Recent colonization by a coastal plant of inland habitats at an ancient freshwater lake, L ake B iwa: multilocus sequencing and a demographic history of L athyrus japonicus ( F abaceae)

Ancient lakes have been recognized as “long‐term isolated islands” in terrestrial ecosystems. L ake B iwa, one of the few ancient lakes that formed around 4 million years ago, harbors many coastal species that commonly inhabit seashores. The beach pea, L athyrus japonicus , is a typical coastal species of this freshwater lake, where morphological, physiological, and genetic differentiations have been reported between B iwa and coastal populations. Whether B iwa populations were isolated for long periods throughout P leistocene climatic oscillations and subsequent range shifts is unclear. We assessed population genetic structure and demography of beach pea in this ancient freshwater lake using the sequences of eight nuclear loci. The results of STRUCTURE analyses showed evidence of admixture between B iwa and coastal populations, reflecting recent gene flow. The estimated demographic parameters implemented by the isolation with migration model ( IM model) revealed a recent divergence (postglacial period) of B iwa populations, with some gene flow from B iwa to coastal populations. In addition, B iwa populations were significantly smaller in size than the ancestral or coastal populations. Our study suggests that a H olocene thermal maximum, when transgression could allow seeds from coastal plants to access L ake B iwa, was involved in the origin of the B iwa populations and their genetic divergence. Thus, coastal populations might have migrated to L ake B iwa relatively recently. Our study concluded that ancestral migrants in L ake B iwa were derived from small founding populations and accelerated genetic isolation of B iwa populations during short‐term isolation.