The Mechanism of Secondary Nondisjunction in Drosophila melanogaster females

Abstract: 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<->Y segregational events in oocytes in which the two X chromosomes had failed to undergo crossingover. 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 Y chromosome directs the segregation of two achiasmate X chromosomes to the opposite pole on the first meiotic spindle. Using in situ hybridization to X and YL chromosomal satellite sequences, we demonstrate that XX<->Y segregations are indeed presaged by physical associations of the X and Y chromosomal heterochromatin. The physical co-localization of the three sex chromosomes is observed in virtually all oocytes in early prophase, and maintained at high frequency until mid-prophase in all genotypes examined. Although these XXY associations are usually dissolved by late prophase in XXY oocytes that undergo crossingover, they are maintained throughout prophase in XXY oocytes with nonexchange X chromosomes. The perdurance of such XXY associations in the absence of exchange presumably facilitates the segregation of the two X chromosomes and the Y chromosome to opposite poles on the developing meiotic spindle. Moreover, the observation that XXY pairings are dissolved by the end of pachytene in exchange-competent oocytes demonstrates that exchanges can alter heterochromatic (and thus presumably centromeric) associations during meiotic prophase.

Key Words: achiasmate segregation, centromere-co-orientation, heterochromatic pairing, meiosis, secondary nondisjunction

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				C. Wu, V. Singaram, and K. S. McKim mei-38 Is Required for Chromosome Segregation During Meiosis in Drosophila Females Genetics, September 1, 2008; 180(1): 61 - 72. [Abstract] [Full Text] [PDF]

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