Biochemical analysis of genetic differences in the growth of Drosophila.

Abstract

1. Lines ofDrosophila melanogaster, which differ greatly in body size and/or development time, have been created by selecting for either large or small body size or longer or shorter development time on different chemically defined axenic media. The ten lines studied here have been selected either on the optimum medium or on media deficient in either protein, ribonucleic acid or choline.2. The lines show a variable degree of correlation between adult size and the duration of the larval period. Selection for one trait may involve a positively correlated change in the other or have no such effect. There is a fairly regular association between the composition of the diet and the presence or absence of evidence for genetic correlation.3. The biochemical composition of the various lines, and also the unselected stock from which they were derived, has been compared at successive stages from egg to adult in animals grown on the optimum axenic diet. Records of wet weight, protein, RNA and DNA content per individual have revealed differences in composition which can be releated to characteristic differences in the response to selection.4. Comparisons between the individual content of the various constituents at the critical size, i.e. at the time in early third instar when larvae can complete development even if no longer allowed to feed, and in newly emerged adults indicate a high positive correlation between DNA content and the time to reach the critical size and also the time to pupation. Protein and RNA content, on the other hand, may vary within wide limits without alteration of the development time. The association between DNA content and development time which underlies the correlation between body size and development time, is related to the rate of DNA synthesis in early larval life, such that the rate of synthesis tends to be negatively correlated with the absolute DNA content at the critical size and in the adult.5. Since the protein/DNA ratio is comparatively unchanged in lines with correlated changes in body size and development time, but varies widely in lines in which development time is unaltered, the contrasts may reflect, in adults at least, differences in respectively cell number or cell size.6. An earlier hypothesis of the origin of correlated and uncorrelated changes in body size and development time has been modified. It appears that the absolute critical size is positively correlated with the size of the adult, whether or not changes in development time have occurred. The intrinsic differences in growth operate from very early stages of development. This has been demonstrated in the differences in RNA/DNA ratio in early larval life and also in a high correlation between the time to complete development in the egg and the time to reach critical size after hatching from the egg.7. There is also evidence, in the relative deviations from unselected of the various estimated constituents, of partial independence of growth between early and late larval life, and this may reflect differences in the growth of the imaginal discs.8. The data are discussed in relation to earlier experimental evidence relating to the growth ofDrosophila. The point is made that judicious selection for appropriate parameters of growth will often create differences great enough to reveal developmental interrelations which would otherwise be difficult to detect.