Induction of hybrid sterility in nonhybrid males of Drosophila paulistorum.

HREE different kinds of hybrid sterility of males occur within the superspecies Drosophila ptzulistorum Dobzhansky and Pavan, and a fourth one will he described in the present paper. The superspecies consists of at least five races or incipient species (DOBZHANSKY and SPASSKY 1959). The F, hybrids between these races are fertile if females and sterile if males. This is a sterility of the genic type, since the degenerative changes begin before meiosis, and the meiotic metaphases show at least some paired chromosomes ( DOBZHANSKY, unpublished observations). The second kind of sterility occurs in males in backcross progenies. This sterility depends upon the genetic constitution not of the males themselves but rather of their mothers. As a rule, all the sons of a female carrying any mixture of the chromosomes of the parental races are sterile, even though some of these sons themselves carry only the chromosomes of a single race (EHRMAN 1960). This sterility operates via a maternal effect, the genes responsible being distributed in all three pairs of chromosomes which D. paulistorum possesses. The sterilities of the F, hybrid and of the backcross males are due obviously to different causes, since F, hybrids are sons of pure rather than of hybrid mothers. The different causation is shown also by the fact that the two kinds of sterility are sometimes dissociated; although in most crosses both the F, and the backcross males are sterile. in some exceptional cases the F, may be sterile and backcrosses fertile, or vice versa ( EHRMAN 1962a). The third kind of sterility is known thus far only in hybrids between strains of the Transitional race, the ancestors of which were collected at Santa Marta and at Mesitas, Colombia, respectively. The male progeny from the cross Santa Marta female x Mesitas male are sterile, while the reciprocal cross gives fertile progeny of both sexes. When the hybrid females from the Santa Marta female x Mesitas male are backcrossed to Mesitas males, sterile male progenies are obtained in six successive generations (EHRMAN 1963, 1964a). In the seventh backcross to Mesitas, fertile sons are finally obtained. The sterility appears to be due to interactions between the Santa Marta cytoplasm and the Mesitas Y chromosome. Whatever factor is transmitted by the Santa Marta cytoplasm, it is eventually overcome by the Mesitas genome. The following working hypothesis is worthy of testing: Suppose that the Santa Marta strain carries a substance, or an associated microorganism, a symbiont or