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X-Y chromosome dissociation in mouse strains differing in efficiency of spermatogenesis: elevated frequency of univalents in pubertal males.

Dissociation of the X-Y chromosome bivalent in diakinesis-metaphase I spermatocytes of adult mice was significantly more frequent in the CBA strain (29%) than in C57, KP, or KE strains (7-11%). Autosome dissociation (1-5%) involved only the smallest chromosome pairs. Elevated frequency of X-Y dissociation in the CBA strain correlates with significantly lower testes weight and lower yield of spermatogenesis, which suggests that sex bivalent dissociation may be responsible for some loss of spermatogenic cells. However, sperm quality is not affected, the percentage of normal spermatozoa and their fertilizing capacity being higher in CBA than in the remaining strains. Two congenic strains, KE and KE.CBA (the latter with the Y chromosome introduced from CBA), had the same level of X-Y dissociation, suggesting that the Y chromosome plays no role in the determination of this character. In comparison with adult males, pubertal (27-29 day-old) males had twice as high a frequency of X-Y dissociation in KE and KP strains, and combined frequencies of dissociated sex and autosome bivalents were significantly higher in pubertal males of all tested strains. Although the level of chromosome dissociation is not sufficient to explain increased mortality of germ cells observed in pubertal males, it could be one of the contributing factors.