Explanation of sterility in txty male mice

The lethal 't alleles' of the T complex on mouse chromosome 17 (refs 1, 2) have been assigned to six complementation groups (the definitive classes being t0, t9, t12, tw1, tw5 and tw73), each causing embryonic death at a particular stage. Intercomplementation heterozygotes (txty) show 'partial' complementation, so that only some txty offspring survive. These survivors appear morphologically normal; indeed the females, being fertile, seem entirely normal. The txty males, however, are sterile. Among the reasons given for this sterility in some txty males are low motility of the txty spermatozoa, and an inability to effect fertilization either in vitro or in vivo even when already present at the site of fertilization. Spermatozoa from sterile txty males were reported to be apparently present in 'normal' numbers in the uterus, although actual ejaculate numbers have not been quoted for these, or indeed for normal fertile males. In the human, typical ejaculate counts of less than 20 x 10(6) spermatozoa, where the normal ejaculate contains 100-250 x 10(6), are judged to indicate clinical sterility. The comparison of numbers of spermatozoa10 ejaculated into the female tract by sterile t6tw5, t6tw32 and tw5tw32 (all 'strong' mutations2) and control mice clearly shows an even greater oligozoospermia (about 100 times less than controls) occurring in the txty males.