Male sterility and double heterozygosity for chromosomal inversion.

A meiotic analysis has been carried out on male mice heterozygous for one of two inversions in Chromosome 2, In(2)5Rk and In(2)2H, as well as on double heterozygotes for these two overlapping inversions. Electron microscopic observation of synaptonemal complexes revealed that heterosynapsis had occurred in a large number of spermatocytes, producing a small number of cells with an inversion loop. Heterozygous carriers of a single inversion loop reproduced quite normally, whereas doubly heterozygous carriers of a double loop showed a reduction in spermatogenesis. These data shed new light on the role of inversions in speciation.