Studies of human trisomies indicate a remarkable relationship between abnormal meiotic recombination and subsequent nondisjunction at maternal meiosis I or II. Specifically, failure to recombine or recombination events located either too near to or too far from the centromere have been linked to the origin of human trisomies. It should be possible to identify these abnormal crossover configurations by using immunofluorescence methodology to directly examine the meiotic recombination process in the human female. Accordingly, we initiated studies of crossover-associated proteins (e.g., MLH1) in human fetal oocytes to analyze their number and distribution on nondisjunction-prone human chromosomes and, more generally, to characterize genome-wide levels of recombination in the human female. Our analyses indicate that the number of MLH1 foci is lower than predicted from genetic linkage analysis, but its localization pattern conforms to that expected for a crossover-associated protein. In studies of individual chromosomes, our observations provide evidence for the presence of “vulnerable” crossover configurations in the fetal oocyte, consistent with the idea that these are subsequently translated into nondisjunctional events in the adult oocyte.
An immunofluorescence approach was used to directly examine meiotic recombination events in 483 pachytene spermatocytes from 11 male rhesus monkeys. Specifically, we examined the nuclear localization patterns of the DNA mismatch repair protein MLH1, known from analyses of other mammalian species to be a useful marker of meiotic cross-overs. Our results indicated that rhesus pachytene spermatocytes contain approximately 40 cross-overs per cell, corresponding to about one cross-over per chromosome. The chromosomal distribution of these exchanges was consistent with data from human and mouse males but, surprisingly, the overall number of foci was lower, and the number of ‘exchangeless’ bivalents higher, than reported for either humans or mice.
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