Transmission ratio distortion in mouse t-haplotypes is due to multiple distorter genes acting on a responder locus

Transmission ratios of male mice heterozygous for various combinations of partial t-haplotypes provide evidence in support of a model for the genetic basis of ratio distortion, involving two or more distorter genes acting on a responder locus. The t form of the responder locus, Tcr, in the medial part of the haplotype, must be present and heterozygous for distortion to occur. When the responder alone is present, as in t^low haplotypes, the chromosome carrying it is transmitted in a low ratio (<50%). The t forms of the distorter loci act additively, in cis or trans, to raise the transmission of whichever chromosome carries Tcr. Identified distorter loci are Tcd-1, in the proximal part of the haplotype, Tcd-2, distal to Tcr, and probably Tcd-3, lying between Tcr and Tcd-2. In the absence of Tcr the distorters are transmitted normally. The system is compared with the SD system of Drosophila.     

Deletion of mouse t-complex distorter-1 produces an effect like that of the t-form of the distorter.

An allele of the mouse brachyury locus, T22H, had been shown previously to involve a deletion of several markers in the proximal part of chromosome 17, and almost certainly includes deletion of the t-complex distorter gene Tcd-1. The effects of T22H on transmission ratio distortion and male sterility caused by the t-complex were compared with those of a partial t-haplotype th51, which carries the t-form of the distorter Tcd-1t. In combination with the complete haplotype tw32, T22H caused severe impairment of male fertility, but males of genotype T22H/t6 or T22H/th51 were normally fertile. These results were very similar to those obtained when th51 was in combination with the same haplotypes. In effect on transmission ratio T22H was again similar to th51, in that it produced a marked increase in the transmission of the haplotype t6. To test whether the effects of T22H were due to deletion of elements other than Tcd-1, the effect of T22H on transmission of the partial haplotype th2 was compared with that of the deletion Thp. Again T22H markedly increased transmission of the t-haplotype and the effect was significantly greater than the small effect produced by Thp. It is concluded that deletion of the distorter Tcd-1 has an effect like that of the t-form of this distorter, Tcd-1t, and hence that Tcd-1t must be an amorph or hypomorph. It is speculated that other t-complex distorters, Tcd-2t and Tcd-3t, may also be amorphs or hypomorphs.(ABSTRACT TRUNCATED AT 250 WORDS)     

Search for differences among t haplotypes in distorter and responder genes

Transmission ratio distortion due to the mouse t complex is though to be due to harmful effects of trans-acting distorter genes acting on a responder, with the t complex form of the responder being relatively resistant to this harmful action of the distorters. Previous work had indicated that naturally occurring t haplotypes differed in their responders or in distorters lying near the responder, with the result that animals doubly heterozygous for two responder-carrying haplotypes transmitted these haplotypes unequally. In the present work t haplotypes could be divided into three types on the basis of their transmission when doubly heterozygous with the responder-carrying partial haplotype tlowH. The majority, t0, t6, tw1, tw2 and tw73, were transmitted equally with tlowH, a second group, including tw5 and two haplotypes derived from it, were transmitted less frequently than tlowH, and the single member of a third group, tw32, was transmitted in excess of tlowH. This last result suggests that the underlying differences are in the responder itself, rather than in the distorters. Search for differences among t haplotypes in distorters produced some equivocal results possibly resulting from effects of genetic background. In particular, results of others suggesting presence of a fourth distorter, Tcd-4, were not confirmed.     

A Novel Mouse Chromosome 17 Hybrid Sterility Locus: Implications for the Origin of T Haplotypes

The effects of heterospecific combinations of mouse chromosome 17 on male fertility and transmission ratio were investigated through a series of breeding studies. Animals were bred to carry complete chromosome 17 homologs, or portions thereof, from three different sources-Mus domesticus, Mus spretus and t haplotypes. These chromosome 17 combinations were analyzed for fertility within the context of a M. domesticus or M. spretus genetic background. Two new forms of hybrid sterility were identified. First, the heterospecific combination of M. spretus and t haplotype homologs leads to complete male sterility on both M. spretus and M. domesticus genetic backgrounds. This is an example of symmetrical hybrid sterility. Second, the presence of a single M. domesticus chromosome 17 homolog within a M. spretus background causes sterility, however, the same combination of chromosome 17 homologs does not cause sterility within the M. domesticus background. This is a case of asymmetrical hybrid sterility. Through an analysis of recombinant chromosomes, it was possible to map the M. domesticus, M. spretus and t haplotype alleles responsible for these two hybrid sterility phenotypes to the same novel locus (Hybrid sterility-4). Previous structural studies had led to the hypothesis that the ancestral t haplotype originated through an introgression event from M. spretus or a related species. If this were true, one might expect that (1) M. spretus homologs would be transmitted at a non-Mendelian ratio within the M. domesticus background, and (2) t haplotypes would be transmitted at a ratio closer to Mendelian within the M. spretus background.(ABSTRACT TRUNCATED AT 250 WORDS)     

Male sterility of the mouse t-complex is due to homozygosity of the distorter genes

Evidence is presented that the male sterility produced by the mouse t-complex is due to interaction of at least three sterility factors. These factors are carried in the same partial haplotypes as the three distorter genes, Tcd-1, Tcd-2, and Tcd-3 and are suggested to be identical with them. When heterozygous, the distorter/sterility genes act on the wild-type form of the responder gene, rendering sperm carrying it nonfunctional, thus leading to high transmission of the t form of the responder. When homozygous, the harmful effects of the distorter genes are stronger and affect both forms of the responder, leading to sterility. If homozygous sterility is an inescapable part of ratio distortion, then the t-lethals confer a selective advantage in removing sterile males from the population. Thus, the relationship between the various properties of the t-complex can now be understood.     

Genetic Exchange across a Paracentric Inversion of the Mouse T Complex

Mouse t haplotypes are distinguished from wild-type forms of chromosome 17 by four nonoverlapping paracentric inversions which span a genetic distance of 20 cM. These inversion polymorphisms are responsible for a 100-200-fold suppression of recombination which maintains the integrity of complete t haplotypes and has led to their divergence from the wild-type chromosomes of four species of house mice within which t haplotypes reside. As evidence for the long period of recombinational isolation, alleles that distinguish all t haplotypes from all wild-type chromosomes have been established at a number of loci spread across the 20-cM variant region. However, a more complex picture emerges upon analysis of other t-associated loci. In particular, "mosaic haplotypes" have been identified that carry a mixture of wild-type and t-specific alleles. To investigate the genetic basis for mosaic chromosomes, we conducted a comprehensive analysis of eight t complex loci within 76 animals representing 10 taxa in the genus Mus, and including 23 previously characterized t haplotypes. Higher resolution restriction mapping and sequence analysis was also performed for alleles at the Hba-ps4 locus. The results indicate that a short tract of DNA was transferred relatively recently across an inversion from a t haplotype allele of Hba-ps4 to the corresponding locus on a wild-type homolog leading to the creation of a new hybrid allele. Several classes of wild-type Hba-ps4 alleles, including the most common form in inbred strains, appear to be derived from this hybrid allele. The accumulated data suggest that a common form of genetic exchange across one of the four t-associated inversions is gene conversion at isolated loci that do not play a role in the transmission ratio distortion phenotype required for t haplotype propagation. The implications of the results pose questions concerning the evolutionary stability of gene complexes within large paracentric inversions and suggest that recombinational isolation may be best established for loci residing within a short distance from inversion breakpoints.     

Elevated galactosyltransferase activity on t-bearing sperm segregates with T/t-complex distorter loci-2

Sperm bearing complete t-haplotypes are preferentially transmitted during fertilization from heterozygous +/t males, often in excess of 95% relative to their (+)-bearing meiotic partner. Sperm from t-bearing males have an approximate two- to fourfold increase in beta 1,4-galactosyltransferase (GalTase) activity, a cell surface protein that mediates sperm binding to the egg zona pellucida. The elevated GalTase activity strictly correlates with the preferential transmission of t-sperm from +/t males, since eight other enzymes show normal levels of activity on t-sperm. Furthermore, sperm bearing proximal partial t-haplotypes, which are no longer favoured during fertilization, have normal levels of GalTase activity. Nevertheless, it has been unclear whether the elevated sperm GalTase activity on t-sperm is due to specific loci in the distal segment of the T/t-complex, or rather, is an indirect consequence of the abnormal sperm function characteristic of +/t and tx/ty males. In this study, it is shown that the elevated sperm GalTase activity is due specifically to factors that reside within the distal segment of the T/t complex, which also contains Tcd-2, the strongest of the distorter loci. Since the structural locus for GalTase is located on mouse chromosome 4, these results also show that T/t-complex alleles on chromosome 17 are regulatory in nature and affect the expression of sperm surface components critical for normal fertilization. Models are presented to explain how elevated GalTase activity could contribute to sperm transmission distortion.     

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 >=0.90 or higher are typically observed. With transmission ratios of this level, theoretical analyses predict high frequencies of t haplotypes (~ 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.     

The Putative Oncogene Pim-1 in the Mouse: Its Linkage and Variation among t Haplotypes

Pim-1, a putative oncogene involved in T-cell lymphomagenesis, was mapped between the pseudo-alpha globin gene Hba-4ps and the alpha-crystallin gene Crya-1 on mouse chromosome 17 and therefore within the t complex. Pim-1 restriction fragment variants were identified among t haplotypes. Analysis of restriction fragment sizes obtained with 12 endonucleases demonstrated that the Pim-1 genes in some t haplotypes were indistinguishable from the sizes for the Pim-1b allele in BALB/c inbred mice. There are now three genes, Pim-1, Crya-1 and H-2 I-E, that vary among independently derived t haplotypes and that have indistinguishable alleles in t haplotypes and inbred strains. These genes are closely linked within the distal inversion of the t complex. Because it is unlikely that these variants arose independently in t haplotypes and their wild-type homologues, we propose that an exchange of chromosomal segments, probably through double crossingover, was responsible for indistinguishable Pim-1 genes shared by certain t haplotypes and their wild-type homologues. There was, however, no apparent association between variant alleles of these three genes among t haplotypes as would be expected if a single exchange introduced these alleles into t haplotypes. If these variant alleles can be shown to be identical to the wild-type allele, then lack of association suggests that multiple exchanges have occurred during the evolution of the t complex.     

EVOLUTION AT THE MOUSE t COMPLEX: WHY IS THE t HAPLOTYPE PRESERVED AS AN INTEGRAL UNIT?

Abstract Segregation distorters are selfish genetic elements that bias Mendelian segregation in their favor. All well-known segregation distortion systems consist of one or more "distorter" loci that act upon a "responder" locus. At the t complex of the house mouse, segregation distortion is brought about by the harmful effect of t alleles at a number of distorter loci on the wild-type variant of the responder locus. The responder and distorter alleles are closely linked by a number of inversions, thus forming a coherent t haplotype. It has been conjectured that the close integration of the various components into a "complete" t haplotype has been crucial for the evolutionary success of these selfish genetic elements. By means of a population genetical metapopulation model, we show that this intuition may be unfounded. In fact, under most circumstances an "insensitive" t haplotype retaining only the responder did invade and reach a high frequency, despite the fact that this haplotype has a strong segregation disadvantage. For certain population structures, the complete t haplotype was even competitively excluded by partial t haplotypes with lower segregation ratios. Moreover, t haplotypes carrying one or more recessive lethals only prevailed over their nonlethal counterparts if the product of local population size and migration rate ( Nm ) was not much smaller or larger than one. These phenomena occurred for rather realistic fitness, segregation, and recombination values. It is therefore quite puzzling that partial t haplotypes are absent from natural house mousepopulations, and that t haplotypes carrying recessive lethals prevail over nonlethal t haplotypes.     

Distortion of the maternal segregation of the silent alleles of complement factor 4 in normal and diabetic families

54 normal Caucasian families and 169 families in whom at least one child had type I diabetes (IDDM) were genotyped for HLA-A, B, C, DR and for the complement factors Bf and C4. The paternal and maternal transmission of the different alleles and of haplotypes and complotypes in linkage desequilibrium have been analysed. No distortion of the paternal transmission has been observed in the offspring of the two series of families. On the contrary, a distortion of the maternal segregation of the silent alleles at the complement factor C4A and B locus was found: mothers transmitted C4AQ0 more often than expected to their male offspring (p less than 0.04 in normal families, p less than 0.001 in IDDM families) while they transmitted C4BQ0 in excess to their female offspring (p less than 0.01 and p less than 0.03 in normal and IDDM families, respectively).     

Inheritance patterns of maternal alleles in imprinted regions of the mouse genome at different stages of development

Deviations from Mendelian 1:1 transmission ratio have been observed in mice and humans. With few exceptions, the mechanism leading to transmission-ratio distortion (TRD) remains obscure. We proposed that a genomic imprinting mechanism plays a key role in the genesis of grandparental origin-dependent TRD (Naumova et al. 2001). To further test this hypothesis, we analyzed the transmission of grandparental alleles at three imprinted regions of the mouse genome known to contain genes required for embryo development. We found and replicated moderate (58%: 42%) TRD in favor of grandmaternal alleles in the imprinted region of maternal distal Chromosome (Chr) 12 among female offspring. Comparison of transmission ratios at the distorted region of Chr 12 among 3-week-old mice with those in embryos suggests that the distortion in favor of grandmaternal alleles is owing to postimplantation embryo loss. The absence of grandparental origin-dependent TRD for maternal Chr 6 and 7 implies that the relationship between TRD and imprinting is complex. Most likely, multiple conditions are required for TRD to occur. Received: 20 June 2001 / Accepted: 28 August 2001     

Identification of a cryptic lethal mutation in the mouse t(w73) haplotype

t haplotypes are naturally occurring, variant forms of the t complex on mouse chromosome 17, characterized by the presence of four inversions with respect to wild-type. They harbour mutations causing male sterility, male transmission ratio distortion (TRD) and embryonic lethality. Mice carrying t haplotypes have been found throughout the world, and genetic studies of the lethal mutations have identified at least 16 complementation groups. The embryonic lethal phenotypes of many t haplotypes have been characterized in detail, and are thought to be the consequence of homozygosity for single gene mutations. However, the existence of additional mutations in genes that function at later stages of development would be obscured. Here we investigated the possibility of multiple mutations in t haplotypes by screening the t(w73) haplotype for the presence of novel mutations. Since genetic analysis of t haplotype mutations is hindered by recombination suppression due to the inversions, deletion complexes covering the proximal two-thirds of the t complex were used to uncover the presence of any new lethal alleles. This analysis revealed a novel mutation between D17Jcs41 and D17Mit100, causing mice carrying both t(w73) and selected deletions to die at birth, prior to feeding. The finding of a new, cryptic lethal mutation in t haplotypes is an indication that these recombinationally isolated chromosomes, which already contain at least one lethal mutation that prevents homozygosity, may serve as sinks for the accumulation of additional recessive mutations.     

Parental effect of DNA (Cytosine-5) methyltransferase 1 on grandparental-origin-dependent transmission ratio distortion in mouse crosses and human families

Transmission ratio distortion (TRD) is a deviation from the expected Mendelian 1:1 ratio of alleles transmitted from parents to offspring and may arise by different mechanisms. Earlier we described a grandparental-origin-dependent sex-of-offspring-specific TRD of maternal chromosome 12 alleles closely linked to an imprinted region and hypothesized that it resulted from imprint resetting errors in the maternal germline. Here, we report that the genotype of the parents for loss-of-function mutations in the Dnmt1 gene influences the transmission of grandparental chromosome 12 alleles. More specifically, maternal Dnmt1 mutations restore Mendelian transmission ratios of chromosome 12 alleles. Transmission of maternal alleles depends upon the presence of the Dnmt1 mutation in the mother rather than upon the Dnmt1 genotype of the offspring. Paternal transmission mirrors the maternal one: live-born offspring of wild-type fathers display 1:1 transmission ratios, whereas offspring of heterozygous Dnmt1 mutant fathers tend to inherit grandpaternal alleles. Analysis of allelic transmission in the homologous region of human chromosome 14q32 detected preferential transmission of alleles from the paternal grandfather to grandsons. Thus, parental Dnmt1 is a modifier of transmission of alleles at an unlinked chromosomal region and perhaps has a role in the genesis of TRD.     

A premature acrosome reaction is programmed by mouse t haplotypes during sperm differentiation and could play a role in transmission ratio distortion

Mouse t haplotypes are variant forms of chromosome 17 that can be transmitted at non-Mendelian ratios by heterozygous +/t males. The accumulated genetic data indicate that '+-sperm' and 't-sperm' are produced in equal numbers but that most '+-sperm' are rendered dysfunctional, so that 't-sperm' have a relative advantage at fertilization. To date, the basis for this t-induced sperm dysfunction has remained unknown. Here we demonstrate that a high proportion of sperm obtained from certain strains of +/t mice undergo a premature acrosome reaction under in vitro capacitation conditions. The simplest interpretation of these data, in conjunction with previous results, is that developing '+-spermatids' are preprogrammed by 't-spermatids' to undergo this premature reaction. Since acrosome-reacted sperm are unable to participate in the process of fertilization, this defect could account for the extreme distortion of transmission ratio observed from mice heterozygous for a class of complete t haplotypes.     

Phylogenetic analysis of the alpha-globin pseudogene-4 (Hba-ps4) locus in the house mouse species complex reveals a stepwise evolution of t haplotypes [published erratum appears in Mol Biol Evol 1994 Jan;11(1):158]

A parsimony analysis was performed on restriction sites at the Hba-ps4 pseudogene locus within one of four inversions associated with mouse t haplotypes. The results suggest that all t haplotypes form a monophyletic group and that the in (17)4 inversion originated before the radiation of the Mus musculus species complex but after the divergence of the lineages leading to M. spretus, M. abbotti, and M. hortulanus. A time frame based on the evolutionary rate of mouse pseudogenes places the origin of this t haplotype inversion at 1.5 Mya, or approximately 1.5 Myr after the origin of the more proximal t complex inversion, in (17)2. The accumulated evidence indicates that complete t haplotypes have been assembled in a stepwise manner, with each of these inversions occurring on separate chromosomal lineages and at different evolutionary times. In addition, the evolutionary relationships of pseudogene sequences resulting from genetic exchange between wild-type and t haplotype alleles were examined. Analysis of sequences from the 5' and 3' sides of a putative site of recombination resulted in cladograms with different topologies. The implications for hypotheses concerning the evolutionary forces acting on t haplotypes and their rapid propagation throughout worldwide populations of mice are discussed.      

Pattern of segmental recombination in the distal inversion of mouse t haplotypes

To examine genetic exchange between t haplotypes and their wild-type homologs, four previously identified mouse Chromosome (Chr) 17 variants termed mosaic haplotypes were analyzed in detail. Three of these haplotypes-one from a Mus musculus population in Bulgaria, one from a Mus domesticus population in Chile, and one from a M. domesticus population in Germany-display properties indicative of the t complex. All four haplotypes are exceptional because they are characterized by the presence of a few wild-type DNA markers in the distal inversion [In(17)4] of a t haplotype chromosome: thus, they are classified as mosaic t haplotypes. The mosaic pattern for each haplotype is distinct, however. We compared the mosaic haplotypes with each other, and with several well-characterized laboratory t haplotypes, by analyzing several DNA markers in the In(17)4 region of the t complex, where all of the mosaicism occurs. We used a combination of high-resolution restriction mapping, DNA sequencing, and analysis of new DNA markers to classify the haplotypes. This analysis shows that segmental exchange, either by gene conversion or double crossing-over, has occurred at molecular markers in the vicinity of a gene, Dnahc8, that is a candidate for the t complex distorter locus Tcd2. While it is unclear whether segmental exchanges have included the Tcd2 gene, it is apparent that several independent recombination events have occurred in In(17)4 during the recent evolution of t haplotypes.     

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 females and C57BL/6 males show transmission ratio distortion at the Om locus on mouse chromosome 11. This result has been replicated in several independent experiments. Here we show that the distortion maps to a single locus on chromosome 11, closely linked to Om, and that gene conversion is not implicated in the origin of this phenomenon. To further investigate the origin of the transmission ratio distortion we generated a test using the well-known effect of recombination on maternal meiotic drive. The genetic test presented here discriminates between unequal segregation of alleles during meiosis 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 determine the cause of the transmission ratio distortion observed at the Om locus. Our results indicate that transmission ratio distortion at Om is due to unequal segregation of alleles to the polar body at the second meiotic division. Because the presence of segregation distortion at Om also depends on the genotype of the sire, our results confirm that the sperm can influence segregation of maternal chromosomes to the second polar body.     

Genetic Analysis of a Mouse t Complex Locus That Is Homologous to a Kidney Cdna Clone

A mouse kidney cDNA clone, pMK174, identifies restriction fragment length polymorphisms (RFLPs) that map to two unlinked loci. One, designated D17Rp17, has been mapped near quaking, (qk), on chromosome 17 using three sets of recombinant inbred (RI) strains. A study of several t haplotypes resulted in the identification of t-specific alleles of D17Rp17 that map to the proximal half of the t complex. Neither t-specific nor wild-type D17Rp17 alleles are present in chromosomes carrying either the T Orleans (TtOrl) or the T hairpin tail (Thp) deletions. Comparison with other molecular markers indicates that pMK174 identifies a new proximal t complex locus, Rp17. The second locus identified by pMK174, termed D4Rp18, is tentatively assigned to chromosome 4 by mouse-Chinese hamster somatic cell hybrid analysis.