From a laboratory stock of Drosophila melanogaster (Oregon), reared for more than 20 years at 18°C, two new populations were derived and maintained at 25° and 28°C for 8 years. The chromosomal and cytoplasmic contribution to genetic divergence between the two more extreme populations was estimated at 18°C and 28°C. Wing shape and two fitness components (fecundity and fertility) were taken into account. Fourier descriptors and the position of the centroid were taken as indicators either of wing shape variation, determined by a different response of the two wing compartments to temperature selection, or of wing shape variation determined by both compartments. The descriptors appear to be good characters: they show a variability which is genetically controlled and ascribable to genes located on specific chromosomes. The third chromosome is responsible for the adaptive difference to temperature. The genes which control wing shape are located on the second and third chromosome, although the contribution of each chromosome depends on the environment in which the flies develop. Cytoplasmic genes display an effect as large as that of chromosomes, and nucleus × cytoplasm interaction is present. The correlation between the genetic contributions to compartment‐dependent wing shape variation and the contributions to fitness is highly significant, especially at 28°C. Wing shape has, therefore, an adaptive significance in relation to temperature, which is particularly expressed in the environment where selection occurred.
From a laboratory stock of Drosophila melanogaster (Oregon), reared for more than 20 years at 18" C, a new population was derived and maintained at 28" C for 8 years. The chromosomal and cytoplasmic contribution to genetic divergence between the two populations was estimated. Six body traits and reproductive fitness were taken into account. The third chromosome is responsible for the adaptive difference for temperature between the two lines. Temperature-selected genes which control body size are located on the second and third chromosomes, although the contribution of each chromosome depends on the environment in which the flies develop. The correlation between the chromosomal and cytoplasmic contributions to different traits and fitness, changes with temperature. At 28" C the correlation between fitness and each body trait is proportional to the response to selection exhibited by each of them, but this is not true at 18" C. Body size has, therefore, an adaptive significance in relation to temperature, which is expressed only in the environment where selection occurs. Cytoplasmic genes affect almost all characters to an extent similar to that of chromosomal genes. Inter-chromosomal and nucleocytoplasmic interactions are present and also change with temperature. In general, genes selected in a given environment produce greater phenotypic changes in that environment than in another. The population that experienced both temperatures is fitter in both environments, suggesting that the capacity to adapt to warm temperatures depends on genes other than those which are involved in the adaptation to cold. 235 236
An analysis of the modifiers affecting the expression of the vg gene was performed. We selected for weak and strong expression of the vg mutant in F2 segregating populations obtained by crossing a vestigial stock with an Oregon laboratory stock (O) and with a wild strain (B) captured near Bologna, Italy. The selection for enlarged wings was more effective in the vg B population where wild wings appeared from the 10th generation. The assay of the three major chromosomes showed that the modifiers are located on chromosomes 2 and 3. The mutant imaginal disc cell death phenotype is evident in vg/vg strains that have a wild-type wing phenotype. It is suggested that the selected modifiers do not prevent cell death but induce regenerative growth.
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