Arsenault-Benoit, A. | bioRxiv Science

bioRxiv2026

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

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.

genomics↗

bioRxiv2026

Climate and urbanization structure West Nile virus vector communities: Tools for surveillance under environmental change

West Nile virus (WNv) in eastern North America is transmitted primarily by Culex pipiens and Culex restuans, two cryptic vector species whose relative abundance shifts seasonally and across urbanization gradients - gradients that are themselves now shifting under continuing climate change and urban expansion. Because the two species are difficult to separate morphologically and are typically pooled in routine surveillance, this turnover is seldom tracked directly, and agencies lack species-resolved tools to anticipate when, where, and how vector communities will reorganize. Yet the timing of this turnover has been linked to the seasonal timing and intensity of human WNv cases, making it a potentially forecastable correlate of risk. We molecularly identified 9,789 Culex collected over six years, of which 5,808 came from a designed multi-year study in the Baltimore-Washington metropolitan region, and the remainder from Philadelphia and Chicago for cross-regional comparison. We combined negative-binomial generalized linear mixed models of species-specific abundance in Baltimore-Washington with Gradient Forest models of compositional turnover across all three regions. The two species diverged along both the urbanization and thermal gradients: Cx. pipiens abundance increased with impervious surface and with temperature, whereas Cx. restuans declined along both. Consequently, highly urbanized areas remained Cx. pipiens-rich across the season, whereas suburban, low-to-moderate-development landscapes exhibited the largest seasonal shifts in community composition. Accumulated degree-days (ADD) and weekly mean temperature were the most consistent drivers of turnover across regions, with the shift from Cx. restuans to Cx. pipiens fastest between 594 and 610 ADD. These patterns translate into three operational tools for WNv management: a climate-based degree-day window that lets agencies forecast the community shift from temperature data before it is detectable in trap composition, identification of suburban landscapes as priority targets for intensified surveillance and early intervention, and species-resolved environmental responses that help anticipate how community composition may shift within these regions as local climate and land-use conditions change. Together they offer a path from reactive surveillance to anticipatory, spatially targeted, and forward-looking management of WNv risk under environmental change. Open Research StatementAll datasets and R scripts used to generate the results and figures in this study will be made publicly available on DRYAD (DOI pending) upon acceptance of the manuscript. Climate data are publicly available from the PRISM Climate Group (PRISM Group, Oregon State University). Landscape predictor datasets are publicly available from the sources cited in Methods (Dewitz 2023; WorldPop 2018; U.S. Census Bureau 2020; Zell & Sanford 2020; Gesch et al., 2018; Didan 2015).

ecology↗

bioRxiv2025

Ancient origin of an urban underground mosquito

Understanding how life is adapting to urban environments represents an important challenge in evolutionary biology. Here we investigate a widely cited example of urban adaptation, Culex pipiens form molestus, also known as the London Underground Mosquito. Population genomic analysis of [~]350 contemporary and historical samples counter the popular hypothesis that molestus originated belowground in London less than 200 years ago. Instead, we show that molestus first adapted to human environments aboveground in the Middle East over the course of >1000 years, likely in concert with the rise of agricultural civilizations. Our results highlight the role of early human society in priming taxa for contemporary urban evolution and have important implications for understanding arbovirus transmission.

evolutionary biology↗