Although organisms belonging to different species and subspecies sometimes produce fertile offspring, a hallmark of the speciation process is reproductive isolation, characterized by hybrid sterility (HS) due to failure in gametogenesis. In mammals, HS is usually exhibited by males, the heterogametic sex. The phenotypic manifestations of HS are complex. The most frequently observed are abnormalities in both autosomal and sex chromosome interactions that are linked to meiotic prophase arrest or postmeiotic spermiogenesis aberrations and lead to defective or absent gametes. The aim of this study was to determine the HS phenotypes in intersubspecific F 1 mice produced by matings between Mus musculus molossinus-derived strains and diverse Mus musculus domesticus-inbred laboratory mouse strains. Most of these crosses produced fertile F 1 offspring. However, when female BALB/cJ (domesticus) mice were mated to male JF1/MsJ (molossinus) mice, the (BALB dom xJF1 mol )F 1 males were sterile, whereas the (JF1 mol xBALB dom )F 1 males produced by the reciprocal crossings were fertile; thus the sterility phenotype was asymmetric. The sterile (BALB dom xJF1 mol ) F 1 males exhibited a high rate of meiotic metaphase arrest with misaligned chromosomes, probably related to a high frequency of XY dissociation. Intriguingly, in the sterile (BALB dom xJF1 mol )F 1 males we observed aberrant allelespecific expression of several meiotic genes, that play critical roles in important meiotic events including chromosome pairing. Together, these observations of an asymmetrical HS phenotype in intersubspecific F 1 males, probably owing to meiotic defects in the meiotic behavior of the XY chromosomes pair and possibly also transcriptional misregulation of meiotic genes, provide new models and directions for understanding speciation mechanisms in mammals.

An unbiased screen for discovering novel genes for fertility identified thespcar3, spermatocyte arrest 3,mutant phenotype. Thespcar3mutation identified a new allele of theSetxgene, encoding senataxin, a DNA/RNA helicase that regulates transcription termination by resolving DNA/RNA hybrid R-loop structures. Although mutations in the humanSETXgene cause neural disorders,Setxspcar3mutant mice do not show any apparent neural phenotype, but instead exhibit male infertility and female subfertility. Histology of theSetxspcar3mutant testes revealed absence of spermatids and mature spermatozoa in the seminiferous tubules. Cytological analysis of chromosome spread preparations of theSetxspcar3mutant spermatocytes revealed normal synapsis, but aberrant DNA damage in the autosomes, and defective formation of the sex body. Furthermore,Setxspcar3testicular cells exhibited abnormal accumulation of R-loops compared to wild type testicular cells. Transient expression assays identified regions of the senataxin protein required for sub-nuclear localization. Together, these results not only confirm that senataxin is required for normal meiosis and spermatogenesis but also provide a new resource for determination of its role in maintaining R-loop formation and genome integrity.