Patterns of genetic variation in a range‐expanding fruit fly Zeugodacus tau

BACKGROUND: Range expansion is often accompanied by reduced genetic diversity and increased environmental stress in populations at range edges, potentially constraining adaptive evolution despite strong selection. This study aimed to investigate these processes during the northward range expansion of the fruit fly Zeugodacus tau, an important agricultural pest. We sampled 217 individuals from 11 geographical populations spanning the species' current distribution across China and genotyped them using double-digest restriction-site-associated DNA sequencing (ddRAD-seq). RESULTS: The surveyed populations were genetically differentiated into southwestern and southeastern groups, with an admixed group located in the longitudinal transition zone between them. Northern populations generally exhibited lower genetic diversity than southern populations, whereas populations with admixed ancestry tended to retain higher genetic diversity than non-admixed populations at comparable latitudes. TreeMix supported two historical migration events, and all populations except Shanghai exhibited broadly similar demographic histories. Environmental variables independently explained more genomic variation than geographic variables, both across all populations (53.01% vs 27.28%) and within the admixed group (50.64% vs 17.25%). Genome scans identified candidate SNPs in the admixed group that were potentially associated with responses to environmental stress. CONCLUSIONS: Our findings show that the northward range expansion of Z. tau is accompanied by a general decline in genetic diversity, while admixture among differentiated lineages may partially offset this loss. These results highlight the joint roles of demographic history and admixture in shaping genomic variation during range expansion, although the adaptive consequences of admixture remain unresolved.