Relationships between light dependent fitness of Drosophila melanogaster and D. simulans and their genotypes for pupation site photopreference

The genetic variation in the preference for pupation sites, under conditions of varying white light intensity, was investigated in two Drosophila sibling species. In parallel, an attempt was made to examine the effect on fecundity of the level of light experienced by the flies immediately after their pupation site choice had been made. Larval photopreferences of several selection lines of Drosophila melanogaster and D. simulans were assessed in a gradient of illumination and the genetic architectures influencing this behaviour were investigated from the responses to artificial selection, reciprocal crosses between divergently-selected lines, and a chromosomal analysis. D. simulans responded only to selection for negative larval photopreference, while D. melanogaster responded strongly to selection for positive but weakly to selection for negative photopreferences. Selected lines of flies that repeatedly experienced light conditions grossly at variance with those they preferred (traumatic lines) sometimes diverged in preference from lines that experienced their chosen conditions (rewarded lines). The traumatic lines always diverged from the corresponding rewarded lines in the direction of preferences that were associated with high fitness, as revealed by low response to selection in the rewarded lines. It is suggested that for a given genotype for phototactic behaviour there is a corresponding particular light dependent fitness, as measured by female fecundity.