DISRUPTIVE SELECTION ON I-MAZE ACTIVITY IN DROSOPHILA MELANOGASTER

Abstract

YMPATRIC speciation as one theoretical consequence of disruptive selection has not gained wide acceptance due, in part, to the paucity of laboratory evi-dence confirming the efficacy of this process. Only one study to date (THODAY and GIBSON 1962) has shown reproductive isolation as a result of disruptive se-lection on a meristic trait (sternopleural chaeta number in Drosophila melano-gaster) ; attempts to corroborate the work have produced some divergence in this character but have shown no evidence of reproductive isolation between selected optima (SCHARLOO, DEN BOER and HOOGMOED 1967; CHABORA 1968; BARKER and CUMMINS 1968). THODAY and GIBSON (1970), reviewing this series of experi-ments, concluded that their results were due to a fortunate choice of experimental fly stock. Behavioral traits are likely candidates for artificial disruptive selection since some have been demonstrated to be correlated with mating behavior and hence possibly with reproductive isolation. GRANT and METTLER (1969) have per-formed disruptive selection on a trait of this type: induced vertical I-maze activ-ity (" escape " reaction) in D. melanogaster. A marked response to directional se-lection for high and low maze activity was accompanied by increased mating discrimination between the diverging lines, but with disruptive selection no evi-dence for either bimodality or nonrandom mating was indicated. The present experiments were undertaken to examine more rigorously the conditions which might be necessary to attain sympatric divergence with disruptive selection on vertical I-maze performance in D. melanogaster; several aspects of the experi-mental design of GRANT and METTLER were modified in order to increase the probability of detecting incipient divergence. Much of the design of the experiment and the base population used were those of GRANT and METTLER. For each maze trial, fifty flies were run through a vertical I-maze constructed from 21 one-ounce polyethylene funneled dropping bottles numbered from 0 (starting chamber) to 20. As an excitatory stimulus the entire maze was manually raised to a height of 8 cm and then dropped vertically to the floor. This tapping was performed twice every twenty seconds for the duration of the trial (five minutes). The score of each fly was the numbered chamber in which it was located at the end of a run. (See GRANT and METTLER [I9691 for further details of the procedure.) Two replicate lines were maintained for the ten generations of selection. In each line the ten Genetics 71 : 185-188 May, 1972.