Comparative phylogeography of three Cletus species pairs reveals shared responses to quaternary environmental change

Understanding how co-distributed species respond to shared environmental histories is a central question in evolutionary biology. Although comparative phylogeography has advanced rapidly in vertebrates, insects-despite comprising most terrestrial biodiversity-remain underrepresented. Here, we conducted a comparative phylogeographic study of three closely related phytophagous insect species pairs in the genus Cletus (Heteroptera: Coreidae) distributed across eastern Asia. Using genome-wide SNPs generated by ddRAD-seq, we integrated coalescent-based demographic modeling, ecological niche modeling, and full-likelihood Bayesian tests to explore patterns of divergence and demographic history. Our results revealed strong phylogeographic structure in all three species pairs, with genetic clusters largely partitioned along a north-south axis. Demographic modeling supported initial divergence during the Early Pleistocene followed by recent secondary contact during Late Pleistocene climatic oscillations. Bayesian model choice identified a single synchronous divergence event among all three lineages, indicating a shared response to Quaternary climatic changes. Species distribution modeling and Stairway Plot analyses further revealed asymmetrical demographic histories between northern and southern species, with northern taxa showing glacial contractions followed by recent expansion. Additionally, isolation-by-distance (IBD) was consistently detected across all lineages, whereas isolation-by-environment (IBE) was significant only in lineage I. Together, these findings suggest that historical climatic change was associated with parallel diversification and lineage-specific traits also contributed to differences in demographic responses. This study advances our understanding of insect diversification in eastern Asia and highlights the value of comparative genomic approaches for resolving complex evolutionary histories.