Distributive pairing: mechanism for segregation of compound autosomal chromosomes in oocytes of Drosophila melanogaster.

HE segregational behavior of Drosophila chromosomes in the first meiotic Tdivision of the oocyte is quite different from that expected from the classical picture of the independent assortment of nonhomologous chromosomes and the regular segregation of homologous chromosomes. Under appropriate circumstances, nonhomologous chromosomes may segregate very regularly to opposite poles of the spindle of the first meiotic division, and conversely, homologs may frequently pass to the same pole (R. F. GRELL 1957, l959,1962a, 1962b, 1964a, 1964b). Simple rules which reliably predict the behavior of chromosomes in oocytes have been formulated and incorporated into the distributive pairing model of meiosis (R. F. GRELL 1962a, 1962b, 1964a, 1964b; E. H. GRELL 1963). According to the distributive pairing hypothesis, after the crossing over process noncrossovers and compound chromosomes make up the “distributive Chromosomes in this pool are distributed to the two poles without being influenced by homology but according to certain rules. The relative sizes of the members of the pool determine their segregation (see reviews by R. F. GRELL 1965, 1967). Nonhomologous distributive pairing occurs only in oocytes and does not occur during meiosis in males (R. F. GRELL 195 7) . Compound autosomes are a class of useful and interesting chromosomes in Drosophila. They consist of two identical autosomal arms attached to one centromere, and are sometimes called isochromosomes in the cytological literature. They were first synthesized in the laboratory of E. B. LEWIS. Compound 2L, compound 2R and compound 3L were synthesized by INGE RASMUSSEN; compound 3R by E. ORIAS and P. DEAL (RASMUSSEN 1960); and compound 4 by LEWIS and A. ROBERTS. Compound autosomes have been used in a number of experiments. LEWIS (1967) used compound 3R chromosomes to recover both reciprocal products of crossing over between pseudoalleles of the bx complex. RASMUSSEN (unpublished) recovered the hZ h double mutant with a compound 3L. HEXTER, LOZNER and BUNN (1967) obtained reciprocal crossovers between members of the ss complex. BATEMAN (1968) used compound autosomes to study radiation-induced nondisjunction and compound formation. MCCLOSKEY (1 966) used them to demonstrate that sperm lacking a whole chromosome arm are functional. LINDSLEY and GRELL (1969) extended the demonstration to show that