The mechanism of secondary nondisjunction in Drosophila melanogaster females

BRIDGES (1916) observed that X chromosome nondisjunction was much more frequent in XXY females than it was in genetically normal XX females. In addition, virtually all cases of X nondisjunction in XXY females were due to XX <-> Ysegregational events in oocytes in which the two Xchromosomes had failed to undergo crossing over. He referred to these XX <-> Y segregation events as secondary nondisjunction. COOPER (1948) proposed that secondary nondisjunction results from the formation of an X-Y-X trivalent, such that the Ychromosome directs the segregation of two achiasmate Xchromosomes to opposite poles on the first meiotic spindle. Using in situ hybridization to X and YL chromosomal satellite sequences, we demonstrate that XX <-> Ysegregations are indeed presaged by physical associations of the X and Ychromosomal heterochromatin. The physical colocalization of the three sex chromosomes is observed in virtually all oocytes in early prophase and maintained at high frequency until midprophase in all genotypes examined. Although these XXYassociations are usually dissolved by late prophase in oocytes that undergo Xchromosomal crossing over, they are maintained throughout prophase in oocytes with nonexchange X chromosomes. The persistence of such XXY associations in the absence of exchange presumably facilitates the segregation of the two Xchromosomes and the Ychromosome to opposite poles on the developing meiotic spindle. Moreover, the observation that XXYpairings are dissolved at the end of pachytene in oocytes that do undergo X chromosomal crossing over demonstrates that exchanges can alter heterochromatic (and thus presumably centromeric) associations during meiotic prophase.