Washington State University, Vancouver Vancouver, Washington, United States
Abstract: Mutualism, cooperation between species, is an essential driver of biodiversity and is critical to productivity in agriculture. Agricultural crops have undergone artificial selection via domestication of wild species. The genetic costs of domestication include an increased burden of deleterious alleles and decreased genetic variation in domesticated plants compared to their wild relatives. However, the genetic costs of domestication on the genes involved in symbiosis in crop plants are not well understood. Legumes are well known for their mutualistic symbiosis with rhizobia, soil bacteria which fix nitrogen for their plant host. As an off-target trait of domestication, there is a potential for genes involved in symbiosis to accumulate deleterious alleles in symbiosis genes because these variants would not be purged by strong selection. The aim of this study is to provide a genetic perspective on the evolution of symbiosis under domestication by examining the impacts of genetic costs of domestication on symbiosis. Our approach was to compare deleterious variants in symbiosis genes between domesticated soybean (Glycine max) and wild soybean (Glycine soja). We quantified the number and severity of deleterious substitutions in 27K single nucleotide polymorphisms (SNPs) within 201 genes involved in rhizobial symbiosis for 1500 domesticated and wild soybean genotypes. We also examined genetic population structuring and tested for signals of selection in the symbiosis genes. Our preliminary analyses indicate that domesticated soybean may harbor a fewer number of putative deleterious substitutions in its symbiosis genes than its closest wild relative. For example, for the resistance gene analogue (Rfg1), essential to rhizobial symbiotic specificity, there are 59 potential sites that may contain putative deleterious substitutions across the gene’s coding region (3156bp). Twelve of these sites can be observed to be homozygous for the alternate allele in the wild soybean population, while sixteen can be observed in the domesticated soybean population, and eight sites in both populations. In conclusion, this study suggests that domesticated soybean house a lower accumulation of deleterious substitutions in Rfg1 when compared to their wild relatives, though many more symbiosis genes have yet to be examined. This continued research will generate the first assessment of genetic costs of domestication in multiple symbiosis genes. Understanding how crop plants evolve under domestication is vital to cultivar development, especially for off-target traits like symbiosis. Improving upon symbiosis is crucial for supporting the movement towards more sustainable agroecosystems and decreased dependence on fertilizers like nitrogen.
