Frequency-dependent viabilities of Drosophila pseudoobscura karyotypes

Abstract

The viabilities between egg and adult life stages of Drosophila pseudoobscura karyotypes were studied at low, intermediate, and high frequencies. The viabilities of pairs of karyotypes were compared at each frequency and the viabilities of the three karyotypes, at one combination of frequencies. Eggs were counted into vials and samples taken of the adults emerging after viability selection. ST and CH gene arrangements of the third chromosome carrying different amylase alleles were used, and the karyotypes of adult flies were scored by gel electrophoresis. A statistical method related to the loglinear model was developed for estimating viabilities. This method takes account of the additional variability between replicates common in experiments of this kind and allows testing of nested hypotheses about the mechanism of selection. The viabilities of the homokaryotypes relative to the heterokaryotype were significantly higher at the low homokaryotypic frequency than at the higher ones. These viabilities do not show a consistent heterozygote advantage. This pattern of frequency-dependent viabilities will lead to a protected polymorphism for the gene arrangements, even in the absence of heterozygote advantage.