atlantic salmon influenced by inadvertent gene flow from hatchery releases

Many salmonid fish populations are threatened by genetic homogenization, primarily due to introgressive hybridization with hatchery‐reared conspecifics. By applying genomewide analysis using two molecular marker types (1986 SNPs and 17 microsatellites), we assessed the genetic impacts of inadvertent gene flow via straying from hatchery releases on wild populations of Atlantic salmon in the Gulf of Finland, Baltic Sea, over 16 years (1996–2012). Both microsatellites and SNPs revealed congruent population genetic structuring, indicating that introgression changed the genetic make‐up of wild populations by increasing genetic diversity and reducing genetic divergence. However, the degree of genetic introgression varied among studied populations, being higher in the eastern part and lower in the western part of Estonia, which most likely reflects the history of past stocking activities. Using kernel smoothing and permutation testing, we detected considerable heterogeneity in introgression patterns across the genome, with a large number of regions exhibiting nonrandom introgression widely dispersed across the genome. We also observed substantial variation in nonrandom introgression patterns within populations, as the majority of genomic regions showing elevated or reduced introgression were not consistently detected among temporal samples. This suggests that recombination, selection and stochastic processes may contribute to complex nonrandom introgression patterns. Our results suggest that (i) some genomic regions in Atlantic salmon are more vulnerable to introgressive hybridization, while others show greater resistance to unidirectional gene flow; and (ii) the hybridization of previously separated populations leads to complex and dynamic nonrandom introgression patterns that most likely have functional consequences for indigenous populations.     

Dissimilarity of contemporary and historical gene flow in a wild carrot (Daucus carota) metapopulation under contrasting levels of human disturbance: implications for risk assessment and management of transgene introgression

Background and Aims

Transgene introgression from crops into wild relatives may increase the resistance of wild plants to herbicides, insects, etc. The chance of transgene introgression depends not only on the rate of hybridization and the establishment of hybrids in local wild populations, but also on the metapopulation dynamics of the wild relative. The aim of the study was to estimate gene flow in a metapopulation for assessing and managing the risks of transgene introgression.

Methods

Wild carrots (Daucus carota) were sampled from 12 patches in a metapopulation. Eleven microsatellites were used to genotype wild carrots. Genetic structure was estimated based on the F_ST statistic. Contemporary (over the last several generations) and historical (over many generations) gene flow was estimated with assignment and coalescent methods, respectively.

Key Results

The genetic structure in the wild carrot metapopulation was moderate (F_ST = 0·082) and most of the genetic variation resided within patches. A pattern of isolation by distance was detected, suggesting that most of the gene flow occurred between neighbouring patches (≤1 km). The mean contemporary gene flow was 5 times higher than the historical estimate, and the correlation between them was very low. Moreover, the contemporary gene flow in roadsides was twice that in a nature reserve, and the correlation between contemporary and historical estimates was much higher in the nature reserve. Mowing of roadsides may contribute to the increase in contemporary gene flow. Simulations demonstrated that the higher contemporary gene flow could accelerate the process of transgene introgression in the metapopulation.

Conclusions

Human disturbance such as mowing may alter gene flow patterns in wild populations, affecting the metapopulation dynamics of wild plants and the processes of transgene introgression in the metapopulation. The risk assessment and management of transgene introgression and the control of weeds need to take metapopulation dynamics into consideration.     

Sequence evidence for sporadic intergeneric DNA introgression from wheat into a wild Aegilops species

Introgressive hybridization has played a crucial role in the evolution of many plant species, especially polyploids. The duplicated genetic material and wide geographical distribution facilitate hybridization and introgression among polyploid species having either homologous or homoeologous genomes. Such introgression may lead to the production of recombinant genomes that are more difficult to form at the diploid level. Crop genes that have introgressed into wild relatives can increase the capability of the wild relatives to adapt to agricultural environments and compete with crops or to compete with other wild species. Although the transfer of genes from crops into their conspecific immediate wild progenitors has been reported, little is known about spontaneous gene movement from crops to more distantly related species. We describe recent spontaneous DNA introgression from domesticated polyploid wheat into distantly related, wild tetraploid Aegilops peregrina (syn. Aegilops variabilis) and the stabilization of this sequence in wild populations despite not having homologous chromosomes. Our results show that DNA can spontaneously introgress between homoeologous genomes of species of the tribe Triticeae and, in the case of crop-wild relatives, possibly enrich the wild population. These results also emphasize the need for fail-safe mechanisms in transgenic crops to prevent gene flow where there may be ecological risks.     

Impact of gene flow from cultivated beet on genetic diversity of wild sea beet populations

Gene flow and introgression from cultivated plants may have important consequences for the conservation of wild plant populations. Cultivated beets (sugar beet, red beet and Swiss chard: Beta vulgaris ssp. vulgaris) are of particular concern because they are cross-compatible with the wild taxon, sea beet (B.vs. ssp. maritima). Cultivated beet seed production areas are sometimes adjacent to sea beet populations; the numbers of flowering individuals in the former typically outnumber those in the populations of the latter. In such situations, gene flow from cultivated beets has the potential to alter the genetic composition of the nearby wild populations. In this study we measured isozyme allele frequencies of 11 polymorphic loci in 26 accessions of cultivated beet, in 20 sea beet accessions growing near a cultivated beet seed production region in northeastern Italy, and 19 wild beet accessions growing far from seed production areas. We found one allele that is specific to sugar beet, relative to other cultivated types, and a second that has a much higher frequency in Swiss chard and red beet than in sugar beet. Both alleles are typically rare in sea beet populations that are distant from seed production areas, but both are common in those that are near the Italian cultivated beet seed production region, supporting the contention that gene flow from the crop to the wild species can be substantial when both grow in proximity. Interestingly, the introgressed populations have higher genetic diversity than those that are isolated from the crop. The crop-to-wild gene flow rates are unknown, as are the fitness consequences of such alleles in the wild. Thus, we are unable to assess the long-term impact of such introgression. However, it is clear that gene flow from a crop to a wild taxon does not necessarily result in a decrease in the genetic diversity of the native plant.     

Insights on the evolution of a vegetatively propagated crop species

The opportunity for gene flow between a vegetatively propagated crop and its wild relatives is expected to be much lower than for seed-propagated crops, since sexual reproduction in the crop occurs only infrequently. A study by Duputié and colleagues now demonstrates evidence of sexual reproduction between a vegetatively propagated crop and a closely related wild congener. Working in French Guiana, these workers have documented a hybrid zone arising from introgression between cassava (Manihot esculenta ssp. esculenta, Euphorbiaceae), which is propagated by stem cuttings, and wild Manihot populations growing in close proximity. Patterns of heterozygosity suggest that there are little-to-no barriers to reproduction between the crop and these wild populations. Previous work by these researchers has documented the importance of occasional sexual reproduction for the development of cassava varieties in traditional Amerindian farming systems. Taken together with their previous work, these new findings suggest that gene flow between wild Manihot populations and cassava plants could potentially play a much greater role in the crop's evolution than previously thought.     

Introgression and persistence of cultivar alleles in wild carrot (Daucus carota) populations in the United States

Premise

Cultivated species and their wild relatives often hybridize in the wild, and the hybrids can survive and reproduce in some environments. However, it is unclear whether cultivar alleles are permanently incorporated into the wild genomes or whether they are purged by natural selection. This question is key to accurately assessing the risk of escape and spread of cultivar genes into wild populations.

Methods

We used genomic data and population genomic methods to study hybridization and introgression between cultivated and wild carrot (Daucus carota) in the United States. We used single nucleotide polymorphisms (SNPs) obtained via genotyping by sequencing for 450 wild individuals from 29 wild georeferenced populations in seven states and 144 cultivars from the United States, Europe, and Asia.

Results

Cultivated and wild carrot formed two genetically differentiated groups, and evidence of crop–wild admixture was detected in several but not all wild carrot populations in the United States. Two regions were identified where cultivar alleles were present in wild carrots: California and Nantucket Island (Massachusetts). Surprisingly, there was no evidence of introgression in some populations with a long-known history of sympatry with the crop, suggesting that post-hybridization barriers might prevent introgression in some areas.

Conclusions

Our results provide support for the introgression and long-term persistence of cultivar alleles in wild carrots populations. We thus anticipate that the release of genetically engineered (GE) cultivars would lead to the introduction and spread of GE alleles in wild carrot populations.     

Contrasting population genetic structure and gene flow between <Emphasis Type="Italic">Oryza rufipogon</Emphasis> and <Emphasis Type="Italic">Oryza nivara</Emphasis>

The cross compatible wild relatives of crops have furnished valuable genes for crop improvement. Understanding the genetics of these wild species may enhance their further use in breeding. In this study, sequence variation of the nuclear Lhs1 gene was used to investigate the population genetic structure and gene flow of Oryza rufipogon and O. nivara, two wild species most closely related to O. sativa. The two species diverge markedly in life history and mating system, with O. rufipogon being perennial and outcrossing and O. nivara being annual and predominantly inbreeding. Based on sequence data from 105 plants representing 11 wild populations covering the entire geographic range of these wild species, we detected significantly higher nucleotide variation in O. rufipogon than in O. nivara at both the population and species levels. At the population level the diversity in O. rufipogon (Hd = 0.712; θ _sil = 0.0017) is 2–3 folds higher than that in O. nivara (Hd = 0.306; θ _sil = 0.0005). AMOVA partitioning indicated that genetic differentiation among O. nivara populations (78.2%) was much higher than that among O. rufipogon populations (52.3%). The different level of genetic diversity and contrasting population genetic structure between O. rufipogon and O. nivara might be explained by their distinct life histories and mating systems. Our simulation using IM models demonstrated significant gene flow from O. nivara to O. rufipogon, indicating a directional introgression from the annual and selfing species into the perennial and outcrossing species. The ongoing introgression has played an important role in shaping current patterns of genetic diversity of these two wild species. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Genome‐specific introgression between wheat and its wild relative Aegilops triuncialis

Introgression of sequences from crop species in wild relatives is of fundamental and practical concern. Here, we address gene flow between cultivated wheat and its widespread polyploid relative, Aegilops triuncialis, using 12 EST‐SSR markers mapped on wheat chromosomes. The presence of wheat diagnostic alleles in natural populations of the barbed goatgrass growing in proximity to cultivated fields highlights that substantial gene flow occurred when both species coexisted. Furthermore, loci from the A subgenome of wheat were significantly less introgressed than sequences from other subgenomes, indicating differential introgression into Ae. triuncialis. Gene flow between such species sharing nonhomeologous chromosomes addresses the evolutionary outcomes of hybridization and may be important for efficient gene containment.     

Transgene introgression in crop relatives: molecular evidence and mitigation strategies

Incorporation of crop genes into wild and weedy relative populations (i.e. introgression) has long been of interest to ecologists and weed scientists. Potential negative outcomes that result from crop transgene introgression (e.g. extinction of native wild relative populations; invasive spread by wild or weedy hosts) have not been documented, and few examples of transgene introgression exist. However, molecular evidence of introgression from non-transgenic crops to their relatives continues to emerge, even for crops deemed low-risk candidates for transgene introgression. We posit that transgene introgression monitoring and mitigation strategies are warranted in cases in which transgenes are predicted to confer selective advantages and disadvantages to recipient hosts. The utility and consequences of such strategies are examined, and future directions provided.     

Stress and domestication traits increase the relative fitness of crop-wild hybrids in sunflower

After a decade of transgenic crop production, the dynamics of gene introgression into wild relatives remain unclear. Taking an ecological genetics approach to investigating fitness in crop-wild hybrid zones, we uncovered both conditions and characteristics that may promote introgression. We compared diverse crop-wild hybrid genotypes relative to wild Helianthus annuus under one benign and three stressful agricultural environments. Whereas relative fitness of crop-wild hybrids averaged 0.25 under benign conditions, with herbicide application or competition it reached 0.45 and was more variable. In some instances, hybrid fitness matched wild fitness (approximately 1). Thus, wild populations under agronomic stress may be more susceptible to introgression. Although 'domestication' traits are typically considered unlikely to persist in wild populations, we found some (e.g. rapid growth and early flowering) that may enhance hybrid fitness, especially in stressful environments. Rigorous assessment of how particular genotypes, phenotypes, and environments affect introgression will improve risk assessment for transgenic crops.