QUANTITATIVE GENETICS OF OVARIOLE NUMBER IN DROSOPHILA-MELANOGASTER .1. SEGREGATING VARIATION AND FITNESS

The number of ovarioles of the Drosophila melanogaster ovary is a trai t thought to be associated with female fecundity, and therefore is exp ected to be under strong natural selection. This hypothesis may be tes ted by examining patterns of genetic and environmental variation for o variole number in natural populations, and by determining the associat ion between ovariole number and fitness in isogenic lines derived from a natural population. We measured ovariole number, and competitive fi tness and its components, for 48 homozygous chromosome 3 substitution lines in a standard inbred background; and body size in a sample of 15 chromosome 3 substitution lines. We found significant segregating gen etic variation for ovariole number, with a broad-sense heritability (H -2) of 0.403 and correspondingly high coefficients of genetic variatio n (CVG = 20.8) and residual variation (CVR = 25.3). Estimates of quant itative genetic parameters for body size (H-2 = 0.191, CVG = 2.15, and CVR = 3.87) are similar to those previously reported for this trait. Although the isogenic chromosome 3 substitution lines varied significa ntly for components of fitness, there was no significant linear or qua dratic association of ovariole number and body size with fitness. Ther e was, however, highly significant sex x genotype interaction for fitn ess among these lines. This special case of genotype x environment int eraction for fitness may contribute to the maintenance of genetic vari ation for fitness in natural populations.