Sexual dimorphism in the meiotic requirement for PRDM9: A mammalian evolutionary safeguard.

In many mammals, genomic sites for recombination are determined by the histone methyltransferase PRMD9. Some mouse strains lacking PRDM9 are infertile, but instances of fertility or semifertility in the absence of PRDM9 have been reported in mice, canines, and a human female. Such findings raise the question of how the loss of PRDM9 is circumvented to maintain fertility. We show that genetic background and sex-specific modifiers can obviate the requirement for PRDM9 in mice. Specifically, the meiotic DNA damage checkpoint protein CHK2 acts as a modifier allowing female-specific fertility in the absence of PRDM9. We also report that, in the absence of PRDM9, a PRDM9-independent recombination system is compatible with female meiosis and fertility, suggesting sex-specific regulation of meiotic recombination, a finding with implications for speciation.

Prdm9 and Meiotic Cohesin Proteins Cooperatively Promote DNA Double-Strand Break Formation in Mammalian Spermatocytes.

Meiotic recombination is required for correct segregation of chromosomes to gametes and to generate genetic diversity. In mice and humans, DNA double-strand breaks (DSBs) are initiated by SPO11 at recombination hotspots activated by PRDM9-catalyzed histone modifications on open chromatin. However, the DSB-initiating and repair proteins are associated with a linear proteinaceous scaffold called the chromosome axis, the core of which is composed of cohesin proteins. STAG3 is a stromalin subunit common to all meiosis-specific cohesin complexes. Mutations of meiotic cohesin proteins, especially STAG3, perturb both axis formation and recombination in the mouse, prompting determination of how the processes are mechanistically related. Protein interaction and genetic analyses revealed that PRDM9 interacts with STAG3 and REC8 in cooperative relationships that promote normal levels of meiotic DSBs at recombination hotspots in spermatocytes. The efficacy of the Prdm9-Stag3 genetic interaction in promoting DSB formation depends on PRDM9-mediated histone methyltransferase activity. Moreover, STAG3 deficiency has a major effect on DSB number even in the absence of PRDM9, showing that its role is not restricted to canonical PRDM9-activated hotspots. STAG3 and REC8 promote axis localization of the DSB-promoting proteins HORMAD1, IHO1, and MEI4, as well as SPO11 activity. These results establish that PRDM9 and axis-associated cohesin complexes together coordinate and facilitate meiotic recombination by recruiting key proteins for initiation of DSBs, thereby associating activated hotspots with DSB-initiating complexes on the axis.

Modulation of Prdm9-controlled meiotic chromosome asynapsis overrides hybrid sterility in mice.

Hybrid sterility is one of the reproductive isolation mechanisms leading to speciation. Prdm9, the only known vertebrate hybrid-sterility gene, causes failure of meiotic chromosome synapsis and infertility in male hybrids that are the offspring of two mouse subspecies. Within species, Prdm9 determines the sites of programmed DNA double-strand breaks (DSBs) and meiotic recombination hotspots. To investigate the relation between Prdm9-controlled meiotic arrest and asynapsis, we inserted random stretches of consubspecific homology on several autosomal pairs in sterile hybrids, and analyzed their ability to form synaptonemal complexes and to rescue male fertility. Twenty-seven or more megabases of consubspecific (belonging to the same subspecies) homology fully restored synapsis in a given autosomal pair, and we predicted that two or more DSBs within symmetric hotspots per chromosome are necessary for successful meiosis. We hypothesize that impaired recombination between evolutionarily diverged chromosomes could function as one of the mechanisms of hybrid sterility occurring in various sexually reproducing species.

Hybrid Sterility Locus on Chromosome X Controls Meiotic Recombination Rate in Mouse.

Meiotic recombination safeguards proper segregation of homologous chromosomes into gametes, affects genetic variation within species, and contributes to meiotic chromosome recognition, pairing and synapsis. The Prdm9 gene has a dual role, it controls meiotic recombination by determining the genomic position of crossover hotspots and, in infertile hybrids of house mouse subspecies Mus m. musculus (Mmm) and Mus m. domesticus (Mmd), it further functions as the major hybrid sterility gene. In the latter role Prdm9 interacts with the hybrid sterility X 2 (Hstx2) genomic locus on Chromosome X (Chr X) by a still unknown mechanism. Here we investigated the meiotic recombination rate at the genome-wide level and its possible relation to hybrid sterility. Using immunofluorescence microscopy we quantified the foci of MLH1 DNA mismatch repair protein, the cytological counterparts of reciprocal crossovers, in a panel of inter-subspecific chromosome substitution strains. Two autosomes, Chr 7 and Chr 11, significantly modified the meiotic recombination rate, yet the strongest modifier, designated meiotic recombination 1, Meir1, emerged in the 4.7 Mb Hstx2 genomic locus on Chr X. The male-limited transgressive effect of Meir1 on recombination rate parallels the male-limited transgressive role of Hstx2 in hybrid male sterility. Thus, both genetic factors, the Prdm9 gene and the Hstx2/Meir1 genomic locus, indicate a link between meiotic recombination and hybrid sterility. A strong female-specific modifier of meiotic recombination rate with the effect opposite to Meir1 was localized on Chr X, distally to Meir1. Mapping Meir1 to a narrow candidate interval on Chr X is an important first step towards positional cloning of the respective gene(s) responsible for variation in the global recombination rate between closely related mouse subspecies.

Retinoic acid activates two pathways required for meiosis in mice.

In all sexually reproducing organisms, cells of the germ line must transition from mitosis to meiosis. In mice, retinoic acid (RA), the extrinsic signal for meiotic initiation, activates transcription of Stra8, which is required for meiotic DNA replication and the subsequent processes of meiotic prophase. Here we report that RA also activates transcription of Rec8, which encodes a component of the cohesin complex that accumulates during meiotic S phase, and which is essential for chromosome synapsis and segregation. This RA induction of Rec8 occurs in parallel with the induction of Stra8, and independently of Stra8 function, and it is conserved between the sexes. Further, RA induction of Rec8, like that of Stra8, requires the germ-cell-intrinsic competence factor Dazl. Our findings strengthen the importance of RA and Dazl in the meiotic transition, provide important details about the Stra8 pathway, and open avenues to investigate early meiosis through analysis of Rec8 induction and function.

Prdm9 incompatibility controls oligospermia and delayed fertility but no selfish transmission in mouse intersubspecific hybrids.

PR-domain 9 (Prdm9) is the first hybrid sterility gene identified in mammals. The incompatibility between Prdm9 from Mus musculus domesticus (Mmd; the B6 strain) and the Hstx2 region of chromosome (Chr) X from M. m. musculus (Mmm; the PWD strain) participates in the complete meiotic arrest of mouse intersubspecific (PWD×B6)F1 hybrid males. Other studies suggest that also semisterile intersubspecific hybrids are relevant for mouse speciation, but the genes responsible remain unknown. To investigate the causes of this semisterility, we analyzed the role of Prdm9 and Chr X in hybrids resulting from the crosses of PWK, another Mmm-derived inbred strain. We demonstrate that Prdm9 and Chr X control the partial meiotic arrest and reduced sperm count in (PWK×B6)F1 males. Asynapsis of heterosubspecific chromosomes and semisterility were partially suppressed by removal of the B6 allele of Prdm9. Polymorphisms between PWK and PWD on Chr X but not in the Prdm9 region were responsible for the modification of the outcome of Prdm9-Chr X F1 hybrid incompatibility. Furthermore, (PWK×B6)F1 hybrid males displayed delayed fertility dependent on the Prdm9 incompatibility. While the Drosophila hybrid sterility gene Overdrive causes both delayed fertility and increased transmission of its own chromosome to the offspring, the segregation of Chr X and the Prdm9 region from the mouse (PWK×B6)F1 males was normal. Our results indicate extended functional consequences of Prdm9-Chr X intersubspecific incompatibility on the fertility of hybrids and should influence the design of fertility analyses in hybrid zones and of laboratory crosses between Mmm and Mmd strains.

X chromosome control of meiotic chromosome synapsis in mouse inter-subspecific hybrids.

Hybrid sterility (HS) belongs to reproductive isolation barriers that safeguard the integrity of species in statu nascendi. Although hybrid sterility occurs almost universally among animal and plant species, most of our current knowledge comes from the classical genetic studies on Drosophila interspecific crosses or introgressions. With the house mouse subspecies Mus m. musculus and Mus m. domesticus as a model, new research tools have become available for studies of the molecular mechanisms and genetic networks underlying HS. Here we used QTL analysis and intersubspecific chromosome substitution strains to identify a 4.7 Mb critical region on Chromosome X (Chr X) harboring the Hstx2 HS locus, which causes asymmetrical spermatogenic arrest in reciprocal intersubspecific F1 hybrids. Subsequently, we mapped autosomal loci on Chrs 3, 9 and 13 that can abolish this asymmetry. Combination of immunofluorescent visualization of the proteins of synaptonemal complexes with whole-chromosome DNA FISH on pachytene spreads revealed that heterosubspecific, unlike consubspecific, homologous chromosomes are predisposed to asynapsis in F1 hybrid male and female meiosis. The asynapsis is under the trans- control of Hstx2 and Hst1/Prdm9 hybrid sterility genes in pachynemas of male but not female hybrids. The finding concurred with the fertility of intersubpecific F1 hybrid females homozygous for the Hstx2(Mmm) allele and resolved the apparent conflict with the dominance theory of Haldane's rule. We propose that meiotic asynapsis in intersubspecific hybrids is a consequence of cis-acting mismatch between homologous chromosomes modulated by the trans-acting Hstx2 and Prdm9 hybrid male sterility genes.

Mechanistic basis of infertility of mouse intersubspecific hybrids.

According to the Dobzhansky-Muller model, hybrid sterility is a consequence of the independent evolution of related taxa resulting in incompatible genomic interactions of their hybrids. The model implies that the incompatibilities evolve randomly, unless a particular gene or nongenic sequence diverges much faster than the rest of the genome. Here we propose that asynapsis of heterospecific chromosomes in meiotic prophase provides a recurrently evolving trigger for the meiotic arrest of interspecific F1 hybrids. We observed extensive asynapsis of chromosomes and disturbance of the sex body in >95% of pachynemas of Mus m. musculus × Mus m. domesticus sterile F1 males. Asynapsis was not preceded by a failure of double-strand break induction, and the rate of meiotic crossing over was not affected in synapsed chromosomes. DNA double-strand break repair was delayed or failed in unsynapsed autosomes, and misexpression of chromosome X and chromosome Y genes was detected in single pachynemas and by genome-wide expression profiling. Oocytes of F1 hybrid females showed the same kind of synaptic problems but with the incidence reduced to half. Most of the oocytes with pachytene asynapsis were eliminated before birth. We propose the heterospecific pairing of homologous chromosomes as a preexisting condition of asynapsis in interspecific hybrids. The asynapsis may represent a universal mechanistic basis of F1 hybrid sterility manifested by pachytene arrest. It is tempting to speculate that a fast-evolving subset of the noncoding genomic sequence important for chromosome pairing and synapsis may be the culprit.

Dissecting the genetic architecture of F1 hybrid sterility in house mice.

Hybrid sterility as a postzygotic reproductive isolation mechanism has been studied for over 80 years, yet the first identifications of hybrid sterility genes in Drosophila and mouse are quite recent. To study the genetic architecture of F(1) hybrid sterility between young subspecies of house mouse Mus m. domesticus and M. m. musculus, we conducted QTL analysis of a backcross between inbred strains representing these two subspecies and probed the role of individual chromosomes in hybrid sterility using the intersubspecific chromosome substitution strains. We provide direct evidence that the asymmetry in male infertility between reciprocal crosses is conferred by the middle region of M. m. musculus Chr X, thus excluding other potential candidates such as Y, imprinted genes, and mitochondrial DNA. QTL analysis identified strong hybrid sterility loci on Chr 17 and Chr X and predicted a set of interchangeable autosomal loci, a subset of which is sufficient to activate the Dobzhansky-Muller incompatibility of the strong loci. Overall, our results indicate the oligogenic nature of F(1) hybrid sterility, which should be amenable to reconstruction by proper combination of chromosome substitution strains. Such a prefabricated model system should help to uncover the gene networks and molecular mechanisms underlying hybrid sterility.