MEIOSIS IN CARRIERS OF HETEROMORPHIC BIVALENTS - SEX-DIFFERENCES AND IMPLICATIONS FOR MALE-FERTILITY

Mice that are double heterozygous for the semi-identical T(1;13)70H an d T(1;13)1Wa reciprocal translocations display a great variation in ma le fertility. The synaptic behaviour of the different translocation ch romosomes of adult males was studied in relation to this parameter. Ju venile males and embryonic females (16 and 18 days old) were included for comparison. In agreement with the minor differences in the translo cation breakpoint positions, two differently sized heteromorphic bival ents are formed in meiotic prophase of both sexes (a quadrivalent was never encountered). Synaptonemal complex (SC) configurations of both b ivalents in either sex are characterized by a high degree of non-homol ogous synapsis at zygotene-early pachytene. The rate of synaptic adjus tment during pachytene is dependent on the size of the heteromorphic b ivalent and varies between the sexes. Differences in SC configuration and morphology of the small heteromorphic bivalent in particular exist between the sexes and between animals. In males, this correlates with different degrees of fertility. Normal SC morphology in a fully synap sed small heteromorphic bivalent is an important determinant of succes sful meiosis and spermatogenesis. Moreover, aberrant synapsis favours the 'unsaturated pairing site' model as the primary cause for male ste rility.