How Did a Size-Divergent Water Strider Species Group Exhibiting an East Asia-Eastern North America Disjunction Originate and Diversify?

The classic East Asia-Eastern North America (EA-ENA) disjunction, well-documented in temperate plants, is typically attributed to the fragmentation of once-continuous high-latitude ranges by climatic cooling and regional extinction. Whether this model extends to insects, particularly aquatic groups, remains largely untested with genomic data. Here, we integrate whole-genome sequencing, ddRAD-seq and morphology with phylogenetic, phylogeographic, genotype-environment association (GEA) and niche modelling analyses to reconstruct the origin and diversification of the Aquarius conformis species group (Hemiptera: Gerridae). Phylogenomic and demographic analyses reject a simple widespread-ancestor vicariance scenario, instead supporting an East Asian origin followed by intercontinental dispersal to North America and subsequent divergence at approximately 460 ka, with no detectable post-divergence gene flow. The two continental lineages exhibit deep genetic divergence, marked body size differentiation and contrasting demographic histories, with precipitation seasonality (BIO15) emerging as the primary climatic axis separating their niches. GEA identifies candidate SNPs along this axis that map to conserved growth-regulating pathways (Hippo, mTOR, FoxO, Toll and Imd), suggesting a genomic basis for environment-associated divergence. Within East Asia, the widespread species A. elongatus comprises four phylogeographic lineages that diverged sequentially during the Pleistocene, accompanied by climate-associated allele-frequency variation and successful colonisation of highly heterogeneous environments. Collectively, our results reveal an alternative route to the EA-ENA disjunction, specifically source-area dispersal rather than relictual fragmentation, and demonstrate that post-isolation evolution in this system involves hydroclimatic differentiation, candidate pathways linked to growth and stress response, and lineage-specific demographic trajectories. This study provides a rare insect-based test of the EA-ENA disjunction and offers an integrative framework for linking biogeographic origin to post-isolation trait evolution in aquatic organisms.