Cryptic genetic divergence of the red dwarf rasbora, Microrasbora rubescens, in and around Inle Lake: implications for the origin of endemicity in the ancient lake in Myanmar.

Inle Lake, an ancient lake located in the Shan Plateau of Myanmar, is a biogeographically attractive region with high fish endemism. Some endemic species inhabit the lake as well as the surrounding areas. The genetic and ecological relationships between populations in the lake and surrounding areas provide important insights into the process underlying ichthyofaunal formation in Inle Lake. In this study, the authors focused on red dwarf rasbora Microrasbora rubescens, an endemic genus and species in this region, and estimated its population structure and evolutionary scenario based on genome-wide polymorphism, mtDNA and geometric morphometric analyses using samples from Inle Lake and three areas surrounding the lake. The results showed that M. rubescens comprises at least three genetically divergent lineages (Inle, Heho and Hopong) with distinct geographic structures consistent with nuclear and mtDNA data. In contrast, there was no clear regional differentiation in morphology. The divergence time estimation based on mtDNA suggests that the Hopong lineage diverged at 2.7 Ma and the Inle and Heho lineages diverged at 1.9 Ma - consistent with the nuclear DNA results. The deep divergence observed in the endemic species supports the ancient history of ichthyofaunal development in this region. The distinct regional differentiation and morphological conservatism of this species might have been shaped by niche conservatism in stagnant water environments that limit dispersal and morphological diversification. Future comprehensive genetic and morphological analyses and comparisons for other native species should reveal the geographic and ecological processes that shaped the ichthyofauna in this region.