Laboratory of Mouse Molecular Genetics is interested in genetics and genomics of meiotic chromosome pairing in hybrids between closely related mouse subspecies. We identified mouse Hybrid sterility 1, Hst1 with the Prdm9 gene encoding a histone H3K4me3 and H3K36me3 methyltransferase as the first hybrid sterility gene in vertebrates (Forejt and Ivanyi, 1974, Mihola et al. 2009).

Prdm9 controls hybrid sterility and meiotic recombination
Hybrid sterility is studied in inter-species hybrids of C57BL6/J and PWD/Ph inbred strains as model representatives of Mus m. domesticus and Mus m. musculus subspecies. Intra-meiotic arrest and full male sterility of (PWD x B6)F1 hybrid males is controlled by interaction between Prdm9 gene and a 2,7 Mb locus (Hstx2) on Chromosome X (Dzur-Gejdosova et al. 2012, Bhattacharyya et al. 2014). Meiotic arrest in sterile hybrids is associated with a failure to properly synapse homologous chromosomes coming from different mouse subspecies and with failure of male sex chromosome inactivation (MSCI). More than 90% of early pachytene spermatocytes show one or more pairs of autosomes with synapsis problems, no sperm is produced. The asynapsis and consequent sterility is controlled in cis and in trans. In cis, the synapsis depends on genomic sequence diversity between homologs. Particular role in cis control is proposed to the sequence diversity between the Prdm9 binding sites, due to recombination hotspot paradox (Davies et al. 2016, Forejt 2016). The overall frequency of pachytene spermatocytes with synapsis defects is influenced in trans by interaction of Prdm9 with Hstx2 (Bhattacharyya et al. 2013, 2014, Flachs et al., 2012, 2014). Prdm9 in intraspecific crosses decides about positioning the recombination hotspots in the genome.

X-linked Hybrid sterility locus Hstx2 resides in a recombination cold-spot and controls genome-wide meiotic recombination rate
Recently we have found a dual role also in case of the X-linked hybrid Hstx2 locus (Lustyk et al. 2019). Besides interacting with Prdm9 to control hybrid sterility it carries a major genetic factor, designated Meir1, which regulates the genome-wide rate of meiotic recombination (Balcova et al. 2016). The relation between Hstx2 and Meir1 will be clarified after identification of their causative genes.

Decreased genomic divergence overrides hybrid sterility in mouse subspecific hybrids
Using chromosome substitution strans we inserted random stretches of consubspecific homology on several autosomal pairs in (PWD x B6)F1 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 hypothesized that impaired recombination between evolutionarily diverged genomes could function as one of the mechanisms of hybrid sterility occurring in various sexually reproducing species (Gregorova et al. 2018, Wang et al. 2018).

Meiotic noncrossovers identified using chromosome substitution strains
We identified and characterized ten mouse chromosome-wide sets of noncrossovers. Based on 94 identified noncrossovers we determined the mean length of a conversion tract to 32 bp. The chromosome-wide distribution of noncrossovers and crossovers significantly differed, though both sets overlapped the known hotspots of PRDM9-directed histone methylation and DNA DSBs, thus proving 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 (Gergelits et al. 2019).

Prdm9 controls hybrid sterility of mice from from different natural populations
We showed, for the first time, that the Prdm9 allelic incompatibilities represent the primary cause of reduced fertility in intersubspecific hybrids between M. m. musculus and M. m. domesticus based on 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 Prdm9^dom2 allele was substituted with the Prdm9^dom2H humanized variant. To conclude, our data show the Prdm9 as the major hybrid sterility gene controlling male infertility of wild-derived musculus and domesticus subspecies F1 hybrids by (Mukaj et al. 2020).