Polymorphic inversion 2La frequencies associated with ecotypes in populations of Anopheles coluzzii from Southwest Nigeria

Chromosomal inversion 2La play a vital role in local adaptation of the malaria vector Anopheles. This polymorphic inversion, linked with ecological and behavioural plasticity, ensures adaptation of these species and continued transmission of the malaria parasite. However, the adaptive significance of this inversion in several Anopheles populations is still not well understood. Here, we determined the frequencies of polymorphic inversion 2La in natural populations of Anopheles coluzzii, the second major malaria vector in Southwest Nigeria, and its association with ecotypes. Anopheles larvae were collected from eleven localities across the mangrove/forest and forest/savannah ecozones of Southwest Nigeria. Adults were identified morphologically and with Polymerase Chain Reaction (PCR). Chromosomal inversion frequencies were determined by PCR. Relationship between genotype frequencies and ecotypes was determined using linear regression analysis and chi square. A total of 333 Anopheles coluzzii were scored for inversion 2La karyotypes. Frequencies of the 2La/2La homokaryotype increased progressively Northwards (F = 10.9; p = 0.01; R2=0.54) corresponding to a significant decline in the frequencies of 2L+a/2L+a (linear regression: F = 7.03; p = 0.03; R2=0.44), an indication of association with ecotypes. The heterokaryotype (2L+a/2La) however, showed high frequencies across ecotypes (F = 0.15; p = 0.71; R2=0.02). There was no significant deviation of the karyotype frequencies from Hardy-Weinberg estimates, an indication of a possible heterozygous advantage