Still another attempt to achieve assortative mating by disruptive selection of Drosophila

Disruptive selection for sternopleural bristle number in Drosophila melanogaster was shown by other workers two decades ago to produce two sexually isolated populations from a single population. The disruptive selection technique adopted now differs from that of previous investigators in that the focus of selection is shifted from the progeny of mated pairs to the mated pairs themselves. Two inbred wild type strains of contrasting bristle number were crossed; their heterozygous progeny being mass mated, a base population segregating for high and low bristle numbers was provided both for the selected line (mated pairs with highest or lowest sum of sternopleural numbers) and the control line (unselected). Although disruptive selection for the morphological trait was effective in producing a bimodal distribution within a few generations, no change in assortative tendencies for mating (reproductive isolation) was observed, as measured by correlation between bristle numbers of mated pairs after 16 generations of selection; in fact, a preliminary positive assortative proportion (20%) found in earlier tests with these strains failed to be demonstrated. From the distribution of bristle numbers among mated pairs in the last generations, it was apparent that mating may not have been strictly random; and further, from extra tests done with single females after the last selected generation, there was a tendency for one-sided assortative (L x L) mating. However, we cannot conclude that the selection procedure was effective in changing from random mating to assortative in significant degree. Two possibilities for this failure remain: either genetic variation for assortative mating behavior was not available in the base population or it was structured in such a way that the selection procedure was not effective in making it available. Other selection schemes that could be more effective suggested by genetic models of premating isolation from other authors are discussed as reasonable for further experiments.