Evidence for selective differences in single-band polytene chromosome polymorphism in Sciara ocellaris.

Sciara ocellaris (Bradysia tritici) is a dipteran species characterized by an extensive polymorphism of structural modifications of single bands occurring in the polytene chromosomes. In this experiment we obtained estimates of egg-to-adult viabilities in relation to the genotypes determined by 15 different loci. In spite of the presence of considerable levels of linkage disequilibrium possibly generated by random sampling genetic drift, it is suggested that at least one out of 15 polymorphic modified chromomeric loci is maintained in the population by an overdominance mechanism.