Studies on Hybrid Sterility. II. Localization of Sterility Factors in Drosophila Pseudoobscura Hybrids.

HE causation of hybrid sterility has long been one of the unsolved T problems of biology. To date, probably the greatest advance in this field has been made by FEDERLEY who discovered a failure of the meiotic chromosome pairing in the sterile hybrids between moths of the genus Pygaera. This finding has since been amply corroborated by other investigators in sterile hybrids both in animals and in plants. Naturally enough it became tempting to suppose that the failure of the meiotic pairing is the cause of hybrid sterility. The restitution of the normal meiotic pairing as well as of fertility following the reduplication of the chromosome complement in allotetraploid hybrids seems to be further evidence in favor of this view. And yet, this view proves to be inadequate as a general explanation of hybrid sterility. Two difficulties deserve particular attention. First, some hybrids are sterile despite the fact that the meiotic pairing in their gametogenesis is apparently normal (MICHAELIS 1933 in Epilobium, DOBZHANSKY 1934 in some crosses in Drosophila pseudoobscura) ; while in other sterile hybrids the gametogenesis does not reach the meiotic stages (KERKIS 1933, in Drosophila melanogaster X D. simulans). Second, the failure of the meiotic pairing in sterile hybrids is usually attributed to an “incompatibility” of the chromosomes of species or races entering the cross. This, clearly, is a restatement of facts and not a causal explanation. It remains possible that suppression of meiotic pairing may be caused by different mechanisms in different cases, and that sometimes there is no cause and effect relation between the failure of paking and the sterility. In my previous publications (DOBZHANSKY 1933, 1934) a hypothesis was suggested according to which there exist a t least two different types of hybrid sterility-the chromosomal type and the genic type. The chromosomal type is caused by differences in the gross structure (gene alignment) of the chromosomes of the parental forms, preventing, through competition in pairing, the normal conjugation of the chromosomes a t meiosis, and causing their irregular disjunction. The genic type of sterility is due to interactions between complementary genetic factors contributed by both parents. If the genetic constitution of one of the parental forms is SStt, and of the other ssTT, the hybrid is SsTt. The assumption is made that the presence of the factor (or the group of factors) S alone, or of the