Estimation of Fitness Components in DROSOPHILA MELANOGASTER . I. Heterozygote Viability Indices.

We examine the assumption of "dominance" with regard to viability of the Cy and Pm marker chromosomes in D. melanogaster . This assumption is often invoked for the extraction of wild-type second chromosomes from natural populations and for the calculation of relative viability indices. Significant genotypic variances for viability are found among both Cy/+(j) and Pm/+(i) heterozygotes in California and Japanese populations. The magnitude of the Pm/+( i) genotypic variance is substantially less than that of the Cy/+(j) heterozygotes (less than one half). Significant reciprocal effects are also found to influence Cy/+(j), Pm/+(i) and +(i)/+(j) viabilities. We conclude that viability indices of heterozygotes based on the Curly method are biased. We suggest that viability indices in the future be expressed relative to the viability of the Cy/Pm genotype (Curly-Plum method) or possibly that of the Pm/+(i) genotype (Plum method).