Extracellular vesicles secreted by cumulus cells contain microRNAs that are potential regulatory factors of mouse oocyte developmental competence.

The role of cumulus cells (CCs) in the acquisition of oocyte developmental competence is not yet fully understood. In a previous study, we matured cumulus-denuded fully-grown mouse oocytes to metaphase II (MII) on a feeder layer of CCs (FL-CCs) isolated from developmentally competent (FL-SN-CCs) or incompetent (FL-NSN-CCs) SN (surrounded nucleolus) or NSN (not surrounding nucleolus) oocytes, respectively. We observed that oocytes cultured on the former could develop into blastocysts, while those matured on the latter arrested at the 2-cell stage. To investigate the CC factors contributing to oocyte developmental competence, here we focused on the CCs' release into the medium of extracellular vesicles (EVs) and on their miRNA content. We found that, during the 15-h transition to MII, both FL-SN-CCs and FL-NSN-CCs release EVs that can be detected, by confocal microscopy, inside the zona pellucida (ZP) or the ooplasm. The majority of EVs are <200 nm in size, which is compatible with their ability to cross the ZP. Next-generation sequencing of the miRNome of FL-SN-CC versus FL-NSN-CC EVs highlighted 74 differentially expressed miRNAs, with 43 up- and 31 down-regulated. Although most of these miRNAs do not have known roles in the ovary, in silico functional analysis showed that seven of these miRNAs regulate 71 target genes with specific roles in meiosis resumption (N = 24), follicle growth (N = 23), fertilization (N = 1), and the acquisition of oocyte developmental competence (N = 23). Overall, our results indicate CC EVs as emerging candidates of the CC-to-oocyte communication axis and uncover a group of miRNAs as potential regulatory factors.

MALDI mass spectrometry imaging shows a gradual change in the proteome landscape during mouse ovarian folliculogenesis.

Our knowledge regarding the role proteins play in the mutual relationship among oocytes, surrounding follicle cells, stroma, and the vascular network inside the ovary is still poor and obtaining insights into this context would significantly aid our understanding of folliculogenesis. Here, we describe a spatial proteomics approach to characterize the proteome of individual follicles at different growth stages in a whole prepubertal 25-day-old mouse ovary. A total of 401 proteins were identified by nano-scale liquid chromatography-electrospray ionization-tandem mass spectrometry (nLC-ESI-MS/MS), 69 with a known function in ovary biology, as demonstrated by earlier proteomics studies. Enrichment analysis highlighted significant KEGG and Reactome pathways, with apoptosis, developmental biology, PI3K-Akt, epigenetic regulation of gene expression, and extracellular matrix organization being well represented. Then, correlating these data with the spatial information provided by matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) on 276 follicles enabled the protein profiles of single follicle types to be mapped within their native context, highlighting 94 proteins that were detected throughout the secondary to the pre-ovulatory transition. Statistical analyses identified a group of 37 proteins that showed a gradual quantitative change during follicle differentiation, comprising 10 with a known role in follicle growth (NUMA1, TPM2), oocyte germinal vesicle-to-metaphase II transition (SFPQ, ACTBL, MARCS, NUCL), ovulation (GELS, CO1A2), and preimplantation development (TIF1B, KHDC3). The proteome landscape identified includes molecules of known function in the ovary, but also those whose specific role is emerging. Altogether, this work demonstrates the utility of performing spatial proteomics in the context of the ovary and offers sound bases for more in-depth investigations that aim to further unravel its spatial proteome.

Maternal effect factors that contribute to oocytes developmental competence: an update.

Oocyte developmental competence is defined as the capacity of the female gamete to be fertilized and sustain development to the blastocyst stage. Epigenetic reprogramming, a correct cell division pattern, and an efficient DNA damage response are all critical events that, before embryonic genome activation, are governed by maternally inherited factors such as maternal-effect gene (MEG) products. Although these molecules are stored inside the oocyte until ovulation and exert their main role during fertilization and preimplantation development, some of them are already functioning during folliculogenesis and oocyte meiosis resumption. This mini review summarizes the crucial roles played by MEGs during oocyte maturation, fertilization, and preimplantation development with a direct/indirect effect on the acquisition or maintenance of oocyte competence. Our aim is to inspire future research on a topic with potential clinical perspectives for the prediction and treatment of female infertility.

Three-dimensional imaging and reconstruction of the whole ovary and testis: a new frontier for the reproductive scientist.

The 3D functional reconstruction of a whole organ or organism down to the single cell level and to the subcellular components and molecules is a major future scientific challenge. The recent convergence of advanced imaging techniques with an impressively increased computing power allowed early attempts to translate and combine 2D images and functional data to obtain in-silico organ 3D models. This review first describes the experimental pipeline required for organ 3D reconstruction: from the collection of 2D serial images obtained with light, confocal, light-sheet microscopy or tomography, followed by their registration, segmentation and subsequent 3D rendering. Then, we summarise the results of investigations performed so far by applying these 3D image analyses to the study of the female and male mammalian gonads. These studies highlight the importance of working towards a 3D in-silico model of the ovary and testis as a tool to gain insights into their biology during the phases of differentiation or adulthood, in normal or pathological conditions. Furthermore, the use of 3D imaging approaches opens to key technical improvements, ranging from image acquisition to optimisation and development of new processing tools, and unfolds novel possibilities for multidisciplinary research.

Cytoplasmic movements of the early human embryo: imaging and artificial intelligence to predict blastocyst development.

RESEARCH QUESTION: Can artificial intelligence and advanced image analysis extract and harness novel information derived from cytoplasmic movements of the early human embryo to predict development to blastocyst? DESIGN: In a proof-of-principle study, 230 human preimplantation embryos were retrospectively assessed using an artificial neural network. After intracytoplasmic sperm injection, embryos underwent time-lapse monitoring for 44 h. For comparison, standard embryo assessment of each embryo by a single embryologist was carried out to predict development to blastocyst stage based on a single picture frame taken at 42 h of development. In the experimental approach, in embryos that developed to blastocyst or destined to arrest, cytoplasm movement velocity was recorded by time-lapse monitoring during the first 44 h of culture and analysed with a Particle Image Velocimetry algorithm to extract quantitative information. Three main artificial intelligence approaches, the k-Nearest Neighbour, the Long-Short Term Memory Neural Network and the hybrid ensemble classifier were used to classify the embryos. RESULTS: Blind operator assessment classified each embryo in terms of ability to develop to blastocyst, with 75.4% accuracy, 76.5% sensitivity, 74.3% specificity, 74.3% precision and 75.4% F1 score. Integration of results from artificial intelligence models with the blind operator classification, resulted in 82.6% accuracy, 79.4% sensitivity, 85.7% specificity, 84.4% precision and 81.8% F1 score. CONCLUSIONS: The present study suggests the possibility of predicting human blastocyst development at early cleavage stages by detection of cytoplasm movement velocity and artificial intelligence analysis. This indicates the importance of the dynamics of the cytoplasm as a novel and valuable source of data to assess embryo viability.

Inhibition of lysyl oxidase stimulates TGF-ß signaling and metalloproteinases-2 and -9 expression and contributes to the disruption of ascending aorta in rats: protection by propylthiouracil.

Mutations in lysyl oxidase (LOX) genes cause severe vascular anomalies in mice and humans. LOX activity can be irreversibly inhibited by the administration of ß-aminoproprionitrile (BAPN). We investigated the mechanisms underlying the damage to the ascending thoracic aorta induced by LOX deficiency and evaluated whether 6-propylthiouracil (PTU) can afford protection in rats. BAPN administration caused disruption of the ascending aortic wall, increased the number of apoptotic cells, stimulated TGF-ß signaling (increase of nuclear p-SMAD2 staining), and up-regulated the expression of metalloproteinases-2 and -9. In BAPN-treated animals, PTU reduced apoptosis, p-SMAD2 staining, MMP-2, and -9 expression, and markedly decreased the damage to the aortic wall. Our results suggest that, as in some heritable vascular diseases, enhanced TGF-ß signaling and upregulation of MMP-2 and -9 can contribute to the pathogenesis of ascending aorta damage caused by LOX deficiency. We have also shown that PTU, a drug already in clinical use, protects against the effects of LOX inhibition. MMP-2 and -9 might be potential targets of new therapeutic strategies for the treatment of vascular diseases caused by LOX deficiency.

X-Chromosome Inactivation during Preimplantation Development and in Pluripotent Stem Cells.

X dosage compensation between XX female and XY male mammalian cells is achieved by a process known as X-chromosome inactivation (XCI). XCI initiates early during preimplantation development in female cells, and it is subsequently stably maintained in somatic cells. However, XCI is a reversible process that occurs in vivo in the inner cell mass of the blastocyst, in primordial germ cells or in spermatids during reprogramming. Erasure of transcriptional gene silencing can occur though a mechanism named X-chromosome reactivation (XCR). XCI and XCR have been substantially deciphered in the mouse, whereas they still remain debated in the human. In this review, we summarized the recent advances in the knowledge of X-linked gene dosage compensation during mouse and human preimplantation development and in pluripotent stem cells.

Germ cell cysts, a fetal feature in mammals, are constitutively present in the adult armadillo.

Formation and subsequent break down of ovarian germ cell (GC) cysts is a key and an evolutionary-conserved developmental event, described in phylogenetically diverse species of invertebrates and vertebrates. In mammals, cyst break down (CBD) ends at the time of, or soon after, birth with the formation of primordial follicles enclosing single oocytes, which constitute the sole reservoir of gametes available through the whole female's reproductive life. In this study, we challenge this paradigm demonstrating the constitutive presence of a large number of cysts, enclosing two-thirty GCs, in the ovary of the adult armadillo Chaetophractus villosus, belonging to the superorder Xenarthra, one of the earliest offshoots among placentals. We also describe that (a) GCs enclosed within cysts are connected by intercellular bridges-intercellular bridges-markers of their clonal origin; (b) CBD occurs through four main phases, ending with primordial follicles containing single oocytes; (c) GCs encompass meiotic prophase I stages, from leptotene to diplotene; (d) seasonal variations in the number of GCs enclosed within cysts, suggesting the presence of a GC multiplying activity. The armadillo C. villosus''s ovary emerges as an extraordinary resource to investigate folliculogenesis and to explore the evolutionary past of the mammalian ovary.

Sertoli-immature spermatids disengagement during testis regression in the armadillo.

In nature, mammalian seasonal breeders undergo spermatogenetic arrest during the non-breeding season. In the large hairy armadillo Chaetophractus villosus, testis regression initiates with immature post-meiotic germ cells sloughing into the tubule lumen and continues with the death of the remaining spermatocytes. At the end of the regression period, only spermatogonia and Sertoli cells persist in the seminiferous epithelium. It has been suggested that cell sloughing is determined by changes in the adhesion complexes between Sertoli cells and spermatids, which are mediated by low intra-testicular testosterone levels. By immunofluorescence and Western blotting we studied key proteins of the N-cadherin/N-cadherin and A6B1-integrin/laminin interlocks that contribute to the complex Sertoli/spermatid adhesion system throughout the eight stages of the seminiferous epithelium cycle in the comparison between active and regressing testes. In active testis, B1-integrin, laminin G3, N-cadherin, B-catenin, P-B-catenin-Tyr654, FAK, P-FAK-Tyr397, SRC, P-SRC-Tyr416 proteins present a spermatogenetic cycle-dependent localisation pattern, unmaintained in regressing testes. In the latter, quantitative variations and changes in the phosphorylation state of protein FAK, SRC and B-catenin contribute to the disassembly of the N-cadherin/N-cadherin and A6B1-integrin/laminin interlocks, thus promoting the massive release of immature spermatids.

IVM of mouse fully grown germinal vesicle oocytes upon a feeder layer of selected cumulus cells enhances their developmental competence.

In the ovary, acquisition of oocyte developmental competence depends on a bidirectional exchange between the gamete and its companion cumulus cells (CCs). In this study we investigated the contribution of CCs surrounding oocytes of known developmental competence or incompetence to the acquisition of oocyte developmental competence. To this end, feeder layers of CCs (FL-CCs) were prepared using CCs isolated either from: (1) developmentally competent mouse oocytes whose nucleolus was surrounded by a chromatin ring (FL-SN-CCs); or (2) developmentally incompetent mouse oocytes whose nucleolus was not surrounded by a chromatin ring (FL-NSN-CCs). Denuded, fully grown oocytes (DOs) were matured to the MII stage on either FL-SN-CCs or FL-NSN-CCs, inseminated with spermatozoa and cultured throughout preimplantation development. FL-SN-CCs significantly improved the acquisition of oocyte developmental competence, with a blastocyst development rate equal to that for maturation of intact cumulus-oocyte-complexes. In contrast, DOs matured on FL-NSN-CCs or in the absence of CCs exhibited developmental failure, with embryos arresting at either the 4-cell or morula stage. These results set a culture platform to further improve the protocols for the maturation of DOs and to unravel the molecules involved in the cross-talk between the gamete and its companion CCs during the germinal vesicle to MII transition.

Cytoplasmic movement profiles of mouse surrounding nucleolus and not-surrounding nucleolus antral oocytes during meiotic resumption.

Full-grown mouse antral oocytes are classified as surrounding nucleolus (SN) or not-surrounding nucleolus (NSN), depending on the respective presence or absence of a ring of Hoechst-positive chromatin surrounding the nucleolus. In culture, both types of oocytes resume meiosis and reach the metaphase II (MII) stage, but following insemination, NSN oocytes arrest at the two-cell stage whereas SN oocytes may develop to term. By coupling time-lapse bright-field microscopy with image analysis based on particle image velocimetry, we provide the first systematic measure of the changes to the cytoplasmic movement velocity (CMV) occurring during the germinal vesicle-to-MII (GV-to-MII) transition of these two types of oocytes. Compared to SN oocytes, NSN oocytes display a delayed GV-to-MII transition, which can be mostly explained by retarded germinal vesicle break down and first polar body extrusion. SN and NSN oocytes also exhibit significantly different CMV profiles at four main time-lapse intervals, although this difference was not predictive of SN or NSN oocyte origin because of the high variability in CMV. When CMV profile was analyzed through a trained artificial neural network, however, each single SN or NSN oocyte was blindly identified with a probability of 92.2% and 88.7%, respectively. Thus, the CMV profile recorded during meiotic resumption may be exploited as a cytological signature for the non-invasive assessment of the oocyte developmental potential, and could be informative for the analysis of the GV-to-MII transition of oocytes of other species.

A Half-Century of Studies on a Chromosomal Hybrid Zone of the House Mouse.

The first natural chromosomal variation in the house mouse was described nearly 50 years ago in Val Poschiavo on the Swiss side of the Swiss-Italian border in the Central Eastern Alps. Studies have extended into neighboring Valtellina, and the house mice of the Poschiavo-Valtellina area have been subject to detailed analysis, reviewed here. The maximum extent of this area is 70 km, yet it has 4 metacentric races and the standard 40-chromosome telocentric race distributed in a patchwork fashion. The metacentric races are characterized by highly reduced diploid numbers (2n = 22-26) resulting from Robertsonian fusions, perhaps modified by whole-arm reciprocal translocations. The races hybridize and the whole Poschiavo-Valtellina area can be considered a "hybrid zone." The studies of this area have provided insights into origin of races within hybrid zones, gene flow within hybrid zones and the possibility of speciation in hybrid zones. This provides a case study of how chromosomal rearrangements may impact the genetic structure of populations and their diversification.

Achilles' heel of pluripotent stem cells: genetic, genomic and epigenetic variations during prolonged culture.

Pluripotent stem cells differentiate into almost any specialized adult cell type of an organism. PSCs can be derived either from the inner cell mass of a blastocyst-giving rise to embryonic stem cells-or after reprogramming of somatic terminally differentiated cells to obtain ES-like cells, named induced pluripotent stem cells. The potential use of these cells in the clinic, for investigating in vitro early embryonic development or for screening the effects of new drugs or xenobiotics, depends on capability to maintain their genome integrity during prolonged culture and differentiation. Both human and mouse PSCs are prone to genomic and (epi)genetic instability during in vitro culture, a feature that seriously limits their real potential use. Culture-induced variations of specific chromosomes or genes, are almost all unpredictable and, as a whole, differ among independent cell lines. They may arise at different culture passages, suggesting the absence of a safe passage number maintaining genome integrity and rendering the control of genomic stability mandatory since the very early culture passages. The present review highlights the urgency for further studies on the mechanisms involved in determining (epi)genetic and chromosome instability, exploiting the knowledge acquired earlier on other cell types.

Impact of the number of Robertsonian chromosomes on germ cell death in wild male house mice.

Previous studies in the house mouse have shown that the presence of Robertsonian (Rb) metacentric chromosomes in heterozygous condition affects the process of spermatogenesis. This detrimental effect mainly depends on the number of metacentrics involved and the complexity of the resulting meiotic figures. In this study, we aimed at elucidating the relationship between the chromosomal composition and spermatogenesis impairment in mice present in an area of chromosomal polymorphism (the so-called Barcelona system BRbS) in which Rb mice are surrounded by all acrocentric animals, no established metacentric races are present and the level of structural heterozygosity is relatively low. Using the terminal deoxinucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) assay, we report higher frequency of apoptotic spermatogenetic cells in mice carrying six pairs of metacentrics at the homozygous state than in those carrying two or three fusions at the heterozygous state. Specifically, we detected a higher frequency of TUNEL-positive (T+) tubules and of T+ cells per tubule cross section and also a lower spermatid/spermatocyte ratio. These results indicate that the number of metacentrics at the homozygous state is more influential in determining apoptotic germ cell death than that of moderate chromosome heterozygosity. The percentage of germ cell death lower than 50 % found in our samples and the geographic distribution of the set of metacentrics within the BRbS indicate that although the spermatogenic alterations detected in this area could act as a partial barrier to gene flow, they are not sufficient to prevent Rb chromosomes from spreading in nature.

Formulation and stability evaluation of 3D alginate beads potentially useful for cumulus-oocyte complexes culture.

Ovarian follicle encapsulation in synthetic or natural matrixes based on biopolymers is potentially a promising approach to in vitro maturation (IVM) process, since it maintains follicle 3D organisation by preventing its flattening and consequent disruption of gap junctions, preserving the functional relationship between oocyte and companion follicle cells. The aim of the work was to optimise physico-chemical parameters of alginate microcapsules for perspective IVM under 3D environments. On this purpose alginate and cross-linking agent concentrations were investigated. Alginate concentration between 0.75% and 0.125% w/w and Mg(2+), Ba(2+), Ca(2+ )at concentration between 100 and 20 mM were tested. Follicle encapsulation was obtained by on purpose modified diffusion setting gelation technique, and evaluated together with beads, chemical and mechanical stability in standard and stressing conditions. Beads permeability was tested towards albumin, fetuin, pyruvate, glucose, pullulan. Results demonstrated that 0.25% alginate cross-linked in 100 mM CaCl2 beads is suitable to follicle encapsulation.

The Robertsonian phenomenon in the house mouse: mutation, meiosis and speciation.

Many different chromosomal races with reduced chromosome number due to the presence of Robertsonian fusion metacentrics have been described in western Europe and northern Africa, within the distribution area of the western house mouse Mus musculus domesticus. This subspecies of house mouse has become the ideal model for studies to elucidate the processes of chromosome mutation and fixation that lead to the formation of chromosomal races and for studies on the impact of chromosome heterozygosities on reproductive isolation and speciation. In this review, we briefly describe the history of the discovery of the first and subsequent metacentric races in house mice; then, we focus on the molecular composition of the centromeric regions involved in chromosome fusion to examine the molecular characteristics that may explain the great variability of the karyotype that house mice show. The influence that metacentrics exert on the nuclear architecture of the male meiocytes and the consequences on meiotic progression are described to illustrate the impact that chromosomal heterozygosities exert on fertility of house mice-of relevance to reproductive isolation and speciation. The evolutionary significance of the Robertsonian phenomenon in the house mouse is discussed in the final section of this review.

Chromosomal Abnormalities in Embryonic and Somatic Stem Cells.

The potential use of stem cells (SCs) for tissue engineering, regenerative medicine, disease modeling, toxicological studies, drug delivery, and as in vitro model for the study of basic developmental processes implies large-scale in vitro culture. Here, after a brief description of the main techniques used for karyotype analysis, we will give a detailed overview of the chromosome abnormalities described in pluripotent (embryonic and induced pluripotent SCs) and somatic SCs, and the possible causes of their origin during culture.

FSH and LH receptors are differentially expressed in cumulus cells surrounding developmentally competent and incompetent mouse fully grown antral oocytes.

Cumulus cells (CCs) maintain strict functional relationships with the enclosed antral oocyte and are thought to reflect its developmental competence. Several studies have described a correlation between CC gene expression and oocyte quality. Herein, we tested whether CC-specific FSH and LH receptors (FSHR and LHR, respectively) are differentially expressed in CCs enclosing developmentally competent or incompetent oocytes. To this end, mouse fully grown cumulus-oocyte complexes were isolated and their CCs and oocytes analysed separately. Based on their chromatin organisation, oocytes were classified as those with a surrounded nucleolus (SN) or a non-surrounded nucleolus (NSN), the former being developmentally competent, whereas the latter arrest at the 2-cell stage. The CCs were then analysed to compare the pattern of expression of the Fshr and Lhr genes and their proteins. Quantitative reverse transcription-polymerase chain reaction analysis revealed that only Lhr is significantly differentially expressed. Immunofluorescence analysis revealed that both FSHR and LHR proteins are significantly upregulated in CCs surrounding oocytes arrested at the 2-cell stage, reflecting their developmental incompetence.

Loss of Sertoli-germ cell adhesion determines the rapid germ cell elimination during the seasonal regression of the seminiferous epithelium of the large hairy armadillo Chaetophractus villosus.

The armadillo Chaetophractus villosus is a seasonal breeder whose seminiferous epithelium undergoes rapid regression with massive germ cell loss, leaving the tubules with only Sertoli cells and spermatogonia. Here, we addressed the question of whether this regression entails 1) the disassembly of cell junctions (immunolocalization of nectin-3, Cadm1, N-cadherin, and beta-catenin, and transmission electron microscopy [TEM]); 2) apoptosis (immunolocalization of cytochrome c and caspase 3; TUNEL assay); and 3) the involvement of Sertoli cells in germ cell phagocytosis (TEM). We showed a dramatic reduction in the extension of vimentin filaments associated with desmosomelike junctions at the interface between Sertoli and germ cells, and an increased diffusion of the immunosignals of nectin-3, Cadm1, N-cadherin, and beta-catenin. Together, these results suggest loss of Sertoli-germ cell adhesion, which in turn might determine postmeiotic cell sloughing at the beginning of epithelium regression. Then, loss of Sertoli-germ cell adhesion triggers cell death. Cytochrome c is released from mitochondria, but although postmeiotic cells were negative for late apoptotic markers, at advanced regression spermatocytes were positive for all apoptotic markers. Transmission electron microscopy analysis showed cytoplasmic engulfment of cell debris and lipid droplets within Sertoli cells, a sign of their phagocytic activity, which contributes to the elimination of the residual meiocytes still present in the latest regression phases. These findings are novel and add new players to the mechanisms of seminiferous epithelium regression occurring in seasonal breeders, and they introduce the armadillo as an interesting model for studying seasonal spermatogenesis.

Chromosomal speciation in mice: a cytogenetic analysis of recombination.

Within species, populations differing by chromosomal rearrangements ("chromosomal races") may become reproductively isolated in association with reduced hybrid fertility due to meiotic aberrations. Speciation is also possible if hybridizing chromosomal races accumulate genetic differences because of reduced meiotic recombination in the heterozygous configuration in hybrids. Here, we examine recombination in pure races and hybrids within a model system for chromosomal speciation: the hybridization of the Poschiavo (CHPO) and Upper Valtellina (IUVA) chromosomal races of house mouse in Upper Valtellina, Italy. These races differ by Robertsonian fusions/whole-arm reciprocal translocations, such that hybrids produce a pentavalent meiotic configuration. We determined the number and position of the recombination points (using an antibody against the MutL homolog 1 [MLH1] protein) on synaptonemal complexes at pachytene in laboratory-reared CHPO, IUVA, and hybrid males, analyzing at least 112 spermatocytes per karyotypic group, up to a total of 534 spermatocytes. The mean ± standard deviation numbers of MLH1 foci per spermatocyte were 22.2 ± 3.2, 20.1 ± 2.9, 20.7 ± 2.3, and 21.9 ± 2.9 for CHPO, IUVA, CHPO × IUVA, and IUVA × CHPO, respectively. Altogether, 10,146 chromosome arms were examined, allowing multiple comparisons. Overall, recombination events were more frequently distal than proximal or interstitial. The average number of proximal MLH1 foci per chromosome arm decreased going from telocentric to metacentric bivalents to pentavalents (when present), which (together with other factors) influenced the average number of MLH1 foci per cell between CHPO, IUVA, and hybrid mice. The low frequency of proximal recombination in pentavalents of CHPO-IUVA hybrids may promote reproductive isolation between the CHPO and IUVA races, when coupled with reduced hybrid unfitness.