Pachytene pairing and sperm counts in mice with single Robertsonian translocations and monobrachial compounds

Data on testis weights, sperm counts, and synaptonemal complexes are presented for mice carrying the following Robertsonian translocations: Rb(6.15)lAld; Rb(4.6)2Bnr; Rb(4.15)4Rma; Rb(6.15)lAld/Rb(4.6))2Bnr, which is male-sterile; and Rb(6.15)lAld/Rb(4.15)4Rma, which is male-fertile. In RblAld/Rb2Bnr sperm were absent or sparse, whereas the sperm count in RblAld/Rb4Rma was just over 50% of the parental value. The translocated chromosomes appeared as fully paired bivalents in homozygotes, as trivalents in single heterozygotes, and as quadrivalents in compounds. About 20-40% of trivalents had unsynapsed ends. The proportion of quadrivalents with unsynapsed ends was about 85% in the male-sterile compound, compared with 75% in the male-fertile compound. The proportion of quadrivalents associated with XY was about 70% in both. Testis weights, but not sperm counts, were found to differ in two of three reciprocal crosses. It is concluded that, in addition to pairing failure and autosome/XY association, the effect of translocations on spermatogenesis is affected by other factors, including genetic background, inbreeding, and perhaps environmental factors. It remains to be elucidated whether pairing failure and XY association are primary or secondary effects.