An ecological approach to measuring the evolutionary consequences of gene flow from crops to wild or weedy relatives

Crop-Weed Introgression Plays Critical Roles in Genetic Differentiation and Diversity of Weedy Rice: A Case Study of Human-Influenced Weed Evolution

As an important driving force, introgression plays an essential role in shaping the evolution of plant species. However, knowledge concerning how introgression affects plant evolution in agroecosystems with strong human influences is still limited. To generate such knowledge, we used InDel (insertion/deletion) molecular fingerprints to determine the level of introgression from japonica rice cultivars into the indica type of weedy rice. We also analyzed the impact of crop-to-weed introgression on the genetic differentiation and diversity of weedy rice, using InDel (insertion/deletion) and SSR (simple sequence repeat) molecular fingerprints. Results based on the STRUCTURE analysis indicated an evident admixture of some weedy rice samples with indica and japonica components, suggesting different levels of introgression from japonica rice cultivars to the indica type of weedy rice. The principal coordinate analyses indicated indica-japonica genetic differentiation among weedy rice samples, which was positively correlated with the introgression of japonica-specific alleles from the rice cultivars. In addition, increased crop-to-weed introgression formed a parabola pattern of dynamic genetic diversity in weedy rice. Our findings based on this case study provide evidence that human activities, such as the frequent change in crop varieties, can strongly influence weed evolution by altering genetic differentiation and genetic diversity through crop-weed introgression in agroecosystems.

Recent and Historical Gene Flow in Cultivars, Landraces, and a Wild Taxon of Cucurbita pepo in Mexico

Gene flow among crops and their wild relatives is an active study area in evolutionary biology and horticulture, because genetic exchange between them may impact their evolutionary trajectories and increase the genetic variation of the cultivated lineages. Mexico is a center of diversity for the genus Cucurbita that includes pumpkins, squash and gourds. Gene flow between domesticated and wild species has been reported as common in Cucurbita; but gene flow among populations of C. pepo ssp. pepo from Mexico and its wild relative has not been studied. We used 2,061 SNPs, derived from tunable genotyping by sequencing (tGBS) to estimate gene flow among 14 Mexican traditional landraces of C. pepo ssp. pepo, also including individuals from five improved cultivars of C. pepo ssp. pepo and C. pepo ssp. ovifera var. ovifera, and individuals of their wild relative C. pepo ssp. fraterna. We found moderate to high levels of genetic diversity, and low to moderate genetic differentiation. In the test of introgression between lineages, we found that all possible arrangements for ancestral and derived sites between the lineages showed similar frequencies; thus, incomplete lineage sorting, but also gene flow, might be taking place in C. pepo. Overall, our results suggest that gene flow between these subspecies and cultigens, incomplete lineage sorting and the retention of ancestral characters shaped the evolutionary trajectory of C. pepo in its area of origin and diversification. In addition, we found evidence of the use of Mexican landraces as genetic material for the improvement of commercial cultivars. The landraces of Mexico are an important source of genetic diversity for C. pepo, which has been preserved both by management practices of small farmers and by the natural gene flow that exists between the different crop fields of the region.

Clonal diversity and spatial genetic structure in the long-lived herb, Prairie trillium

Combining population genetic studies with demographic surveys in long-lived clonal herbs can yield insight into the population dynamics of clonal plant populations. In this study, we assayed clonal diversity and spatial genetic structure in a population of a long-lived understory herb, Trillium recurvatum, that has been the focus of a demographic study spanning 26 years at the Meeman Biological Station in Memphis, Tennessee, USA. Using a set of five newly developed simple sequence repeat markers first reported here, we assessed 1) the extent of clonal diversity within the Meeman site, 2) the degree to which genetic diversity varies with stage class (juvenile, non-flowering, and flowering adults) at this site, 3) whether there is spatial genetic structure at the Meeman site, and 4) how measures of genetic diversity and inbreeding at the Meeman site compare to two additional nearby populations. Along with these analyses, we calculated and compared traditional population genetic metrics with information theory-based diversity indices. Although clonal propagation was present, the focal population displayed moderate levels of clonal diversity comprising 81 genets from the 174 individuals sampled. In the focal site, we also found that genetic diversity was highest in the flowering stage class when compared to the non-flowering and juvenile classes. We report that genets exhibited spatial genetic structure in the focal site exhibiting values for the Sp statistic of 0.00199 for linear distance and 0.0271 for log distance. Measures of unbiased gene diversity and the inbreeding coefficient were comparable across the sampled populations. Our results provide complementary genetic data to previous demographic studies in T. recurvatum, and these findings provide data for future studies aimed at integrating the degree of clonality, genetic variation, and population dynamics in this species. Our findings suggest that T. recurvatum at the focal Meeman site displays higher levels of sexual reproduction than were previously suggested, and spatial genetic structure estimates were comparable to other plant species with mixed and outcrossing mating strategies.

Little White Lies: Pericarp Color Provides Insights into the Origins and Evolution of Southeast Asian Weedy Rice

Weedy rice is a conspecific form of cultivated rice (Oryza sativa L.) that infests rice fields and results in severe crop losses. Weed strains in different world regions appear to have originated multiple times from different domesticated and/or wild rice progenitors. In the case of Malaysian weedy rice, a multiple-origin model has been proposed based on neutral markers and analyses of domestication genes for hull color and seed shattering. Here, we examined variation in pericarp (bran) color and its molecular basis to address how this trait evolved in Malaysian weeds and its possible role in weed adaptation. Functional alleles of the Rc gene confer proanthocyanidin pigmentation of the pericarp, a trait found in most wild and weedy Oryzas and associated with seed dormancy; nonfunctional rc alleles were strongly favored during rice domestication, and most cultivated varieties have nonpigmented pericarps. Phenotypic characterizations of 52 Malaysian weeds revealed that most strains are characterized by the pigmented pericarp; however, some weeds have white pericarps, suggesting close relationships to cultivated rice. Phylogenetic analyses indicate that the Rc haplotypes present in Malaysian weeds likely have at least three distinct origins: wild O. rufipogon, white-pericarp cultivated rice, and red-pericarp cultivated rice. These diverse origins contribute to high Rc nucleotide diversity in the Malaysian weeds. Comparison of Rc allelic distributions with other rice domestication genes suggests that functional Rc alleles may confer particular fitness benefits in weedy rice populations, for example, by conferring seed dormancy. This may promote functional Rc introgression from local wild Oryza populations.

Patterns of Gene Flow between Crop and Wild Carrot, Daucus carota (Apiaceae) in the United States

Studies of gene flow between crops and their wild relatives have implications for both management practices for cultivation and understanding the risk of transgene escape. These types of studies may also yield insight into population dynamics and the evolutionary consequences of gene flow for wild relatives of crop species. Moreover, the comparison of genetic markers with different modes of inheritance, or transmission, such as those of the nuclear and chloroplast genomes, can inform the relative risk of transgene escape via pollen versus seed. Here we investigate patterns of gene flow between crop and wild carrot, Daucus carota (Apiaceae) in two regions of the United States. We employed 15 nuclear simple sequence repeat (SSR) markers and one polymorphic chloroplast marker. Further, we utilized both conventional population genetic metrics along with Shannon diversity indices as the latter have been proposed to be more sensitive to allele frequency changes and differentiation. We found that populations in both regions that were proximal to crop fields showed lower levels of differentiation to the crops than populations that were located farther away. We also found that Shannon measures were more sensitive to differences in both genetic diversity and differentiation in our study. Finally, we found indirect evidence of paternal transmission of chloroplast DNA and accompanying lower than expected levels of chloroplast genetic structure amongst populations as might be expected if chloroplast DNA genes flow through both seed and pollen. Our findings of substantial gene flow for both nuclear and chloroplast markers demonstrate the efficiency of both pollen and seed to transfer genetic information amongst populations of carrot.