Molecular and chromosomal studies on the origin of t-haplotypes in mice

Mouse t haplotypes are variant forms of the proximal third of chromosome 17 that enhance their representation in the gene pool by means of a male-specific transmission-ratio distortion. As with other systems of meiotic drive, they are maintained as independent genetic entities by inversions that suppress recombination with their wild-type homologues. This article reviews some of the evidence for the origin of this complex system based on DNA and chromosomal comparisons. Such comparisons suggest a stepwise evolution of t haplotypes, whereby the proximal and distal inversion occurred on separate chromosomal lineages and at different times. The initial event leading to the spread of t haplotypes may have been an inversion occurring on the wild-type chromosome (nondriving) 2-4 million yr ago. An additional implication of the data is that meiotic drive gives t haplotypes the ability to traverse species boundaries and spread rapidly around the world.