Synapsis and obligate recombination between the sex chromosomes of male laboratory mice carrying the Y* rearrangement.

The synaptic and recombinational behavior of the sex chromosomes in male laboratory mice carrying the Y* rearrangement was analyzed by light and electron microscopy. Examination of zygotene and pachytene X-Y* configurations revealed a surprising paucity of the staggered pairing configuration predicted from the distal position of the X pseudoautosomal region and the subcentromeric position of the Y* pseudoautosomal region. When paired at pachynema, the X and Y* chromosomes usually assumed configurations similar to those of typical sex bivalents from normal male laboratory mice. The X and Y* chromosomes were present as univalents in more than half of the early- and mid-pachytene nuclei, presumably as a result of steric difficulties associated with homologous alignment of the pseudoautosomal regions. When paired at diakinesis and metaphase I, the X and Y* chromosomes exhibited an asymmetrical chiasmatic association indicative of recombination within the staggered synaptic configuration. Both pairing disruption and recombinational failure apparently contribute to diakinesis/metaphase I sex-chromosome univalency, as most cells at these stages possessed X and Y* univalents lacking evidence of prior recombination. Recombinant X or Y* chromosomes were detected in all metaphase II complements examined, thus substantiating the hypothesis that X-Y recombination is a prerequisite for the normal progression of male meiosis.