Sexual isolation caused by selection for positive and negative phototaxis and geotaxis in Drosophila pseudoobscura.

Hirsch and his associates'-have selected positively and negatively geotactic, and positively and negatively phototactic, strains in Drosophila melanogaster. Dobzhansky and Spassky6 obtained by similar selection positively and negatively geotactic strains in Drosophila pseudoobscura, as well as positively and negatively phototactic strains in the same species (unpublished data). The selection was made with the aid of classification mazes described by the authors referred to above.1-' The wild and laboratory strains of both species of Drosophila are mostly neutral on the average with respect to geotaxis as well as to phototaxis; however, they contain both positive and negative variants that are genetically conditioned, and the heritability is high enough for selection to make fairly rapid progress. The geotactically and phototactically positive and negative strains appear to be indistinguishable in external morphology. The size, coloration, and other traits of the selected strains seem to be unchanged (no biometric study has, however, been made). Nevertheless, as the experiments described in this report show, they differ in inating preference. A slight, but statistically quite significalit, sexuial isolation has arisen as a by-product of selection for geoand phototaxis. Material and Technique. -Geoand photopositive and negative strains were obtained through the courtesy of Mr. Boris Spassky. They were derived from the same basic population, which was in turn obtained by hybridizing 12 strains of D. pseudoobscura homokaryotypic for AR gene arrangement and 12 strains homokaryotypic for CH gene arrangement in the third chromosome, The ancestors of these flies were collected at Pifion Flats, Mount San Jacinto, California, and the strains were kept in the laboratory for several years. The first generation in which the selection was practiced was the F2 from the crossing of AR and CH strains. The selection was made (by MIr. Spassky) as follows. In each generation, 300 virgin females and 300 males, aged separately for 3-4 days, were run, also separately, through either the geotaxis or the phototaxis maze. The 25 most positive, or the 25 most negative, individuals were selected, and allowed to reproduce in a population cage. Four lines were thus formed, selected for positive or negative geotaxis or phototaxis, respectively. Samples of the flies from these lines were obtained after Mr. Spassky had made his fifth selection. (S-5), and again after he had made his eleventh selection (S-11). The progenies of these flies, grown in ordinary half-pint culture bottles, served as material for observation of the sexual behavior. The observation chambers constructed by Ellens7' 8 were ised. Virgin females and males were aged for 5 days at 240C. Ten females and ten males of each of two strains were introduced into an observation chamber without etherization. To make the strains distinguishable, one wing was clipped in one or in the other strain; in the repeated runs the strain which had a wing clipped was alternated. The flies in the chambers were observed continuously for 100 min under the light of a desk