Phylogeography and genetic diversity of Adineta beysunae and Rotaria sordida species complexes (Rotifera, Bdelloidea): a combined pattern of widespread and localized distribution driven by climate factors

Understanding the driving factors of the biogeography of microscopic animals is crucial for global biodiversity research. However, their geographical distribution patterns remain poorly resolved, with ongoing debate regarding the primary roles of climate, geography, and human activities. In this study, we investigated the genetic diversity and lineage distribution of two understudied bdelloid rotifer species complexes— Adineta beysunae Örstan, 2018 and Rotaria sordida (Western, 1893)—across 15 and 35 geographic regions (primarily in China and Europe), respectively. Phylogenetic analyses based on mitochondrial COI markers identified 10 distinct clades in A. beysunae and 58 in R. sordida , revealing high cryptic diversity. Analysis of Molecular Variance (AMOVA) indicated that genetic variation in both species occurred predominantly within populations ( A. beysunae : 75.26%; R. sordida : 75.55%) rather than among populations. These results demonstrate a heterogeneous phylogeographic structure characterized by widespread lineages and fragmented endemic clades (e.g., island-restricted lineages). Hierarchical partitioning analysis identified climatic factors—particularly seasonal temperature variation, annual precipitation, and mean wind speed—as the dominant drivers of phylogeographic patterns in Chinese populations, surpassing the effects of spatial variables, including longitude and elevation. Collectively, this study uncovers high cryptic diversity in A. beysunae and R. sordida , quantifies the relative contributions of key driving factors, and supplements existing knowledge of microfaunal biogeographic patterns, providing foundational data to elucidate global mechanisms of micro-metazoan diversification and adaptive evolution.