Mechanisms of male sterility in hybrids of the Drosophila paulistorum group.

ROSOPHILA PAULISTORUM is a superspecies composed of five semispecies (the Centroamerican, Amazonian, Andean, Orinocan, and Interior), and a heterogeneous group of Transitional populations (DOBZHANSKY, PAVLOVSKY and EHRMAN 1969; SPASSKY et al. 1971 ) . External morphological, physiological, and ecological differences between the semispecies are too small to be of use in distinguishing the semispecies (PASTEUR 1970). The semispecies can be diagnosed by examination of the gene arrangements in their chromosomes (KASTRITSIS 1969). By far the easiest way to identify the semispecies is, however, by outcrossing the strains to be tested to tester strains known to belong to the respective semispecies. As a rule, fertile progeny are obtained only in crosses to a single tester strain. The gene flow between the semispecies is limited or prevented, presumably in nature as well as in laboratory experiments, by ethological isolation and by sterility of male hybrids. Hybrid females are always vigorous as well as fertile. Male sterility is of at least two kinds, that of the F, hybrid males which is due to their own genetic constitutions, and that of the backcross males, which apparently is induced by the genetic constitution of their mothers. The F, hybrid males have chromosome sets of two different semispecies, and the cytoplasm of one of them. The sterility is genic, since chromolsome pairing at meiosis is usually normal, while degenerative changes take place in the spermatocytes and spermatids before and after meiosis. ( DOBZHANSKY, unpublished observations). The sterility of males from the backcross progenies between the Centroamerican, Amazonian. and Andean semispecies was found by EHRMAN (1960) to be due not to the genetic constitution of these males themselves but to that of their mothers. What seems to be involved here is a maternal effect: sons of hybrid mothers are sterile even if they have chromosomes of only a single semispecies, and are in this sense nonhybrids. As shown by EHRMAN (1962), the sterility of the F, and backcross males is due to different causes. A few crosses even give fertile F, but sterile backcross males, and others sterile F, but fertile males in the backcrosses. Furthermore, one instance of hybrid sterility in Drosophila paulistorum, i.e. between the Santa Marta and Mesitas strains, is due to an incompatibility of the Y chromosome with the cytoplasms of the alternate strain. For