Urban enrichment of molestus ancestry and opposing temperature clines in a metropolitan Culex pipiens hybrid zone

Abstract

Hybridization between ecologically distinct lineages can fuel rapid adaptation when introgressed alleles are locally favored. The North American Culex pipiens assemblage, a principal West Nile virus vector east of the Mississippi, comprises three lineages---temperate Cx. pipiens f. pipiens, urban-specialist Cx. pipiens f. molestus, and subtropical Cx. quinquefasciatus---that meet in a mid-Atlantic hybrid zone where temperature and urbanization gradients intersect. Using whole-genome Pool-seq of 24 field populations and three lineage-reference pools across the Baltimore--Washington corridor, we find a weakly differentiated, Cx. pipiens f. pipiens-like background, richer in molestus- and quinquefasciatus-like ancestry toward the warmer end of the corridor. Urbanization impacts allele frequencies across the genome: molestus-like ancestry rises with impervious surface, independently of temperature and position, while Cx. quinquefasciatus-like ancestry does not. An 11.6 Mb chromosome 3 inversion is molestus-like in every population, most of all in urban ones, and tracks genome-wide molestus-like ancestry. Temperature acts locus by locus. On chromosome 1, a cluster of six CYP6 paralogs, a family implicated in insecticide resistance and thermal-stress response, shows rising Cx. quinquefasciatus-like ancestry and read depth with temperature. A chromosome 2 interval centered on a Yan ortholog, a repressor of cell differentiation, shows non-quinquefasciatus ancestry increasing with temperature, consistent with selection against Cx. quinquefasciatus-like introgression. Urban populations therefore carry more ancestry from the human-commensal lineage, while temperature moves ancestry both ways at individual loci. Both molestus and Cx. quinquefasciatus donors carry human-contact traits, so these patterns bear on how warming and urbanization will reshape arboviral risk.