A pachytene analysis of two male-fertile paracentric inversions in chromosome 1 of the mouse and in the male-sterile double heterozygote.

Meiotic studies have been made at pachytene on two paracentric inversions in chromosome 1 of the mouse. Surface-spread preparations of primary spermatocytes have been analysed at the light microscope level in males heterozygous for the inversions In (1) 1 Rk and In (1) 12 Rk and in the double heterozygote In (1) 1 RK/In (1) 12 Rk. In singly heterozygous form, neither inversion produces any serious effect on male fertility. In the double heterozygote, spermatogenesis is arrested in the majority of cells at the spermatocyte stage and males are rendered totally sterile by azoospermia. In the double heterozygote, a complex loop, indicating the inversion bivalent, is found in 90% of pachytene cells analysed. In the In (1) Rk/+ heterozygote, a looped bivalent was seen in 47 per cent of pachytene cells but in In (1) 12 Rk/+ no cells containing loops could be found. -- 80% of pachytene spermatocytes from the In (1) 1 Rk/In (1) 12 Rk double heterozygote showed apposition of the inversion bivalent to the sex bivalent. Such an association was rarely seen in pachytene cells of either of the fertile single heterozygotes. Spermatogenic failure in the double heterozygote may be related to interference, by the inversion bivalent, with X chromosome inactivation at meiotic prophase.