Quantitative inheritance of red eye pigment inDrosophila melanogaster

Abstract

1. The genetic and environmental variation of red eye pigment in individuals of a wild population ofDrosophila melanogasterhas been studied by extracting and measuring the pigment content of individual flies, which were also scored for eye and body size.2. Comparison of such variability in the wild population with the individual variation in crosses between inbred lines suggested that 60% of the phenotypic variance is genetic. About 75% of both genetic and environmental variance is due to intrinsic variation of pigment content while the remainder is correlated with eye size, which shows appreciable variation, independent of general body size, as measured by thorax length.3. Selection for high and low pigment content by both phenotypic and family selection led to 40–50% differences between high and low lines after eight genera tions. The response was asymmetrical and proceeded further and faster with selection for lower pigment content. Crosses between high and low lines showed positive departure from intermediacy, suggesting that more or less recessive effects had contributed to the selection for lower pigment content.4. There was some evidence of lower viability in the selected lines but no evidence of lower fertility or gamete viability in more extreme individuals of either sex.5. Comparison of pigment content, eye and body size at different temperatures and under different levels of crowding suggested that the pigment content per ommatidium is subject to a high degree of genetic determination.6. The average pigment content in strains derived from widely separated localities showed substantial variation, independent of both eye and body size.7. Inbred lines, derived from the same population, were found to differ greatly in pigment content. Crosses and the exchange of homologous pairs of chromosomes between two of these lines suggested that one or more completely recessive genes were fixed in both the second and third chromosome of one line, while the same second chromosome effect was also fixed in another line. The second and third chromosome difference reduced pigment content by respectively 30% and 50% and combined additively, judging by the effects of single and joint substitutions of homologous pairs.8. The possibility of combining genetic and biochemical analysis of pigment content is discussed.