Post-glacial distribution of a Mongolian mayfly inferred from population genetic analysis

The polymitarcyid burrowing mayfly, Ephoron nigridorsum, adapts to extreme continental climate by undergoing egg diapause during the long and cold winter and rapid growth during the short and hot summer. We performed genetic analyses of the mitochondrial COI gene of E. nigridorsum from 10 local populations of the Selenge River basin in Mongolia to examine the historical population dynamics during the last glacial period. We observed high overall genetic diversity and high intra-population variation. However, we could find no geographic cluster of haplotypes and no correlation between genetic differentiation and geographic distance within the river basin. The long-chain haplotype network and multimodal mismatch distribution implied that the population size has remained constant for a long period of time. The Bayesian skyline plot indicated an expansion of the population size during cooling through the last glacial period and a stable population size from the post-glacial period to the present day. Our results suggest that the Selenge River basin provided a comparatively stable habitat for E. nigridorsum during the last glacial period when the decrease in mean air temperature in summer (1–7 °C colder) is smaller than that of winter temperature (7–15 °C colder) according to botanical records. The mayfly has been probably capable of adapting to a more extreme climate – i.e. with a larger temperature difference between winter and summer – during the last glacial period by undergoing egg diapause, as montane and arctic species of cold-adapted aquatic insects have expanded their distributions and population sizes during this period.