Three new t-haplotypes of Mus musculus reveal structural similarities to t-haplotypes of Mus domesticus

A fascination with tailless mice: a scientific historical review of studies of the T/t complex

The T/t complex of the mouse attracted many of the major figures of mouse genetics to perform genetic, cytogenetic, physiological, biochemical and molecular biological studies of it. These studies started with the discovery of short tailed mutants (Ts) and recessive lethal developmental mutations (ts) which mapped to the same "locus" in the early 1920s in France. However, due to the non-receptivity of French scientists to genetics, they continued to be studied in mostly Anglophone countries to be joined by a wider international community in the 1970s. These discoveries led to developmental studies of the lethal mutants which provided the origin of mammalian developmental genetics. The fascinating property of transmission ratio distortion (non-50/50 segregation of alleles in offspring of males) elicited tremendous interest. There were false leads (that the region consisted of unusual DNA, that the alleles controlled cell surface antigens on embryonic cells and spermatozoa) and exciting discoveries. This historical review provides a review of this extensive area of research and some of the individuals involved in it.

Natural variation in the zinc-finger-encoding exon of Prdm9 affects hybrid sterility phenotypes in mice

PRDM9-mediated reproductive isolation was first described in the progeny of Mus musculus musculus (MUS) PWD/Ph and Mus musculus domesticus (DOM) C57BL/6J inbred strains. These male F1 hybrids fail to complete chromosome synapsis and arrest meiosis at prophase I, due to incompatibilities between the Prdm9 gene and hybrid sterility locus Hstx2. We identified 14 alleles of Prdm9 in exon 12, encoding the DNA-binding domain of the PRDM9 protein in outcrossed wild mouse populations from Europe, Asia, and the Middle East, 8 of which are novel. The same allele was found in all mice bearing introgressed t-haplotypes encompassing Prdm9. We asked whether 7 novel Prdm9 alleles in MUS populations and the t-haplotype allele in 1 MUS and 3 DOM populations induce Prdm9-mediated reproductive isolation. The results show that only combinations of the dom2 allele of DOM origin and the MUS msc1 allele ensure complete infertility of intersubspecific hybrids in outcrossed wild populations and inbred mouse strains examined so far. The results further indicate that MUS mice may share the erasure of PRDM9msc1 binding motifs in populations with different Prdm9 alleles, which implies that erased PRDM9 binding motifs may be uncoupled from their corresponding Prdm9 alleles at the population level. Our data corroborate the model of Prdm9-mediated hybrid sterility beyond inbred strains of mice and suggest that sterility alleles of Prdm9 may be rare.

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.

Fine haplotype structure of a chromosome 17 region in the laboratory and wild mouse

Extensive linkage disequilibrium among classical laboratory strains represents an obstacle in the high-resolution haplotype mapping of mouse quantitative trait loci (QTL). To determine the potential of wild-derived mouse strains for fine QTL mapping, we constructed a haplotype map of a 250-kb region of the t-complex on chromosome 17 containing the Hybrid sterility 1 (Hst1) gene. We resequenced 33 loci from up to 80 chromosomes of five mouse (sub)species. Trans-species single-nucleotide polymorphisms (SNPs) were rare between Mus m. musculus (Mmmu) and Mus m. domesticus (Mmd). The haplotypes in Mmmu and Mmd differed and therefore strains from these subspecies should not be combined for haplotype-associated mapping. The haplotypes of t-chromosomes differed from all non-t Mmmu and Mmd haplotypes. Half of the SNPs and SN indels but only one of seven longer rearrangements found in classical laboratory strains were useful for haplotype mapping in the wild-derived M. m. domesticus. The largest Mmd haplotype block contained three genes of a highly conserved synteny. The lengths of the haplotype blocks deduced from 36 domesticus chromosomes were in tens of kilobases, suggesting that the wild-derived Mmd strains are suitable for fine interval-specific mapping.

Sub-milliMorgan map of the proximal part of mouse Chromosome 17 including the hybrid sterility 1 gene

We have generated a high-resolution genetic map, 0.071 cM per backcross animal, of the 13 cM T-H2 region of the mouse Chromosome (Chr) 17. The map contains two phenotypic loci, T and Hst1, 12 RFLP markers, and 24 microsatellite loci. The Hst1 gene was mapped to a chromosomal interval contained within a single 580-kb YAC clone. The FFEH11 YAC is 0.44 cM long and carries, besides the Hst1 gene, five polymorphic DNA markers and recombination breakpoints of six backcross animals. Two candidate genes for Hst1 were identified based on their location and testicular expression. These are Tbp and D17Ph4e. The submilliMorgan map of the T-H2 region revealed significant clustering of (CA)n loci. The clustering, if shown to be a common feature in the mouse genome, may cause gaps in the physical map of the mouse genome.

Genetic analysis of genomic imprinting: an Imprintor-1 gene controls inactivation of the paternal copy of the mouse Tme locus

The Thp deletion on mouse chromosome 17 is lethal when inherited from the mother, because it deletes the T-associated maternal effect (Tme) locus, the paternal copy of which is inactivated by genomic imprinting. We have found a paternally nonimprinted Tme variant in crosses of Thp females with Mus m. musculus males. The data are consistent with the existence of a single Tme-unlinked gene, Imprintor-1 (Imp-1), with two alleles, one of which only causes imprinting at the Tme locus. Imp-1 is unlinked to the gene for cation-dependent Man-6-P receptor and acts prezygotically. Although Tme and Igf2r were thought to be identical, they show different patterns of imprinting in interspecies hybrids. The apparent nonequivalence of the Igf2r gene and Tme results in occurrence of viable mice lacking an active Igf2r gene. These mice are bigger at birth than their normal littermates, in accord with the proposed function of the IGF-II/Man-6-P receptor.