Population Genetics of an Endemic Species Mongoloniscus sinensis (Isopoda: Oniscidea) and Diversification Dynamics Across Northern China.

Endemic to temperate northern China, Mongoloniscus sinensis is a soil-dwelling isopod that provides a powerful system for investigating how Pleistocene climatic fluctuations and mountainous barriers have shaped genetic divergence and regional endemism. Through fine-scale phylogeographic and demographic analyses, we explored the evolutionary processes shaping genetic differentiation and biodiversity in this region. We obtained three mitochondrial genes (COI, ND5, and 12S rRNA) in 305 specimens collected from 48 localities spanning the species' distribution. Population structure analyses identified 3 clusters corresponding to geographic locations-East, West, and Midland. The Midland group exhibited the highest genetic diversity, possibly representing ancestral populations, followed by the West group and the lowest in the East group. Limited gene flow was observed between eastern and western populations, likely because of the geographical barrier formed by the Taihang and Luliang Mountains, which restrict genetic exchange. Demographic history analyses showed recent expansion in both East and West groups. Divergence time estimation dated the following splits from the Midland group: West (~0.63 Ma) and East (~0.53 Ma), coinciding with Pleistocene climatic oscillations. Ecological niche modeling revealed that M. sinensis currently occupies a wide climate range with highly suitable areas in northern China, and its distribution during the LGM was affected by the ice age. For the climate variables, the mean temperature of the coldest quarter was the most important variable to determine species distribution, followed by precipitation of the coldest quarter and elevation, indicating that cold adaptability may constrain the species to northern China. As an important geographical barrier, the Qinling Mountains have a diverse climate and habitats, which play a role in limiting the distribution of species.