Low frequency of mouse t haplotypes in wild populations is not explained by modifiers of meiotic drive

t haplotypes are naturally occurring forms of mouse chromosome 17 that show non-Mendelian transmission from heterozygous +/t males. In laboratory studies, transmission ratios of greater than or equal to 0.90 or higher are typically observed. With transmission ratios of this level, theoretical analyses predict high frequencies of t haplotypes (similar to 75%) in wild populations. In contrast, empirical frequencies of only 15-25% are typically found. This has led to the suggestion that modifiers of drive may play a role in reducing t frequencies. We have measured transmission ratio distortion (TRD) levels in wild +/t mice to examine this hypothesis. TRD was very high in both litters collected from wild-caught pregnant females, and in wild litters bred in the laboratory (mean = 0.9). Contrary to the results of other studies, we found no difference in TRD levels between semilethal and lethal t haplotypes nor between litters conceived from cycling or postpartum estrus. We found three litters with aberrantly low TRDs that were all multiply sired, although the role this might play in natural populations is unknown. These findings show a general absence of modifiers of drive in natural populations and suggest that other factors are responsible for the low observed frequencies of wild t haplotypes.