Selection for Temperature Sensitivity in Scute Drosophila.

HE concept of canalisation as a system by which potential variation is channelled during development into a fairly uniform phenotype was put forward by WADDINGTON (1942). The term canalisation will here be used to mean a developmental buffering system by which potential variability is reduced and a relatively invariant phenotype is produced. If the canalisation system is upset by environmental shock or genetic mutants, this variability may be expressed. Selection on such exposed variability has demonstrated canalisation for wing-vein pattern ( WADDINGTON 1953, 1955) and scutellar bristle number (RENDEL 1959) in Drosophila. This system should itself be under genetic control and so should manifest genetic variability making it possible to select for good or poor canalisation (i.e. more or less effective reduction of variability) for a particular character. RENDEL and SHELDON (1960) have shown that, in the scute mutant, which has a reduced number of scutellar bristles, selection for low variance around a mean of two bristles produces increased canalisation at that number. There was a reduction in temperature sensitivity as well as the reduction in variance. WADDINGTON (1 960) has selected for temperature insensitivity of several mutants as a means of canalising selection, but he did not select for temperature sensitivity (i.e. for poor canalisation). The aim of the present work is to compare the two methods of canalising selection, to see if reduction in temperature sensitivity is accompanied by reduction in variance and in sensitivity to other environmental variables. and if there are changes in sensitivity of other characters.