Spatial distribution of the 2La chromosomal inversion in Anopheles gambiae populations across Nigeria: ecological associations and implications for malaria control.

BackgroundChromosomal inversions are key drivers of local adaptation and ecological diversification in Anopheles gambiae, the principal malaria vector in sub-Saharan Africa. Among these, the 2La inversion is associated with tolerance to aridity, behavioural variation, and insecticide resistance. However, large-scale data on inversion frequency and ecological association remain scarce in Nigeria. This study investigated the spatial distribution of 2La inversion frequencies in An. gambiae and their relationship with ecological zones across the country.MethodsA total of 1200 Anopheles mosquitoes were collected across 12 states representing Nigeria's major ecozones. 2La inversion karyotypes were determined by PCR, and allele frequencies were analysed in relation to ecological zones, latitude, and ecological gradients. Spatial mapping and statistical analyses were performed using R version 4.4.ResultsMorphological identification and species-specific PCR revealed 999 An. gambiae s.l., comprising An. gambiae s.s. (39.7%), An. coluzzii (51.5%), and An. arabiensis (8.7%). A total of 399 An. gambiae s.s. was karyotyped. From the An. gambiae s.s. population karyotyped, a distinct south-north cline in inversion frequency was observed. The 2La/2La homokaryotype predominated in arid northern ecozones (Sahelian and Sudan savannas), whereas 2La + /2La + was dominant in humid southern forests and mangrove regions. The heterokaryotype 2La/2La + occurred at moderate frequencies in transitional zones (Osun-Taraba corridor). Linear regression analyses confirmed significant correlations between inversion frequency and latitude (R2 = 0.68-0.73, P < 0.001). Generally, the allele frequencies from 2La karyotyping did not deviate significantly from Hardy-Weinberg equilibrium expectations (P > 0.05) except in Bayelsa and Kebbi States.ConclusionThis study provides the first nationwide evidence linking 2La inversion polymorphism to ecological adaptation of An. gambiae in Nigeria. The inversion likely enhances vector survival across diverse habitats, contributing to spatial variation in malaria transmission. These findings have direct operational relevance for Nigeria's National Malaria Elimination Programme (NMEP), indicating that vector-control effectiveness may vary by ecological context. Integrating ecotype- and region-specific evidence into NMEP planning could improve intervention targeting. Continuous genomic surveillance will be essential to sustain malaria control gains.