THE EFFECT OF SELECTION FOR STERNOPLEURAL BRISTLE NUMBER ON MATING BEHAVIOUR IN DROSOPHILA MELANOGASTER

Abstract

I N a separate paper (BARKER and CUMMINS 1969) we have reported the results of disruptive selection for sternopleural bristle number in Drosophila melano-gaster. Experimental procedures were the same as those of THODAY and GIBSON (1 962) and THODAY (1964), but in contrast to their results, disruptive selection did not lead to reproductive isolation between the high and low parts of the population. One explanation for THODAY and GIBSON'S results would be the existence in their population of a genetic correlation between bristle number and isolation tendency, which may even have been expressed in the unselected population as assortative mating for bristle number. If this were so, lines directly selected for high and for low bristle number would show isolation. Such directional selection lines selected from the same base population as our disruptive lines were available to us. Therefore, in the early generations of the disruptive selection, an experiment was done to check for any relationship between sternopleural bristle number and tendency towards isolation in this population. No such relationship was found. MATERIALS A N D METHODS The lines used were made available by MR. A. K. SHERIDAN, and had been selected for high and low sternopleural bristle number from the Canberra base population. Their previous selection history was: XD1 (a low line) had been selected for decreased sternopleural and coxal bristle number simultaneously for 22 generations and then for a further 10 generations had been selected for decreased sternopleural bristle number. ZU2 (a high line) had been selected for decreased coxal and increased sternopleural bristle number simultaneously for 22 generations and then for a further 10 generations had been selected for increased sternopleural bristle number. Selection in these lines was then relaxed and they had been maintained at 20°C for four generations. Samples from the two lines were maintained for two generations under the same environmental conditions (medium, temperature, etc.) as the disruptive lines. In the second generation, 10 bottles (four pairs of parents per bottle) were set up for each line. Virgin progeny from these bottles were collected over three days, and used in the experiment when four days old. Flies from the high and low lines were sufficiently different in sternopleural bristle number to identify the origin of all individuals (Table 1). On each of three consecutive days, mating observation runs were commenced at 1 0 m and 4 PM, with four replicates at each time. All observabons were done at 25°C. Twenty-four hours before each mating observation run, sufficient flies were counted out in