Distributive Pairing of Compound Chromosomes in Females of Drosophila Melanogaster.

HE relation between crossing-over and disjunction in the Drosophila female Thas been studied for many years. ANDERSON (1929), DOBZHANSKY (1932, 1933), BROWN (1940), PIPKIN ( 1940) and others clearly demonstrated that crossing-over has a strong influence on disjunction in translocation heterozygotes. The reciprocal products of an exchange tend to pass to opposite poles of the first meiotic division spindle. On the other hand, COOPER (1945) stressed that normal segregation can occur without crossing-over and strongly corltradicted the hypothesis of DARLINGTON (1929) that chromosome pairs not oriented by a chiasma (the cytological manifestation of a crossover) do not regularly segregate one member to each pole of the spindle. The discoveries of highly nonrandom assortment of nonhomologous chromosomes by R. F. GRELL (1957,1959) and OKSALA (1958) have provided new methods by which the interrelation of crossing-over and disjunction can be reanalyzed. Such an analysis led R. F. GRELL (1962a, b) to a new hypothesis of the nature and sequence of some of the events of meiosis in oocytes of Drosophila melanogmter. The most novel feature of her hypothesis is that meiosis is postulated to contain two distinct types of chromosome pairing. The first pairing is called exchange pairing and is between homologous loci, prior to and necessary for crossing-over. The second is distributive pairing which occurs after crossing-over. This second pairing may involve either homologous or nonhomologous chromosomes. It occurs before the meiotic anaphase so that two chromosomes thus paired will pass to opposite poles of the division spindle. All co-oriented pairs of chromosomes are probably involved in distributive pairing, but distributive pairing becomes evident as separate from exchange pairing only in certain situations of nonrandom assortment of nonhomologous chromosomes or in secondary nondisjunction. In general, a chromosome that has been involved in a crossover event with its homologue will pair distributively with its homologue. A chromosome that has not been involved in an exchange with its homologue may participate in nonhomologous distributive pairing. In the case of the X chromosomes, the same conditions that will allow nonhomologous distributive pairing will also allow distributive pairing with the Y chromosome. Therefore, as demonstrated by BRIDGES (1916) the two maternal X chromosomes in secondary exceptions from XXY mothers are noncrossovers. Similarly, ROBERTS ( 1962) showed that exceptions resulting