Candidate genes and signatures of directional selection on fruit quality traits during apple domestication

PREMISE OF THE STUDY: During plant domestication, traits can be subject to a variety of types of selection, ranging from strong directional selection for traits such as seed or fruit size to diversifying selection for traits like color or flavor. These types of selection interact with other evolutionary processes including genetic bottlenecks and interspecific gene flow to generate different levels of genetic diversity across the genome and at target genes in domesticated lineages, but little is known about the impacts of these processes in perennial fruit crops. METHODS: We used sequence-capture by hybridization to examine patterns of diversity at a suite of candidate domestication and anonymous background genes in domesticated apple (Malus x domestica) in comparison to its wild relatives Malus sieversii and Malus orientalis. KEY RESULTS: We found no change in average diversity at these candidate domestication genes across the three species. However, a subset of the genes did exhibit patterns of very high or very low diversity in M. x domestica compared to its progenitor, M. sieversii. Of the genes with characterized function, the low diversity genes mainly contributed to fruit quality traits like color and flavor, predicted to be under conscious, directional selection relatively late in the domestication process, while the high diversity genes included a variety of functions. CONCLUSIONS: Overall, these results are consistent with predictions based on the likely timing and nature of selection during domestication, and open new avenues for understanding genes with high diversity in a perennial crop compared to its wild relatives.