Genotype—environment interaction III. Interactions in Drosophila melanogaster

The responses of two characters, number of sternopleural chaetae and total yield of offspring (which depends on the mother’s genotype), to change in the temperature at which the flies were raised and type of culture container in which they bred were followed in the two inbred lines, Wellington (Well) and Samarkand (Sam). In respect of chaeta number Well was more sensitive to change of the environment than Sam, which furthermore responded in the opposite direction to Well. In respect of yield of offspring the two lines responded similarly. The genetic control of these responses to environmental change was investigated by using the eight substitution lines which comprise all the possible true breeding combinations of the three major chromosomes (X, II and III) from Well and Sam. Two experiments were carried out, the first a diallel experiment at three temperatures (18, 21.5 and 25 °C), and the second an experiment in which the eight lines were raised in nine environments comprising all combinations of the three temperatures and three types of culture. Chaeta number changes more with temperature than with type of culture, whereas the reverse is true of yield of offspring. In respect of chaeta number the genes chiefly responsible for response to environmental change are borne on a different chromosome (II) than those chiefly responsible for variation in mean chaeta number (III), and there are indications of a similar situation in respect of yield of offspring. It is concluded that different characters are separately adjustable by selection in their responses to enviromental change, that sensitivity of a character to environmental change is adjustable separately from mean expression of the character, and that the detailed patterns of response to a range of environments (e. g. temperatures) are separable from the magnitude of the overall change induced by these environments.