Hybrid sterility is a reproductive isolation barrier between diverging taxa securing the early steps of speciation. Hybrid sterility is ubiquitous in the animal and plant kingdoms, but its genetic control is poorly understood. In our previous studies, we have uncovered the sterility of hybrids between musculus and domesticus subspecies of the house mouse, which is controlled by the Prdm9 gene, the X-linked Hstx2 locus, and subspecific heterozygosity for genetic background. To further investigate this form of genic-driven chromosomal sterility, we constructed a simplified hybrid sterility model within the genome of the domesticus subspecies by swapping domesticus autosomes with their homologous partners from the musculus subspecies. We show that the "sterility" allelic combination of Prdm9 and Hstx2 can be activated by a musculus/domesticus heterozygosity of as few as two autosomes, Chromosome 17 (Chr 17) and Chr 18 and is further enhanced when another heterosubspecific autosomal pair is present, whereas it has no effect on meiotic progression in the pure domesticus genome. In addition, we identify a new X-linked hybrid sterility locus, Hstx3, at the centromeric end of Chr X, which modulates the incompatibility between Prdm9 and Hstx2. These results further support our concept of chromosomal hybrid sterility based on evolutionarily accumulated divergence between homologous sequences. Based on these and previous results, we believe that future studies should include more information on the mutual recognition of homologous chromosomes at or before the first meiotic prophase in interspecific hybrids, as this may serve as a general reproductive isolation checkpoint in mice and other species.

• Keywords
• Mus musculus, chromosome, genomes, hybrid, meiosis, speciation,
• MeSH
• Genome MeSH
• Histone-Lysine N-Methyltransferase * genetics   MeSH
• Hybridization, Genetic * MeSH
• Infertility genetics   MeSH
• Mice MeSH
• Reproductive Isolation MeSH
• Genetic Speciation MeSH
• Animals MeSH
• Check Tag
• Male MeSH
• Mice MeSH
• Female MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Histone-Lysine N-Methyltransferase * MeSH
• prdm9 protein, mouse MeSH Browser

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.

• Keywords
• Hstx2, Mus musculus, Prdm9, t-haplotype, asynapsis, fertility, reproductive isolation,
• MeSH
• Exons MeSH
• Phenotype MeSH
• Histone-Lysine N-Methyltransferase genetics   metabolism   MeSH
• Infertility * genetics   MeSH
• Humans MeSH
• Mice, Inbred C57BL MeSH
• Mice MeSH
• Zinc MeSH
• Animals MeSH
• Check Tag
• Humans MeSH
• Male MeSH
• Mice MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Histone-Lysine N-Methyltransferase MeSH
• PRDM9 protein, human MeSH Browser
• prdm9 protein, mouse MeSH Browser
• Zinc MeSH

Intrinsic postzygotic isolation typically appears as reduced viability or fertility of interspecific hybrids caused by genetic incompatibilities between diverged parental genomes. Dobzhansky-Muller interactions among individual genes, and chromosomal rearrangements causing problems with chromosome synapsis and recombination in meiosis, have both long been considered as major mechanisms behind intrinsic postzygotic isolation. Recent research has, however, suggested that the genetic basis of intrinsic postzygotic isolation can be more complex and involves, for example, overall divergence of the DNA sequence or epigenetic changes. Here, we review the mechanisms of intrinsic postzygotic isolation from genic, chromosomal, genomic, and epigenetic perspectives across diverse taxa. We provide empirical evidence for these mechanisms, discuss their importance in the speciation process, and highlight questions that remain unanswered.

• MeSH
• Chromosomes * MeSH
• Genome MeSH
• Genomics * MeSH
• Hybridization, Genetic MeSH
• Reproductive Isolation MeSH
• Genetic Speciation MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Review MeSH

Hybrid sterility (HS) is an early postzygotic reproductive isolation mechanism observed in all sexually reproducing species. Infertility of hybrids prevents gene flow between incipient species and leads to speciation. While Drosophila studies have focused almost exclusively on the genic control of HS, two other model species, Mus musculus and budding yeast, provided the first experimental evidence of hybrid sterility governed by the nongenic effects of DNA sequence divergence. Here, we propose that the nongenic effect of increasing DNA divergence between closely related species may impair mutual recognition of homologous chromosomes and disrupt their synapsis. Unsynapsed or mispaired homologs can induce early meiotic arrest, or their random segregation can cause aneuploidy of spermatids and sperm cells. Impaired recognition of homologs may thus act as a universal chromosomal checkpoint contributing to the complexity of genetic control of HS. Chromosomal HS controlled by the Prdm9 gene in mice and HS driven by the mismatch repair machinery in yeast are currently the most advanced examples of chromosomal homology search-based HS. More focus on the cellular and molecular phenotypes of meiosis will be needed to further validate the role of homolog recognition in hybrid sterility and speciation.

• Keywords
• Prdm9, antirecombination, chromosomal sterility, meiotic pairing, reproductive isolation, speciation,
• MeSH
• Chromosomes MeSH
• Histone-Lysine N-Methyltransferase genetics   MeSH
• Hybridization, Genetic MeSH
• Infertility * genetics   MeSH
• Humans MeSH
• Meiosis MeSH
• Infertility, Male * genetics   MeSH
• Mice MeSH
• Saccharomyces cerevisiae genetics   MeSH
• Seeds MeSH
• Animals MeSH
• Check Tag
• Humans MeSH
• Male MeSH
• Mice MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Histone-Lysine N-Methyltransferase MeSH
• PRDM9 protein, human MeSH Browser
• prdm9 protein, mouse MeSH Browser

Hybrid sterility contributes to speciation by preventing gene flow between related taxa. Prdm9, the first and only hybrid male sterility gene known in vertebrates, predetermines the sites of recombination between homologous chromosomes and their synapsis in early meiotic prophase. The asymmetric binding of PRDM9 to heterosubspecific homologs of Mus musculus musculus × Mus musculus domesticus F1 hybrids and increase of PRDM9-independent DNA double-strand break hotspots results indificult- to- repair double-strand breaks, incomplete synapsis of homologous chromosomes, and meiotic arrest at the first meiotic prophase. Here, we show that Prdm9 behaves as a major hybrid male sterility gene in mice outside the Mus musculus musculus × Mus musculus domesticus F1 hybrids, in the genomes composed of Mus musculus castaneus and Mus musculus musculus chromosomes segregating on the Mus musculus domesticus background. The Prdm9cst/dom2 (castaneus/domesticus) allelic combination secures meiotic synapsis, testes weight, and sperm count within physiological limits, while the Prdm9msc1/dom2 (musculus/domesticus) males show a range of fertility impairment. Out of 5 quantitative trait loci contributing to the Prdm9msc1/dom2-related infertility, 4 control either meiotic synapsis or fertility phenotypes and 1 controls both, synapsis, and fertility. Whole-genome genotyping of individual chromosomes showed preferential involvement of nonrecombinant musculus chromosomes in asynapsis in accordance with the chromosomal character of hybrid male sterility. Moreover, we show that the overall asynapsis rate can be estimated solely from the genotype of individual males by scoring the effect of nonrecombinant musculus chromosomes. Prdm9-controlled hybrid male sterility represents an example of genetic architecture of hybrid male sterility consisting of genic and chromosomal components.

• Keywords
• HORMAD2, SYCP3, homologous synapsis, meiosis, spermatogenesis, synaptonemal complex,
• MeSH
• Chromosomes MeSH
• Histone-Lysine N-Methyltransferase genetics   metabolism   MeSH
• Meiosis * genetics   MeSH
• Infertility, Male * genetics   MeSH
• Mice MeSH
• Semen metabolism   MeSH
• Animals MeSH
• Check Tag
• Male MeSH
• Mice MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Histone-Lysine N-Methyltransferase MeSH
• prdm9 protein, mouse MeSH Browser

We review knowledge about the roles of sex chromosomes in vertebrate hybridization and speciation, exploring a gradient of divergences with increasing reproductive isolation (speciation continuum). Under early divergence, well-differentiated sex chromosomes in meiotic hybrids may cause Haldane-effects and introgress less easily than autosomes. Undifferentiated sex chromosomes are more susceptible to introgression and form multiple (or new) sex chromosome systems with hardly predictable dominance hierarchies. Under increased divergence, most vertebrates reach complete intrinsic reproductive isolation. Slightly earlier, some hybrids (linked in 'the extended speciation continuum') exhibit aberrant gametogenesis, leading towards female clonality. This facilitates the evolution of various allodiploid and allopolyploid clonal ('asexual') hybrid vertebrates, where 'asexuality' might be a form of intrinsic reproductive isolation. A comprehensive list of 'asexual' hybrid vertebrates shows that they all evolved from parents with divergences that were greater than at the intraspecific level (K2P-distances of greater than 5-22% based on mtDNA). These 'asexual' taxa inherited genetic sex determination by mostly undifferentiated sex chromosomes. Among the few known sex-determining systems in hybrid 'asexuals', female heterogamety (ZW) occurred about twice as often as male heterogamety (XY). We hypothesize that pre-/meiotic aberrations in all-female ZW-hybrids present Haldane-effects promoting their evolution. Understanding the preconditions to produce various clonal or meiotic allopolyploids appears crucial for insights into the evolution of sex, 'asexuality' and polyploidy. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part II)'.

• Keywords
• clonal reproduction, evolution, hybridization, sex chromosomes, speciation,
• MeSH
• Hybridization, Genetic * MeSH
• Meiosis * MeSH
• Vertebrates genetics   MeSH
• Sex Chromosomes genetics   MeSH
• Polyploidy * MeSH
• Genetic Speciation * MeSH
• Animals MeSH
• Check Tag
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Review MeSH

BACKGROUND: Vertebrate meiotic recombination events are concentrated in regions (hotspots) that display open chromatin marks, such as trimethylation of lysines 4 and 36 of histone 3 (H3K4me3 and H3K36me3). Mouse and human PRDM9 proteins catalyze H3K4me3 and H3K36me3 and determine hotspot positions, whereas other vertebrates lacking PRDM9 recombine in regions with chromatin already opened for another function, such as gene promoters. While these other vertebrate species lacking PRDM9 remain fertile, inactivation of the mouse Prdm9 gene, which shifts the hotspots to the functional regions (including promoters), typically causes gross fertility reduction; and the reasons for these species differences are not clear. RESULTS: We introduced Prdm9 deletions into the Rattus norvegicus genome and generated the first rat genome-wide maps of recombination-initiating double-strand break hotspots. Rat strains carrying the same wild-type Prdm9 allele shared 88% hotspots but strains with different Prdm9 alleles only 3%. After Prdm9 deletion, rat hotspots relocated to functional regions, about 40% to positions corresponding to Prdm9-independent mouse hotspots, including promoters. Despite the hotspot relocation and decreased fertility, Prdm9-deficient rats of the SHR/OlaIpcv strain produced healthy offspring. The percentage of normal pachytene spermatocytes in SHR-Prdm9 mutants was almost double than in the PWD male mouse oligospermic sterile mutants. We previously found a correlation between the crossover rate and sperm presence in mouse Prdm9 mutants. The crossover rate of SHR is more similar to sperm-carrying mutant mice, but it did not fully explain the fertility of the SHR mutants. Besides mild meiotic arrests at rat tubular stages IV (mid-pachytene) and XIV (metaphase), we also detected postmeiotic apoptosis of round spermatids. We found delayed meiosis and age-dependent fertility in both sexes of the SHR mutants. CONCLUSIONS: We hypothesize that the relative increased fertility of rat versus mouse Prdm9 mutants could be ascribed to extended duration of meiotic prophase I. While rat PRDM9 shapes meiotic recombination landscapes, it is unnecessary for recombination. We suggest that PRDM9 has additional roles in spermatogenesis and speciation-spermatid development and reproductive age-that may help to explain male-specific hybrid sterility.

• Keywords
• Fertility, Meiotic recombination, PRDM9, Rattus norvegicus,
• MeSH
• Chromatin MeSH
• DNA Breaks, Double-Stranded MeSH
• Fertility genetics   MeSH
• Histone-Lysine N-Methyltransferase genetics   MeSH
• Rats MeSH
• Meiosis * genetics   MeSH
• Mice MeSH
• Rats, Inbred SHR MeSH
• Spermatogenesis genetics   MeSH
• Animals MeSH
• Check Tag
• Rats MeSH
• Male MeSH
• Mice MeSH
• Female MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Research Support, N.I.H., Extramural MeSH
• Names of Substances
• Chromatin MeSH
• Histone-Lysine N-Methyltransferase MeSH
• prdm9 protein, mouse MeSH Browser

During meiosis, the recombination-initiating DNA double-strand breaks (DSBs) are repaired by crossovers or noncrossovers (gene conversions). While crossovers are easily detectable, noncrossover identification is hampered by the small size of their converted tracts and the necessity of sequence polymorphism. We report identification and characterization of a mouse chromosome-wide set of noncrossovers by next-generation sequencing of 10 mouse intersubspecific chromosome substitution strains. Based on 94 identified noncrossovers, we determined the mean length of a conversion tract to be 32 bp. The spatial chromosome-wide distribution of noncrossovers and crossovers significantly differed, although both sets overlapped the known hotspots of PRDM9-directed histone methylation and DNA DSBs, thus supporting their origin in the standard DSB repair pathway. A significant deficit of noncrossovers descending from asymmetric DSBs proved their proposed adverse effect on meiotic recombination and pointed to sister chromatids as an alternative template for their repair. The finding has implications for the molecular mechanism of hybrid sterility in mice from crosses between closely related Mus musculus musculus and Mus musculus domesticus subspecies.

• Keywords
• PRDM9 motif erosion, gene conversion, homologous recombination, hybrid sterility, noncrossover-associated GC bias,
• MeSH
• Chromosomes genetics   MeSH
• DNA Breaks, Double-Stranded MeSH
• Genetic Fitness MeSH
• Gene Conversion * MeSH
• Histone-Lysine N-Methyltransferase genetics   metabolism   MeSH
• Histone Code MeSH
• Hybridization, Genetic * MeSH
• Meiosis * MeSH
• Mice, Inbred C57BL MeSH
• Mice MeSH
• Animals MeSH
• Check Tag
• Mice MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Histone-Lysine N-Methyltransferase MeSH
• prdm9 protein, mouse MeSH Browser

The classical definition posits hybrid sterility as a phenomenon when two parental taxa each of which is fertile produce a hybrid that is sterile. The first hybrid sterility gene in vertebrates, Prdm9, coding for a histone methyltransferase, was identified in crosses between two laboratory mouse strains derived from Mus mus musculus and M. m. domesticus subspecies. The unique function of PRDM9 protein in the initiation of meiotic recombination led to the discovery of the basic molecular mechanism of hybrid sterility in laboratory crosses. However, the role of this protein as a component of reproductive barrier outside the laboratory model remained unclear. Here, we show that the Prdm9 allelic incompatibilities represent the primary cause of reduced fertility in intersubspecific hybrids between M. m. musculus and M. m. domesticus including 16 musculus and domesticus wild-derived strains. Disruption of fertility phenotypes correlated with the rate of failure of synapsis between homologous chromosomes in meiosis I and with early meiotic arrest. All phenotypes were restored to normal when the domesticus Prdm9dom2 allele was substituted with the Prdm9dom2H humanized variant. To conclude, our data show for the first time the male infertility of wild-derived musculus and domesticus subspecies F1 hybrids controlled by Prdm9 as the major hybrid sterility gene. The impairment of fertility surrogates, testes weight and sperm count, correlated with increasing difficulties of meiotic synapsis of homologous chromosomes and with meiotic arrest, which we suppose reflect the increasing asymmetry of PRDM9-dependent DNA double-strand breaks.

• Keywords
• Prdm9 polymorphism, HORMAD2, meiotic chromosome synapsis, reproductive isolation, synaptonemal complex,
• MeSH
• Phylogeography MeSH
• Genetic Introgression * MeSH
• Histone-Lysine N-Methyltransferase genetics   MeSH
• Infertility genetics   MeSH
• Meiosis MeSH
• Mice genetics   MeSH
• Reproductive Isolation * MeSH
• Animals MeSH
• Check Tag
• Male MeSH
• Mice genetics   MeSH
• Female MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Histone-Lysine N-Methyltransferase MeSH
• prdm9 protein, mouse MeSH Browser

Hybrid sterility is a hallmark of speciation, but the underlying molecular mechanisms remain poorly understood. Here, we report that speciation may regularly proceed through a stage at which gene flow is completely interrupted, but hybrid sterility occurs only in male hybrids whereas female hybrids reproduce asexually. We analyzed gametogenic pathways in hybrids between the fish species Cobitis elongatoides and C. taenia, and revealed that male hybrids were sterile owing to extensive asynapsis and crossover reduction among heterospecific chromosomal pairs in their gametes, which was subsequently followed by apoptosis. We found that polyploidization allowed pairing between homologous chromosomes and therefore partially rescued the bivalent formation and crossover rates in triploid hybrid males. However, it was not sufficient to overcome sterility. In contrast, both diploid and triploid hybrid females exhibited premeiotic genome endoreplication, thereby ensuring proper bivalent formation between identical chromosomal copies. This endoreplication ultimately restored female fertility but it simultaneously resulted in the obligate production of clonal gametes, preventing any interspecific gene flow. In conclusion, we demonstrate that the emergence of asexuality can remedy hybrid sterility in a sex-specific manner and contributes to the speciation process.

• Keywords
• Cobitis, clonality, endoreplication, gynogenesis, hybridization, meiosis, polyploidy, speciation,
• MeSH
• Biological Evolution MeSH
• Chromosomes MeSH
• Hybrid Cells cytology   physiology   MeSH
• Infertility genetics   MeSH
• Meiosis * MeSH
• Parthenogenesis * MeSH
• Fishes genetics   physiology   MeSH
• Genetic Speciation * MeSH
• Animals MeSH
• Check Tag
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH