Population genomic evidence for radiative divergence of four Orychophragmus (Brassicaceae) species in eastern Asia

Radiation rather than bifurcating divergence has been inferred through a number of phylogenetic analyses using different DNA fragments. However, such inferences have rarely been tested by examining alternative hypotheses based on population genomic data. In this study, we sequenced the transcriptomes of 32 individuals from 13 populations of four Orychophragmus spp. (Brassicaceae) to investigate their divergence history. Cluster and population structure analyses recovered four distinct genetic clusters without any genetic mixture. Most orthologous genes produced unresolved bifurcating interspecific relationships with a star phylogeny. The resolved gene trees were highly inconsistent with each another in reconstructing interspecific relationships. Population genomic analyses suggested unexpectedly high genetic divergence and a lack of gene flow between the four species. We examined radiation vs. bifurcating divergence between these four species based on coalescent modelling tests of population genomic data. Our statistical tests supported a radiation of these species from a common ancestor at almost the same time, rejecting stepwise bifurcating interspecific divergence with time. This nearly simultaneous radiation was dated to the Quaternary, during which climate changes are suggested to have promoted species diversity in eastern Asia. Our results highlight the importance of population genomic data and statistical tests in deciphering interspecific relationships and tracing the divergence histories of closely related species.