A genetic test to determine the origin of maternal transmission ratio distortion: Meiotic drive at the mouse Om locus

We have shown previously that the progeny of crosses between heterozygous f emales and C57BL/G males show transmission ratio distortion at the Om locus on mouse chromosome 11. This result has been replicated in several indepen dent experiments. Here we show that the distortion maps to a single locus o n chromosome 11, closely linked to Om, and that gene conversion is not impl icated in the origin of this phenomenon. To further investigate the origin of the transmission ratio distortion we generated a test using the well-kno wn effect of recombination on maternal meiotic drive. The genetic test pres ented here discriminates between unequal segregation of alleles during meio sis and lethality, based on the analysis of genotype at both the distorted locus and the centromere of the same chromosome. We used this test to deter mine the cause of the transmission ratio distortion observed at the Om locu s. Our results indicate that transmission ratio distortion at Om is due to unequal segregation of alleles to the polar body at the second meiotic divi sion. Because die presence of segregation distortion at Om also depends on rile genotype of the sire, our results confirm that the sperm can influence segregation of maternal chromosomes to the second polar body.