Meiotic nondisjunction in mice and mouse hybrids

Nondisjunction in the first meiotic division was evaluated by the frequency of aneuploid metaphase II (M II) figures in secondary spermatocytes of Mus musculus (N = 40; FN = 40), M. poschiaυinus (N = 26; FN = 40) and M. musculus × M. poschiavinus F₁-hybrids (N = 33; FN = 40), whose karyotypes are related by Robertsonian-type centric fusions. Regular anaphase I disjunction prevailed in M. musculus and M. poschiavinus, where 92.8% and 94.0% of the M II figures examined had a complete haploid chromosome complement. In the F₁-hybrids, however, less than one half of the M II figures examined were euploid (FN/2 = 20), whereas the frequency of hypomodal (FN/2 = 15 to 19; 31.2%) and hypermodal (FN/2 = 21 to 25; 24.2%) was high. These results emphasize the importance of the heterozygous state, and particularly structural heterozygosity within a Robertsonian system of karyotypic variation, in inducing meiotic nondisjunction. From the standpoint of methodology, M. musculus × M. poschiavinus hybrids of different generations should make possible stepwise analysis of the involvement of specific chromosomes in meiotic nondisjunction.