Chromosomal polymorphism is associated with nematode parasitism in a natural population of a tropical midge

The natural population of a tropical midge, Chironomus ramosus (Diptera: Ch ironomidae), was found to be polymorphic for a paracentric inversion (IV: 1 8C-19D). Based on the characteristic banding pattern of the fourth chromoso me in the larval salivary gland polytene nuclei, individuals were classifie d as either structural homozygotes or heterozygotes. Isofemale lines were o btained and subsequently standard (S/S) and inversion (I/I) homozygotes wer e characterised by careful progeny testing in the laboratory. While explori ng various biotic and abiotic factors that might be responsible for the mai ntenance of inversion polymorphism, we detected nematode (Family: Mermithid ae) infections among the larval population. A detailed study indicated that the inversion polymorphism in the natural population of C, ramosus was app arently being maintained as a result of the selective pressure exerted by t he nematode parasite. The corresponding pattern of increase and decrease in genotype frequencies and the relative fitness values indicated a selective advantage of inversion heterozygotes (S/I) over both homozygous types (S/S and I/I). Both empirical and experimental data suggest the strong heteroti c nature of adaptation in this C. ramosus population towards nematode infec tion. This is the first report of its kind where inversion polymorphism has been shown to be associated with nematode parasitism.