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Current meiotic dogma holds that synapsis is required for recombination and that recombination is required for proper disjunction. The mouse chromosome aberration XYSxr [sex reversal; redesignated XY,Tp(Y)1Ct] appears to challenge this assumption, for although chromosomes X and Y often fail to synapse and recombine, there is no dramatic increase in aneuploid progeny. An explanation of this conundrum might be that X-Y univalent spermatocytes do not survive. The phenotype of sex reversal is generated by the "obligatory" crossover between the X and Y chromosomes, which always occurs proximal to a duplicated copy of the testis-determining gene Sry and transfers one copy from one chromatid of the Y chromosome to one chromatid of the X. Animals that inherit an X chromosome with the Sry gene are chromosomally female but phenotypically male. We have used fluorescence in situ hybridization (FISH) to visualize probes for the X and Y chromosomes and for the Sry sequence and chromosome 8 to track the fate of both recombinant and nonrecombinant chromosomes through metaphases I and II into spermatids and sperm. In the 219 gametes examined by multicolor FISH, the frequency of aneuploid products (XY or "O") was low (3.7%) despite a high frequency (66%) of X-Y separation at metaphase I. In balanced gametes, X and Y recombinant chromosomes slightly exceeded nonrecombinants. Both of these observations support the earlier proposal that asynapsis and nondisjunction in primary spermatocytes lead to their developmental arrest and degeneration.

proceedings of the national academy of sciences of the united states of america

Detection of nondisjunction and recombination in meiotic and postmeiotic cells from XYSxr [XY,Tp(Y)1Ct] mice using multicolor fluorescence in situ hybridization.