APPLICATION OF THE “DISTRIBUTIVE PAIRING” HYPOTHESIS TO PROBLEMS OF SEGREGATION IN TRANSLOCATION HETEROZYGOTES OF DROSOPHILA MELANOGASTERl

HE distributive pairing hypothesis of GRELL (1962) aims to elucidate the Tmechanisms involved in chromosome pairing, exchange, and disjunction at meiosis in the female of Drosophila melanogaster. It provides a new interpretation of meiosis which can account for many of the seemingly contradictory reports on chromosome behaviour made by earlier authors. Former ideas on the nature of pairing and disjunction were based largely on the hypothesis of DARLINGTON (1929, 1937). He postulated that chiasmata, formed at points where crossing over had occurred, were responsible for holding bivalents together at meiotic metaphase and ensuring orderly disjunction at anaphase I. However, DOBZHANSKY (1932, 1933) was quick to point out that the example of the Drosophila male showed clearly that orderly disjunction could take place even in the complete absence of crossing over. Since then, COOPER (1944, 1945) has drawn attention to a wide variety of species in which regular segregation can occur in chromosomes devoid of chiasmata. DOBZHANSKY (1931, 1932) put forward the alternative hypothesis of “competitive pairing.” He concluded that crossing over did not determine regular disjunction, but believed that both crossing over and disjunction were determined by a “third factor”-namely the intimacy of pairing of homologous chromosomes prior to crossing over. If this pairing were disrupted, e.g., by translocation, inversion. or duplication, the frequency of crossing over would be decreased and the frequency of nondisjunction increased. However, this hypothesis fails to explain the regular disjunction observed by STURTEVANT and BEADLE (1936) in X chromosomes of the Drosophila female made structurally heterozygous by inversion. and therefore unable to undergo intimate pairing and crossing over. Neither hypothesis can account fully for the phenomenon of “nonhomologous pairing,” the occurrence of which was first postulated by COOPER, ZIMMERING and KRIVSHENKO ( 1955) to explain the increased frequencies of X-chromosome nondisjunction observed in situations where both the X chromosomes and either one of the autosomes were made structurally heterozygous by inversion. Since then SANDLER and NOVITSKI (1956) have postulated the occurrence of pairing between nonhomologues, while the work of GRELL (1957, 1959) and other authors