Speciation genes restrict gene flow between the incipient species and related taxa. Three decades ago, we mapped a mammalian speciation gene, hybrid sterility 1 ( Hst1 ), in the intersubspecific hybrids of house mouse. Here, we identify this gene as Prdm9 , encoding a histone H3 lysine 4 trimethyltransferase. We rescued infertility in male hybrids with bacterial artificial chromosomes carrying Prdm9 from a strain with the “fertility” Hst1 ^f allele. Sterile hybrids display down-regulated microrchidia 2B ( Morc2b ) and fail to compartmentalize γH2AX into the pachynema sex (XY) body. These defects, seen also in Prdm9 -null mutants, are rescued by the Prdm9 transgene. Identification of a vertebrate hybrid sterility gene reveals a role for epigenetics in speciation and opens a window to a hybrid sterility gene network.

SUMMARY: The genetic control of the sterility of male hybrids between certain laboratory and wild mice (Mus musculusL.) is investigated. The observed sterility is, by definition, hybrid sterility since both parental forms (i.e. wild and laboratory mice) are fully fertile, their male offspring displaying small testes with arrest of spermatogenesis at the stage of spermatogenesis or primary spermatocytes. Results of genetic analysis as well as the failure to detect any chromosomal rearrangements point to a genie rather than a chromosomal type of hybrid sterility.Fifty-three wild males were classified into three sets, after mating with C57BL/10 inbred females, according to the fertility of their male progeny (set I – only sterile sons; set II – only fertile sons; set III – both fertile and sterile sons). The wild males of set I, which yield only sterile male offspring with C57BL/10 females, sire sterile sons also with females of the following inbred strains: A/Ph, BALB/c, DBA/1, and AKR/J, whereas the same wild males produce fertile offspring with females of C3H/Di, CBA/J, P/J, I/St and F/St inbred strains.The described hybrid sterility seems to be under the control of several independently segregating genes, one of them (proposed symbol Hst-1) being localized on chromosome 17 (linkage group IX), 6 cM distally from dominantT(Brachyury). A chance to search for the mechanism of hybrid sterility is provided by the finding of two laboratory inbred strains, C57BL/10 and C3H/Di, differing with respect to theHybrid sterilitygenetic system only at theHst-1 gene.Hst-1 is closely linked but apparently not identical with the sterility factor of recessive t alleles of theTlocus.

Significance: Hybrid sterility contributes to speciation by restricting gene flow between related taxa. Although four hybrid sterility genes have been identified in Drosophila and mouse so far, the underlying molecular mechanisms are largely unknown. We describe extensive asynapsis of chromosomes in male and female meiosis of F1 hybrids between two closely related mouse subspecies. Using the intersubspecific chromosome-substitution strains, we demonstrate that the heterospecific pairing of homologous chromosomes is a preexisting condition of asynapsis and may represent a universal mechanism of pachytene arrest in interspecific hybrids. Sex-specific manifestation of asynapsis can explain the mechanism of Haldane’s rule.

The European Mouse Mutagenesis Consortium is the European initiative contributing to the international effort on functional annotation of the mouse genome. Its objectives are to establish and integrate mutagenesis platforms, gene expression resources, phenotyping units, storage and distribution centers and bioinformatics resources. The combined efforts will accelerate our understanding of gene function and of human health and disease.