THE PROBLEM OF DIFFERENCES OF OFFSPRING IN RECIPROCAL CROSSES OF DROSOPHILA

¹ Department of Zoology, University of Rochester, Rochester, New York

1. To test for the existence of maternal effects in Drosophila melanogaster, approximately isogenic stocks of Dichaete, pyd, vg, M-3, M-4 were prepared. Reciprocal crosses were made between each stock and Dichaete, and between Dichaete and non-Dichaete siblings. The results, as indicated by posterior dorso-central "bristle indices" (i.e., average number of dorsocentral bristles per hemithorax), of the Dichaete progeny show that no significant differences arose within six pairs of reciprocal crosses. The reciprocal crosses of four other pairs of experiments yielded significant differences in bristle indices. Two of these pairs were alike in type of crosses, but different in type of culture method. The sign of the differences in bristle indices was reversed in these two sets of experiments, the progeny with the higher indices coming from Dichaete mothers in one case, and from Dichaete fathers in the other.

2. Tests of homogeneity show that within crosses, all bottles of which have had identical treatment, differences in mean posterior dorsocentral bristle frequency occur which are greater than those to be expected on the basis of simple sampling errors.

3. The size of the bristle index of an individual fly depends partly upon its eclosion order, with those flies that eclose earliest having the highest bristle index.

4. The size of the bristle index depends partly on area of substrate available to the individual, although this dependence may be weak in many cases.

5. The lack of homogeneity in indices within crosses, and the reversal in direction of "maternal effect" in the pairs of crosses grown under two different culture conditions, suggest that external conditions during the development of the flies must be made more constant than heretofore, before conclusions as to the presence or absence of a "maternal effect" can be drawn.