Analysis of the testis size inDrosophila pseudo-obscura hybrids

Summary

1. Race A males ofDrosophila pseudo-obscura were found to possess larger tests than males of race B; the difference is statistically significant.

2. Mutant stocks ofD. pseudo-obscura were found to show a slightly greater variation in testis size than normal wild-type strains. In interracial hybrids, “strong” races show greater variation than “weak” races.

3. Testis size shows an increase in intraracial hybrids and a decrease in interracial hybrids as compared with that in parental stocks. The increase in the intraracial hybrids is maintained in successive generations of back-crossing in certain crosses, but disappears in other crosses. The decrease inF ₁ interracial hybrids is great in the B♀ × A♂ cross, but slight and sometimes absent in the reciprocal cross. This decrease disappears with repeated back-crossing.

4. A complex interaction between complementary genetic factors in the hybrid chromosomes and the cytoplasm is assumed to be responsible for the apparently inconsistent behaviour of testis size in the various crosses. The modifying effect of the cytoplasm is assumed to be due to cytoplasmic differentiation conditioned by genic differentiation in the genetic system of the different geographical lines.

5. Sex-linked mutations have no general additive effect on testis size. Particular sex-linked mutant genes, however, do exert some effect on testis size; the data suggest that the qualitative differentiation of the gene complex as expressed by the presence of these genes may be responsible.