Polymorphisms distinguishing different mouse species and t haplotypes

LOW FREQUENCY OF t HAPLOTYPES IN NATURAL POPULATIONS OF HOUSE MICE (MUS MUSCULUS DOMESTICUS)

t haplotypes are a naturally occurring, autosomal, meiotic-drive system found on chromosome 17 of the house mouse. They show non-Mendelian transmission from heterozygous +/t males, such that 90% or more of the male's offspring inherit the t-bearing chromosome. Although they are expected to become rapidly fixed, surveys of natural populations typically report low overall frequencies of only ~15-25% +/t heterozygotes. Generally, such studies of t haplotypes in wild populations have sampled only small numbers of individuals due to the need to genotype mice by breeding, thus we have conducted a large survey of wild mice, Mus musculus domesticus, using DNA markers to examine the frequency and distribution of t haplotypes in natural populations. The overall frequency of +/t heterozygotes from our entire sample was 0.062, which is much lower than all previous estimates of t haplotype frequency. t haplotypes were patchily distributed and rare, and were present in only 46% of the populations we sampled. There were no significant sex-specific differences in the frequency of t haplotypes. Our data suggest that the frequency of +/t heterozygotes in independent populations varies with respect to population size and stability: t haplotypes were at low frequency in all large, relatively persistent populations, whereas they were at more variable, and often higher, frequencies in small, temporally unstable populations. The extinction and recolonization of many of the smaller populations may contribute to the greater variation in t haplotype frequency observed, and small populations may be important reservoirs of t haplotypes in the wild. The highest frequencies of t haplotypes were obtained from populations with semilethal, or complementing lethal, t haplotypes, where t/t homozygous mice were present.

Recent evolution of mouse t haplotypes at polymorphic microsatellites associated with the t complex responder (Tcr) locus

Microsatellites closely associated with each member of the Tcp10 gene family were amplified simultaneously from t haplotype and wild-type forms of mouse chromosome 17, by PCR. The t complex responder (Tcr) locus, which plays a central role in transmission ratio distortion, maps within the Tcp10 cluster on the t haplotype. Thus the amplified set of microsatellite loci (referred to collectively as Tcp10ms) provides a direct marker for this central component of the meiotic drive system associated with all naturally occurring t haplotypes. A unique Tcp10ms pattern of microsatellite alleles was obtained for a number of independent, laboratory-maintained complete and partial t haplotypes. Independent t chromosomes found in wild mice from US populations also had unique patterns, even when they were classified within the same lethal complementation group. Wild and laboratory chromosomes in the tw5 group showed similarly-sized but non-identical Tcp10ms patterns, suggesting they share a recent common ancestor. These chromosomes are likely to have derived from an ancestral chromosome within the founding population of North American house mice. The Tcp10ms pattern was also shown to be useful in field studies for distinguishing among independent t haplotypes, when more than one is present within a single population.