Sex-dependent frequency and type of autosomal univalency at the first meiotic metaphase in mouse germ cells

Univalents at the first meiotic metaphase in mouse spermatocytes occur main ly in the XY pair, making it difficult to compare the amounts of univalency in males and females. In this study, the amounts of autosomal univalency i n male and female meiosis were compared using the model strain CBA-TG, in w hich univalency of the small marker autosome pair T6 has been shown to occu r very frequently in spermatocytes. Mice from inbred CBA and DBA strains we re also analysed. The total frequencies of univalency (sex chromosomes plus autosomes) in metaphase I spermatocytes were 45.6% in CBA, 36.9% in CBA-T6 , and 37.3% in DBA males. The aneuploidy in metaphase II spermatocytes rang ed from 1.4 to 3% in these strains, which was in agreement with previous fi ndings that most primary spermatocytes with abnormal chromosome configurati ons are arrested in their development before metaphase II. In the CBA-T6 st rain, autosomal univalency at metaphase I mostly involved chromosome pair T 6; however, its frequency differed significantly between the sexes, amounti ng to 18.9% in spermatocytes and 4.3% in oocytes. In the CBA strain, autoso mal univalents at metaphase I were seen in 7.7% of the spermatocytes and 1. 4% of the oocytes and, in DBA mice, in 4.9% of the spermatocytes and 3.8% o f the oocytes. However, in DBA oocytes, when univalency occurred it usually concerned a greater number of bivalents in one cell (range: 2-19 disjoined bivalents), a phenomenon very rare in males of this strain. This study sho ws that univalent formation differs between the male and female types of me iosis.