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Hybrid male sterility genes in the mouse subspecific crosses

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Pure species have of course their organs of reproduction in a perfect condition, yet when intercrossed they produce either few or no offspring. Hybrids, on the other hand, have their reproductive organs functionally impotent, as may be clearly seen in the state of the male element in both plants and animals.
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... The same 4.7 Mb locus (X:64,880,641-69,584,093, GRCmm38, see S1 Table in [28]) harbors hybrid sterility genes Hstx1 and Hstx2 [28,44]. Interestingly, the transgressive effect of the Meir1 PWD allele coming from the parent with high recombination rate but acting in the opposite way on B6 background parallels the effect of Hstx2 PWD , which causes small testis size and lack of sperm in (PWD x B6)F1 hybrids [45]. Moreover, Hstx1 PWD causes sperm malformations on the B6 background [44], a phenotype not seen in either parent. ...
... Meir1 maps to the same genomic locus as hybrid sterility gene Hstx2, and both genetic factors operate in a transgressive mode, early at the first meiotic prophase. Hstx2 interacts with Prdm9 to control hybrid sterility of inter-subspecific hybrids in a male-specific manner [28,45]. These facts and the divergent modes of global CO rate regulation in males and females [24,37] led us to investigate the possible activity of Meir1 in female meiosis. ...
... This study compares the global meiotic recombination rate between two closely related subspecies of the house mouse, Mmm represented by the PWD inbred strain and Mmd substituted by the B6 laboratory inbred strain. Previously, both strains served to model inter-subspecific F1 hybrid sterility characterized by disturbed synapsis of homologs, transcriptional dysregulation of the X chromosome and late pachytene arrest [28,45]. The infertility of PWD x B6 F1 hybrid males is under the control of epistatic interaction between the Prdm9 gene on Chr 17 and X-linked Hstx2 genomic locus [39]. ...
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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.
... First, the same interval is associated with multiple correlates of hybrid sterility. Small testes, low sperm count, and morphological abnormalities in sperm observed in F 1 males from crosses between M. m. musculus mothers and M. m. domesticus fathers all map to this genomic location [24,25]. Second, hybrid male sterility due to one of the loci in this X-linked region (Hstx2) involves an incompatibility with Prdm9 [25]. ...
... Small testes, low sperm count, and morphological abnormalities in sperm observed in F 1 males from crosses between M. m. musculus mothers and M. m. domesticus fathers all map to this genomic location [24,25]. Second, hybrid male sterility due to one of the loci in this X-linked region (Hstx2) involves an incompatibility with Prdm9 [25]. Finally, the effects of this piece of the X chromosome on recombination and sterility show similar properties, including male specificity. ...
... (20,21). The former allele was found in inbred strains producing sterile male hybrids when crossed with PWD females, while the Prdm9 Dom3 was observed in the strains that yielded quasi-fertile males in the same type of intersubspecific crosses (22). ...
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The infertility of hybrids between closely related species 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 between mouse strains derived from two mouse subspecies. Within species Prdm9 determines the sites of programmed DNA double-strand breaks 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 rescue male fertility. Twenty-seven or more Mb of consubspecific homology fully restored synapsis in a given autosomal pair and we predicted that two symmetric DSBs or more per chromosome are necessary for successful meiosis. We hypothesize that impaired recombination between evolutionary diverged homologous chromosomes could function as one of the mechanisms of hybrid sterility occurring in various sexually reproducing species.
... The first HS locus in mice, Hst1 was identified as a polymorphic variant on Chr 17 between two laboratory strains, C57BL10/Sn and C6H/Di, both predominantly of Mus musculus domesticus (hereafter Mmd) origin 29 . To identify HS gene on Hst1, Forejt, J. et al. established HS murine recombinant inbred model using PWD/Ph (hereafter PWD) inbred strain derived from Mus musculus musculus (hereafter Mmm) subspecies and C57BL/6 (hereafter B6) inbred strain derived from Mmd subspecies 33,34 . PWD was established from a single pair of wild mice of the Mmm subspecies caught in 1972 in Central Bohemia, Czech Republic 35 . ...
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F1 hybrid progenies between related subspecies often show hybrid sterility (HS) or inviability. HS is caused by failure of meiotic chromosome synapsis and sex body formation in house mouse. Previous studies identified two HS critical genomic regions named Hstx2 on Chr X and Hst1 on Chr 17 by murine forward genetic approaches. HS gene on Hst1 was reported to be Prdm9. Intersubspecific polymorphisms of Prdm9 induce HS in hybrids, and Prdm9 null mutation leads to sterility in the inbred strain. However, HS gene on Hstx2 remains unknown. Here, using knock-out studies, we showed that HS candidate genes on Hstx2 are not individually essential for spermatogenesis in B6 strain. We examined 12 genes on Hstx2: Ctag2, 4930447F04Rik, Mir743, Mir465d, Mir465c-2, Mir465b-1, Mir465c-1, Mir465, Gm1140, Gm14692, 4933436I01Rik, and Gm6812. These genes were expressed in adult testes, and showed intersubspecific polymorphisms on expressed regions. This first reverse genetic approach to identify HS gene on Hstx2 suggested that the loss of function of any one HS candidate gene does not cause complete sterility, unlike Prdm9. Thus, the mechanism(s) of HS by the HS gene on Hstx2 might be different from that of Prdm9.
... Previous studies have also implicated numerous autosomal loci in reproductive isolation in mice (e.g., Forejt and Iványi 1974;Mihola et al. 2009;White et al. 2011;Forejt et al. 2012;Janoušek et al. 2012;Phifer-Rixey et al. 2014). In particular, Mihola et al. (2009) characterized a gene on Chromosome 17, Prdm9, which interacts with the M. m. musculus X Chromosome to drive hybrid sterility; however, sterility in the cross studied here is not associated with known variants of this locus (Good et al. 2008b(Good et al. , 2010. ...
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One approach to understanding the process of speciation is to characterize the genetic architecture of postzygotic isolation. While the majority of work in this area has focused on identifying incompatibilities between protein coding genes, negative epistatic interactions between divergent regulatory elements might also contribute to reproductive isolation. Here we take advantage of a cross between house mouse subspecies, where hybrid dysfunction is largely unidirectional, to test several key predictions about regulatory divergence and reproductive isolation. Regulatory divergence between M. m. musculus and M. m. domesticus was characterized by studying allele-specific expression in fertile hybrid males using mRNA-sequencing of whole testes. We found extensive regulatory divergence between M. m. musculus and M. m. domesticus, largely attributable to cis- regulatory changes. When both cis- and trans- changes occurred, they were observed in opposition much more often than expected under a neutral model, providing strong evidence of widespread compensatory evolution. We also found evidence for lineage-specific positive selection on a subset of genes related to transcriptional regulation. Comparisons of fertile and sterile hybrid males identified a set of genes that were uniquely mis-expressed in sterile individuals. Lastly, we discovered a nonrandom association between these genes and genes showing evidence of compensatory evolution, consistent with the idea that regulatory interactions might contribute to Dobzhansky-Muller incompatibilities and be important in speciation.
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Hybrid sterility is a critical step in the evolution of reproductive barriers between diverging taxa during the process of speciation. Recent studies of young subspecies of the house mouse revealed a multigenic nature and frequent polymorphism of hybrid sterility genes as well as the recurrent engagement of the meiosis-specific gene PR domain-containing 9 (Prdm9) and X-linked loci. Prdm9-controlled hybrid sterility is essentially chromosomal in nature, conditioned by the sequence divergence between subspecies. Depending on the Prdm9 interallelic interactions and the X-linked Hstx2 locus, the same homologs either regularly recombine and synapse, or show impaired DNA DSB repair, asynapsis, and early meiotic arrest. Thus, Prdm9-dependent hybrid sterility points to incompatibilities affecting meiotic recombination as a possible mechanism of reproductive isolation between (sub)species.
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PR domain containing 9 (Prdm9) is a gene specifying hotspots of meiotic recombination but in hybrids between two mouse subspecies Prdm9 controls failure of meiotic chromosome synapsis and hybrid male sterility. We have previously reported that Prdm9-controlled asynapsis and meiotic arrest are conditioned by the inter-subspecific heterozygosity of the hybrid genome and we presumed that the insufficient number of properly repaired PRDM9-dependent DNA double-strand breaks (DSBs) causes asynapsis of chromosomes and meiotic arrest (Gregorova et al. 2018). We now extend the evidence for the lack of properly processed DSBs by improving meiotic chromosome synapsis with exogenous DSBs. A single injection of chemotherapeutic drug cisplatin increased frequency of RPA and DMC1 foci at the zygotene stage of sterile hybrids, enhanced homolog recognition and increased the proportion of spermatocytes with fully synapsed homologs at pachytene. The results bring a new evidence for a DSB-dependent mechanism of synapsis failure and infertility of intersubspecific hybrids.
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