X-CHROMOSOME SEGREGATION DISTORTION IN DROSOPHILA

The sex-ratio trait exhibits both discrete and continuous variation in Drosophila pseudoobscura. The discrete variation is caused by X-chromosome meiotic drive. The evolutionary forces maintaining the meiotic-drive polymorphism include strong viability selection against homozygous females and virility selection in males. The relative importance of the two factors is unresolved. The evidence for a genetic component in the continuous sex-ratio variation comes from artificial-selection experiments, large-scale estimation of segregation ratios corrected for viability effects, and an examination of the rates of spermatid loss. Segregation ratio can be treated as a typical quantitative character subject to major and minor genetic modifiers.