Recombination between two mouse t-haplotypes (tw12tf and tLub-1): segregation of lethal factors relative to centromere and tufted (tf) locus

The T-locus - inspiration and distraction?

The T/t locus was a major focus of study by mouse geneticists during the 20th century. In the 70s, as the study of cell surface antigens controlling transplantation antigens was taking off, several laboratories hypothesized that alleles of this locus would control cell surface antigens important for embryonic development. One such antigen, the embryonal carcinoma F9 antigen was said to be an example. Other antigens were described on sperm and embryos that were said to be controlled by alleles at the T/t complex. These findings were later found to be false. The history of the findings and their refutation is described.

EVOLUTION OF DRIVING X CHROMOSOMES AND RESISTANCE FACTORS IN EXPERIMENTAL POPULATIONS OF DROSOPHILA SIMULANS

Sex-ratio drive is a particular case of meiotic drive, described in several Drosophila species, that causes males bearing driving X chromosome to produce a large excess of females in their progeny. In Drosophila simulans, driving X chromosomes and resistance factors located on the Y chromosome and on the autosomes have been previously reported. In this paper, we report the study of the dynamics of sex-ratio factors in experimental populations. We followed the evolution in frequency of driving X chromosomes in the absence of resistance factors and the evolution of resistance factors in the presence of driving X chromosomes. The driving X chromosome was lost, contrarily to theoretical expectations that predict its rapid invasion. Autosomal resistances increased in frequency, and resistant Y chromosomes invaded the population very quickly, as predicted by theoretical models. Fitness measurements showed that the loss of the driving X chromosome was due to a strong deleterious effect that was expressed only when distorting males were in competition with standard males. However, the spread of autosomal resistances reduced this deleterious effect. Implications for the maintenance of polymorphism in natural populations are discussed.

POLLEN PERFORMANCE AND SEX-RATIO EVOLUTION IN A DIOECIOUS PLANT

There has been a proliferation of studies, in a variety of taxa, that have detected sex-linked or cytoplasmic genes that enhance their own transmission via sex-ratio distortion. One of the most important parameters influencing the dynamics of these elements is the magnitude of their transmission advantage. In many systems, the mechanism of sex-ratio distortion is to abort X- or Y-bearing gametes. With this mechanism, the transmission advantage associated with sex-ratio distortion is diminished when the production of male gametes limits offspring production or when competition among the gametes of different males is intense. In this study, we analyzed the outcome of pollen competition between males that produced different sex ratios in the dioecious plant, Silene alba, and estimated how the sex-ratio bias influenced the transmission properties of the sex chromosomes. We varied the intensity of pollen competition by controlling the quantity of pollen used in crosses and used a combination of single-male pollinations and pollen mixtures to evaluate the effects of multiple paternity. Paternity in pollen mixtures was estimated using allozymes. Sex-ratio bias was directly influenced by the quantity of pollen, but the magnitude of this effect was small. The relative performance of pollen from different males varied substantially, especially when there was multiple paternity. Specifically, males with biased sex ratios sired far fewer offspring of either sex in pollen mixtures. In crosses involving single males, however, these "sex-ratio" males produced the same number of offspring as other males, so the female bias caused a significant transmission advantage for X-linked genes. X-linked genes could enhance their transmission via sex-ratio distortion in Silene populations, but the magnitude of this transmission advantage will depend on the ecological circumstances that influence the opportunity for multiple paternity.

Selfish genetic elements and sexual selection: their impact on male fertility

Females of many species mate with more than one male (polyandry), yet the adaptive significance of polyandry is poorly understood. One hypothesis to explain the widespread occurrence of multiple mating is that it may allow females to utilize post-copulatory mechanisms to reduce the risk of fertilizing their eggs with sperm from incompatible males. Selfish genetic elements (SGEs) are ubiquitous in eukaryotes, frequent sources of reproductive incompatibilities, and associated with fitness costs. However, their impact on sexual selection is largely unexplored. In this review we examine the link between SGEs, male fertility and sperm competitive ability. We show there is widespread evidence that SGEs are associated with reduced fertility in both animals and plants, and present some recent data showing that males carrying SGEs have reduced paternity in sperm competition. We also discuss possible reasons why male gametes are particularly vulnerable to the selfish actions of SGEs. The widespread reduction in male fertility caused by SGEs implies polyandry may be a successful female strategy to bias paternity against SGE-carrying males.

Selfish genetic elements and sexual selection: their impact on male fertility

Females of many species mate with more than one male (polyandry), yet the adaptive significance of polyandry is poorly understood. One hypothesis to explain the widespread occurrence of multiple mating is that it may allow females to utilize post-copulatory mechanisms to reduce the risk of fertilizing their eggs with sperm from incompatible males. Selfish genetic elements (SGEs) are ubiquitous in eukaryotes, frequent sources of reproductive incompatibilities, and associated with fitness costs. However, their impact on sexual selection is largely unexplored. In this review we examine the link between SGEs, male fertility and sperm competitive ability. We show there is widespread evidence that SGEs are associated with reduced fertility in both animals and plants, and present some recent data showing that males carrying SGEs have reduced paternity in sperm competition. We also discuss possible reasons why male gametes are particularly vulnerable to the selfish actions of SGEs. The widespread reduction in male fertility caused by SGEs implies polyandry may be a successful female strategy to bias paternity against SGE-carrying males.

Induction of new mutations in a mouse t-haplotype using ethylnitrosourea mutagenesis

N-ethyl-N-nitrosourea (ENU) was used to induce mutations within thetw5-haplotype of the mouse to make possible a further study of gene arrangement int-mutants and to provide potential landmarks for cloning and sequence studies in the region. Two independent mutants were isolated for each of three loci in thet-region, brachyury (T), quaking (qk), and tufted (tf). The newTktalleles produce tailless mice when atctmutation is present intrans. The newqkktalleles are recessive and homozygous lethal. They are viable, male fertile, and cause seizures and quaking when paired with theqkmutation which previously defined the locus. Thetfktmutations are recessive and phenotypically similar to the mutant alleles available in non-tchromosomes. The mutations were induced in thetw5-haplotype at an average per locus frequency of 1 in 1500. Their isolation demonstrates the power of this technique for obtaining the specific mouse mutants that are needed to genetically dissect a complex mammalian system.

Genetic analysis of the T/t complex

The T/t-complex has held considerable interest for immunologists, primarily because of its close genetic linkage to the major histocompatibility complex (MHC) on mouse chromosome 17. This interest has been heightened recently with the discovery that the MHC is fully contained within the t-complex and that two regions of the MHC, Qa and K, contain t-lethal genes. For a long time, T/t has been an enigmatic system, mainly because classical genetic analysis was not possible. Here the system is defined, recent information is presented, and our understanding of the mouse data to available information about the human MHC is correlated.

Molecular cloning and sequence analysis of a haploid expressed gene encoding t complex polypeptide 1

The mouse t haplotypes show defects in spermatogenesis attributed to multiple loci on chromosome 17. We have cloned the gene for an abundant testicular germ cell protein, t complex polypeptide 1, which has a variant form in t haplotypes, TCP-1A. A cDNA clone, pB1.4, which hybridizes to a 19S mRNA that is abundant in haploid cells during mouse spermatogenesis, derives from the 3' end of the mRNA encoding TCP-1B. The Tcp-1 gene appears to be a member of a novel gene family and shows multiple changes between the predicted amino acid sequences of TCP-1B and TCP-1A. An additional Taq1 site is created by a T to C transition in the predicted open reading frame of the Tcp-1a gene. The resultant RFLP has allowed typing of the Tcp-1 gene cluster in 54 complete and partial t haplotype chromosomes. DNA sequence comparison of the Tcp-1 genes suggests that the t haplotype chromosome arose within the genus Mus more than one million years ago.

Genetic analysis of the proximal portion of the mouse t complex: evidence for a second inversion within t haplotypes

Genomic sequences derived from the mouse t complex by a microdissection cloning technique have been used as tools to obtain high resolution genetic maps of the wild-type and t haplotype forms of the most proximal portion of chromosome 17. Genetic mapping was performed through a recombinant inbred strain analysis and an analysis of partial t haplotypes. The accumulated data demonstrate the existence of a large inversion of genetic material, encompassing the loci of T and qk, within the proximal portion of t haplotypes. This newly described proximal inversion and the previously described distal inversion provide an explanation for the suppression of recombination observed along the length of t haplotype DNA in heterozygous mice.

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.

The arrangement of H-2 class I genes in mouse t haplotypes

The t haplotypes of mouse chromosome 17 bear a number of interesting mutations and rearrangements, some of which map close to the H-2 complex. Since there are many H-2 class I genes of unknown function, we have investigated their arrangement in t haplotypes using genomic Southern blots. We present a detailed chart of the H-2w30 (tw12) complex, and compare it with the arrangement in other t haplotypes and standard mouse haplotypes. The chart shows duplications, deletions, and reshuffling of conserved and divergent regions. The two major features of the t arrangement--large deletions in the Qa and Tla regions--have analogues in some standard strains, so it is unlikely that these deletions are responsible for t-specific phenotypes. The differences between t and standard mouse strains are similar, in nature and in degree, to those between different standard strains.

A new spontaneous mutation at the tufted locus within a mouse t haplotype

We report here the occurrence of a new spontaneous mutation at the tufted locus present within the crossover-suppressed region of the completethaplotypefwLub1This new tufted allele shows complete expressivity of the tufted phenotype without any apparent effect on viability or reproduction. The presence of a tufted mutation within athaplotype that is cytologically marked by a Robertsonian translocation is particularly useful in genetic experiments aimed at understanding the structure and function ofthaplotypes.

Gene mapping within the T/t complex of the mouse. III: t-Lethal genes are arranged in three clusters on chromosome 17

Lethal t mutations belonging to seven different complementation groups have been mapped. The overall picture emerges as three clusters of t-lethal genes: one near the proximally located tail interaction factor (tT) containing tw73; one numerous cluster associated with the MHC and including tw32, t12, tw5, tw18, and tLub-1; and one distal cluster close to the locus of tf encompassing, t0, t6, and tw12. In the two cases examined, members of the same complementation group map to the same location. Evidence is presented that more than one mutant site may be necessary for the lethality of some lethal t "mutations," and that the expression of a specific lethal phenotype may depend on cis-interactions between multiple mutations.

Recombination between two mouse t haplotypes (tw12tf and tLub-1): mapping of the H-2 complex relative to centromere and tufted (tf) locus

A monoclonal antibody known to recognize the H-2.m3 specificity is shown to react with the class I H-2 product of tLub-1 but not tw12tf mice. This reagent was used to study the segregation of the H-2 complex in the progeny of tLub-1 +/tw12tf females. The most straightforward interpretation of the results presented here is that these t haplotypes carry an H-2 complex located between the centromere and tufted locus. Possible consequences of such a location with regard to the recombination between t haplotypes and chromosome 17 from laboratory mice are discussed.