X chromosome effect on maternal recombination and meiotic drive in the mouse

The RING Domain of Rice HEI10 is Essential for Male, But Not Female Fertility

HEI10 is a conserved E3 ubiquitin ligase involved in crossover formation during meiosis, and is thus essential for both male and female gamete development. Here, we have discovered a novel allele of HEI10 in rice that produces a truncated HEI10 protein missing its N-terminal RING domain, namely sh1 (shorter hei10 1). Unlike previously reported hei10 null alleles that are completely sterile, sh1 exhibits complete male sterility but retains partial female fertility. The causative sh1 mutation is a 76 kb inversion between OsFYVE4 and HEI10, which breaks the integrity of both genes. Allelic tests and complementation assays revealed that the gamete developmental defects of sh1 were caused by disruption of HEI10. Further studies demonstrated that short HEI10 can correctly localise to the nucleus, where it could interact with other proteins that direct meiosis; expressing short HEI10 in hei10 null lines partially restores female fertility. Our data reveal an intriguing mutant allele of HEI10 with differential effects on male and female fertility, providing a new tool to explore similarities and differences between male and female meiosis.

A long noncoding RNA influences the choice of the X chromosome to be inactivated

X chromosome inactivation (XCI) is the process of silencing one of the X chromosomes in cells of the female mammal which ensures dosage compensation between the sexes. Although theoretically random in somatic tissues, the choice of which X chromosome is chosen to be inactivated can be biased in mice by genetic element(s) associated with the so-called X-controlling element (Xce). Although the Xce was first described and genetically localized nearly 40 y ago, its mode of action remains elusive. In the approach presented here, we identify a single long noncoding RNA (lncRNA) within the Xce locus, Lppnx, which may be the driving factor in the choice of which X chromosome will be inactivated in the developing female mouse embryo. Comparing weak and strong Xce alleles we show that Lppnx modulates the expression of Xist lncRNA, one of the key factors in XCI, by controlling the occupancy of pluripotency factors at Intron1 of Xist. This effect is counteracted by enhanced binding of Rex1 in DxPas34, another key element in XCI regulating the activity of Tsix lncRNA, the main antagonist of Xist, in the strong but not in the weak Xce allele. These results suggest that the different susceptibility for XCI observed in weak and strong Xce alleles results from differential transcription factor binding of Xist Intron 1 and DxPas34, and that Lppnx represents a decisive factor in explaining the action of the Xce.

Diet effects on mouse meiotic recombination: a warning for recombination studies

Meiotic recombination is a critical process for sexually reproducing organisms. This exchange of genetic information between homologous chromosomes during meiosis is important not only because it generates genetic diversity, but also because it is often required for proper chromosome segregation. Consequently, the frequency and distribution of crossovers are tightly controlled to ensure fertility and offspring viability. However, in many systems, it has been shown that environmental factors can alter the frequency of crossover events. Two studies in flies and yeast point to nutritional status affecting the frequency of crossing over. However, this question remains unexplored in mammals. Here, we test how crossover frequency varies in response to diet in Mus musculus males. We use immunohistochemistry to estimate crossover frequency in multiple genotypes under two diet treatments. Our results indicate that while crossover frequency was unaffected by diet in some strains, other strains were sensitive even to small composition changes between two common laboratory chows. Therefore, recombination is both resistant and sensitive to certain dietary changes in a strain-dependent manner and, hence, this response is genetically determined. Our study is the first to report a nutrition effect on genome-wide levels of recombination. Moreover, our work highlights the importance of controlling diet in recombination studies and may point to diet as a potential source of variability among studies, which is relevant for reproducibility.

Variation in genome-wide levels of meiotic recombination is established at the onset of prophase in mammalian males

Segregation of chromosomes during the first meiotic division relies on crossovers established during prophase. Although crossovers are strictly regulated so that at least one occurs per chromosome, individual variation in crossover levels is not uncommon. In an analysis of different inbred strains of male mice, we identified among-strain variation in the number of foci for the crossover-associated protein MLH1. We report studies of strains with “low” (CAST/EiJ), “medium” (C3H/HeJ), and “high” (C57BL/6J) genome-wide MLH1 values to define factors responsible for this variation. We utilized immunofluorescence to analyze the number and distribution of proteins that function at different stages in the recombination pathway: RAD51 and DMC1, strand invasion proteins acting shortly after double-strand break (DSB) formation, MSH4, part of the complex stabilizing double Holliday junctions, and the Bloom helicase BLM, thought to have anti-crossover activity. For each protein, we identified strain-specific differences that mirrored the results for MLH1; i.e., CAST/EiJ mice had the lowest values, C3H/HeJ mice intermediate values, and C57BL/6J mice the highest values. This indicates that differences in the numbers of DSBs (as identified by RAD51 and DMC1) are translated into differences in the number of crossovers, suggesting that variation in crossover levels is established by the time of DSB formation. However, DSBs per se are unlikely to be the primary determinant, since allelic variation for the DSB-inducing locus Spo11 resulted in differences in the numbers of DSBs but not the number of MLH1 foci. Instead, chromatin conformation appears to be a more important contributor, since analysis of synaptonemal complex length and DNA loop size also identified consistent strain-specific differences; i.e., crossover frequency increased with synaptonemal complex length and was inversely related to chromatin loop size. This indicates a relationship between recombination and chromatin compaction that may develop as DSBs form or earlier during establishment of the meiotic axis.

During prophase of meiosis, homologous chromosomes exchange genetic material, in a process known as crossing-over. Crossovers are thought to be essential for proper separation of chromosomes during meiosis but, surprisingly, most mammalian species exhibit substantial individual variation in the number of crossovers per cell. We investigated the basis for this variation by examining localization patterns of crossover-associated proteins in inbred strains of male mice with differing average numbers of crossovers per spermatocyte. Our results indicate that the strain-specific variation is established early in meiotic prophase, possibly even before the DNA is broken in advance of subsequent exchanges between homologous chromosomes.

Intronic parent-of-origin dependent differential methylation at the Actn1 gene is conserved in rodents but is not associated with imprinted expression

Parent-of-origin differential DNA methylation has been associated with regulation of the preferential expression of paternal or maternal alleles of imprinted genes. Based on this association, recent studies have searched for parent-of-origin dependent differentially methylated regions in order to identify new imprinted genes in their vicinity. In a previous genome-wide analysis of mouse brain DNA methylation, we found a novel differentially methylated region in a CpG island located in the last intron of the alpha 1 Actinin (Actn1) gene. In this region, preferential methylation of the maternal allele was observed; however, there were no reports of imprinted expression of Actn1. Therefore, we have tested if differential methylation of this region is common to other tissues and species and affects the expression of Actn1. We have found that Actn1 differential methylation occurs in diverse mouse tissues. Moreover, it is also present in other murine rodents (rat), but not in the orthologous human region. In contrast, we have found no indication of an imprinted effect on gene expression of Actn1 in mice: expression is always biallelic regardless of sex, tissue type, developmental stage or isoform. Therefore, we have identified a novel parent-of-origin dependent differentially methylated region that has no apparent association with imprinted expression of the closest genes. Our findings sound a cautionary note to genome-wide searches on the use of differentially methylated regions for the identification of imprinted genes and suggest that parent-of-origin dependent differential methylation might be conserved for functions other that the control of imprinted expression.

Genetic analysis of genome-scale recombination rate evolution in house mice

The rate of meiotic recombination varies markedly between species and among individuals. Classical genetic experiments demonstrated a heritable component to population variation in recombination rate, and specific sequence variants that contribute to recombination rate differences between individuals have recently been identified. Despite these advances, the genetic basis of species divergence in recombination rate remains unexplored. Using a cytological assay that allows direct in situ imaging of recombination events in spermatocytes, we report a large (∼30%) difference in global recombination rate between males of two closely related house mouse subspecies (Mus musculus musculus and M. m. castaneus). To characterize the genetic basis of this recombination rate divergence, we generated an F2 panel of inter-subspecific hybrid males (n = 276) from an intercross between wild-derived inbred strains CAST/EiJ (M. m. castaneus) and PWD/PhJ (M. m. musculus). We uncover considerable heritable variation for recombination rate among males from this mapping population. Much of the F2 variance for recombination rate and a substantial portion of the difference in recombination rate between the parental strains is explained by eight moderate- to large-effect quantitative trait loci, including two transgressive loci on the X chromosome. In contrast to the rapid evolution observed in males, female CAST/EiJ and PWD/PhJ animals show minimal divergence in recombination rate (∼5%). The existence of loci on the X chromosome suggests a genetic mechanism to explain this male-biased evolution. Our results provide an initial map of the genetic changes underlying subspecies differences in genome-scale recombination rate and underscore the power of the house mouse system for understanding the evolution of this trait.

Homologous recombination is an indispensable feature of the mammalian meiotic program and an important mechanism for creating genetic diversity. Despite its central significance, recombination rates vary markedly between species and among individuals. Although recent studies have begun to unravel the genetic basis of recombination rate variation within populations, the genetic mechanisms of species divergence in recombination rate remain poorly characterized. In this study, we show that two closely related house mouse subspecies differ in their genomic recombination rates by ∼30%, providing an excellent model system for studying evolutionary divergence in this trait. Using quantitative genetic methods, we identify eight genomic regions that contribute to divergence in global recombination rate between these subspecies, including large effect loci and multiple loci on the X-chromosome. Our study uncovers novel genomic loci contributing to species divergence in global recombination rate and offers simple genetic explanations for rapid phenotypic divergence in this trait.

From mammals to viruses: the Schlafen genes in developmental, proliferative and immune processes

The Schlafen genes have been associated with proliferation control and with several differentiation processes, as well as with disparate phenotypes such as immune response, embryonic lethality and meiotic drive. They constitute a gene family with widespread distribution in mammals, where they are expressed in several tissues, predominantly those of the immune system. Moreover, horizontal transfer of these genes to orthopoxviruses suggests a role of the viral Schlafens in evasion to the host immune response. The expression and functional studies of this gene family will be reviewed under the prism of their evolution and diversification, the challenges they pose and the future avenues of research.

Evolution of the genomic recombination rate in murid rodents

Although very closely related species can differ in their fine-scale patterns of recombination hotspots, variation in the average genomic recombination rate among recently diverged taxa has rarely been surveyed. We measured recombination rates in eight species that collectively represent several temporal scales of divergence within a single rodent family, Muridae. We used a cytological approach that enables in situ visualization of crossovers at meiosis to quantify recombination rates in multiple males from each rodent group. We uncovered large differences in genomic recombination rate between rodent species, which were independent of karyotypic variation. The divergence in genomic recombination rate that we document is not proportional to DNA sequence divergence, suggesting that recombination has evolved at variable rates along the murid phylogeny. Additionally, we document significant variation in genomic recombination rate both within and between subspecies of house mice. Recombination rates estimated in F(1) hybrids reveal evidence for sex-linked loci contributing to the evolution of recombination in house mice. Our results provide one of the first detailed portraits of genomic-scale recombination rate variation within a single mammalian family and demonstrate that the low recombination rates in laboratory mice and rats reflect a more general reduction in recombination rate across murid rodents.

Multiple loci contribute to genome-wide recombination levels in male mice

Recent linkage-based studies in humans suggest the presence of loci that affect either genome-wide recombination rates, utilization of recombination hotspots, or both. We have been interested in utilizing cytological methodology to directly assess recombination in mammalian meiocytes and to identify recombination-associated loci. In the present report we summarize studies in which we combined a cytological assay of recombination in mouse pachytene spermatocytes with QTL analyses to identify loci that contribute to genome-wide levels of recombination in male meiosis. Specifically, we analyzed MLH1 foci, a marker of crossovers, in 194 F2 male mice derived from a subspecific cross between CAST/EiJ and C57BL/6J parental strains. We then used these data to uncover loci associated with individual variation in mean MLH1 values. We identified seven recombination-associated loci across the genome (on chromosomes 2, 3, 4, 14, 15, 17, and X), indicating that there are multiple recombination “setting” loci in mammalian male meiosis.

Evolution of the Schlafen genes, a gene family associated with embryonic lethality, meiotic drive, immune processes and orthopoxvirus virulence

Genes of the Schlafen family, first discovered in mouse, are expressed in hematopoietic cells and are involved in immune processes. Previous results showed that they are candidate genes for two major phenomena: meiotic drive and embryonic lethality (DDK syndrome). However, these genes remain poorly understood, mostly due to the limitations imposed by their similarity, close location and the potential functional redundancy of the gene family members.

Here we use genomic and phylogenetic studies to investigate the evolution and role of this family of genes. Our results show that the Schlafen family is widely distributed in mammals, where we recognize four major clades that experienced lineage-specific expansions or contractions in various orders, including primates and rodents. In addition, we identified members of the Schlafen family in Chondrichthyes and Amphibia, indicating an ancient origin of these genes. We find evidence that positive selection has acted on many Schlafen genes. Moreover, our analyses indicate that a member of the Schlafen family was horizontally transferred from murine rodents to orthopoxviruses, where it is hypothesized to play a role in allowing the virus to survive host immune defense mechanisms. The functional relevance of the viral Schlafen sequences is further underscored by our finding that they are evolving under purifying selection. This is of particular importance, since orthopoxviruses infect mammals and include variola, the causative agent of smallpox, and monkeypox, an emerging virus of great concern for human health.

Characterization of two Mst1-deficient mouse models

Mammalian sterile 20-like kinase 1 (Mst1) is a ubiquitously expressed serine/threonine kinase belonging to the family of Sterile 20-like kinases. MST1 has been inferred to play important roles in apoptosis and in the inhibition of proliferation in mammalian cells. Here, we describe the genetic characterization of Mst1-deficient mice produced by two distinct gene-trap insertions. Animals generated from clone RRT293 exhibit transmission ratio distortion favoring the mutated allele which is amplified with each generation. Inexplicably, while the mutated allele is favored for transmission, its homozygosity is embryonic lethal. By contrast, animals generated from the second Mst1 gene-trap clone, AJ0315, do not show any gross abnormalities. We find that the discrepancy in phenotype is most likely attributable to a second insertion in the RRT293 clone. Thus, a mutation in Mst1 alone does not affect survival. Our results set the stage for identification of the lethal second-site mutation that is paradoxically favored for transmission.

The FG syndromes (Online Mendelian Inheritance in Man 305450): perspective in 2008

Rarely in the history of medicine has an X-linked mental retardation syndrome so thoroughly entered every branch of medicine, at least of pediatrics, but also of internal medicine, on account of its protean manifestations. In such countries as Zambia, malaria, tuberculosis, HIV, and other infections diseases, and many environmental and nutritional disorders still top the list of childhood morbidity and mortality. However, in the more developed nations of the Old and New Worlds, prematurity, birth defects, and genetic conditions constitute the major burden of infant mortality adn chronic childhood handicaps. One of the most pervasive of these is the group of FG syndromes seen in every pediatric clinic and mental health service. Thus, in our experience FGS emerges as the most common yet the least known developmental disabilities condition in our society. FGS imposes a tremendous burden of morbidity, and to some extent also of mortality, on society and families. After successful neonatal adaptation, such recurring problems as otitis, reactive airway disease, and constipation can be routinely treated symptomatically. However, the neurodevelopmental burden represents the greatest challenge that FGS presents for families and to society. Under the best of circumstances, motor and speech development catch up. However, virtually all FGS children, boys and girls, have difficulties in psychologic development, school performance, and ultimate emotional adaptation to adult life and social integration. The many such cases added to those with outright psychiatric disturbances are overwhelming social, psychologic, and psychiatric services and, above all, public and private school systems, which are understaffed, under-funded, beyond formulating individual educational plans, and helpless to deal with the enormous burden of special service needs of these children. It's time that handicapped children receive care according to needs and not according to diagnosis. However, the near absence of information on FGS available to these professionals is a handicap in arriving at a specific diagnosis (allowing state and federal support for special services) and in understanding the prognosis, natural history, and such complications as "autism," seizures, and tethered cord that affect the child's success at home, in school, and out in society. The FGS parent support group has been of enormous help in informing families about all of these "issues," and to this day remains the greatest repository of knowledge on FGS. As they say in baseball, it is time at long last for the professionals "to step up to the plate."

Crossover interference underlies sex differences in recombination rates

In many organisms, recombination rates differ between the two sexes. Here we show that in mice, this is because of a shorter genomic interference distance in females than in males, measured in Mb. However, the interference distance is the same in terms of bivalent length. We propose a model in which the interference distance in the two sexes reflects the compaction of chromosomes at the pachytene stage of meiosis.

Thyroid Epigenetics: X Chromosome Inactivation in Patients with Autoimmune Thyroid Disease

The autoimmune thyroid diseases (AITDs) are female-predominant diseases with a ratio of approximately seven females to each male. X chromosome inactivation (XCI), an epigenetic phenomenon, has been suggested to be skewed in many such female patients with AITD. We analyzed female genomic DNA from 87 patients with Graves' disease (GD), 47 patients with Hashimoto's thyroiditis (HT), and 69 healthy controls. Using an XCI assay based on Hpa II digestion and PCR and DNA sequencing, we found skewed heterozygous XCI (>or=80%) in 20 of 70 GD patients (28.6%) and 11 of 43 HT patients (25.6%), giving a total of 31 of 113 AITD patients (27.4%) with skewed XCI. In contrast, only 5 of 58 healthy controls had skewed XCI (8.6%). Statistical analysis confirmed that XCI skewing was significantly associated with AITD (P = 0.004, OR = 4.0), demonstrating that the degree of XCI is an important contributor to the increased risk of females in developing AITD.

Genetic control of X chromosome inactivation in mice: definition of the Xce candidate interval

In early mammalian development, one of the two X chromosomes is silenced in each female cell as a result of X chromosome inactivation, the mammalian dosage compensation mechanism. In the mouse epiblast, the choice of which chromosome is inactivated is essentially random, but can be biased by alleles at the X-linked X controlling element (Xce). Although this locus was first described nearly four decades ago, the identity and precise genomic localization of Xce remains elusive. Within the X inactivation center region of the X chromosome, previous linkage disequilibrium studies comparing strains of known Xce genotypes have suggested that Xce is physically distinct from Xist, although this has not yet been established by genetic mapping or progeny testing. In this report, we used quantitative trait locus (QTL) mapping strategies to define the minimal Xce candidate interval. Subsequent analysis of recombinant chromosomes allowed for the establishment of a maximum 1.85-Mb candidate region for the Xce locus. Finally, we use QTL approaches in an effort to identify additional modifiers of the X chromosome choice, as we have previously demonstrated that choice in Xce heterozygous females is significantly influenced by genetic variation present on autosomes (Chadwick and Willard 2005). We did not identify any autosomal loci with significant associations and thus show conclusively that Xce is the only major locus to influence X inactivation patterns in the crosses analyzed. This study provides a foundation for future analyses into the genetic control of X chromosome inactivation and defines a 1.85-Mb interval encompassing all the major elements of the Xce locus.

Enhanced adaptive evolution of sperm-expressed genes on the mammalian X chromosome

Genes on the mammalian X chromosome may be under unique evolutionary pressure due to their hemizygous expression in males. Since any recessive deleterious mutation would immediately be expressed in males and, therefore, efficiently removed from the population, selective constraint could be more pronounced in X-linked genes. Conversely, if a recessive mutation were beneficial, its immediate exposure to selection would be advantageous, and would facilitate adaptive evolution. We tested for positive selection in a total of 86 genes using a maximum likelihood approach, including 40 sperm-expressed and 46 non-sperm, tissue-specific genes. We find evidence to suggest that X-linkage enhances the effects of positive selection in sperm-expressed genes in terms of the number of codons affected, and report a general trend for positively selected genes to reside on the X chromosome rather than on the autosomes. Our data suggest that hemizygous expression in males makes the X chromosome a preferred location for positively selected sperm genes that do not require postmeiotic transcription.

Genetic and parent-of-origin influences on X chromosome choice in Xce heterozygous mice

X chromosome inactivation is unique among dosage compensation mechanisms in that the two X chromosomes in females are treated differently within the same cell; one X chromosome is stably silenced while the other remains active. It is widely believed that, when X inactivation is initiated, each cell makes a random choice of which X chromosome will be silenced. In mice, only one genetic locus, the X-linked X controlling element (X ce), is known to influence this choice, because animals that are heterozygous at X ce have X-inactivation patterns that differ markedly from a mean of 0.50. To document other genetic and epigenetic influences on choice, we have performed a population-based study of the effect of X ce genotype on X-inactivation patterns. In B 6 CAST F(1) females (X ce(b)/X ce(c)), the X-inactivation pattern followed a symmetric distribution with a mean of 0.29 (SD=0.08). Surprisingly, however, in a population of X ce(b)/X ce(c) heterozygous B 6 CAST F(2) females, we observed significant differences in both the mean (p=0.004) and variance (p=0.004) of the X-inactivation patterns. This finding is incompatible with a single-locus model and suggests that additional genetic factors also influence X chromosome choice. We show that both parent-of-origin and naturally occurring genetic variation at autosomal loci contribute to these differences. Taken together, these data reveal further genetic complexity in this epigenetic control pathway.

The paternal gene of the DDK syndrome maps to the Schlafen gene cluster on mouse chromosome 11

The DDK syndrome is an early embryonic lethal phenotype observed in crosses between females of the DDK inbred mouse strain and many non-DDK males. Lethality results from an incompatibility between a maternal DDK factor and a non-DDK paternal gene, both of which have been mapped to the Ovum mutant (Om) locus on mouse chromosome 11. Here we define a 465-kb candidate interval for the paternal gene by recombinant progeny testing. To further refine the candidate interval we determined whether males from 17 classical and wild-derived inbred strains are interfertile with DDK females. We conclude that the incompatible paternal allele arose in the Mus musculus domesticus lineage and that incompatible strains should share a common haplotype spanning the paternal gene. We tested for association between paternal allele compatibility/incompatibility and 167 genetic variants located in the candidate interval. Two diallelic SNPs, located in the Schlafen gene cluster, are completely predictive of the polar-lethal phenotype. These SNPs also predict the compatible or incompatible status of males of five additional strains.

Maternal transmission ratio distortion at the mouse Om locus results from meiotic drive at the second meiotic division

We have observed maternal transmission ratio distortion (TRD) in favor of DDK alleles at the Ovum mutant (Om) locus on mouse chromosome 11 among the offspring of (C57BL/6 x DDK) F(1) females and C57BL/6 males. Although significant lethality occurs in this backcross ( approximately 50%), differences in the level of TRD found in recombinant vs. nonrecombinant chromosomes among offspring argue that TRD is due to nonrandom segregation of chromatids at the second meiotic division, i.e., true meiotic drive. We tested this hypothesis directly, by determining the centromere and Om genotypes of individual chromatids in zygote stage embryos. We found similar levels of TRD in favor of DDK alleles at Om in the female pronucleus and TRD in favor of C57BL/6 alleles at Om in the second polar body. In those embryos for which complete dyads have been reconstructed, TRD was present only in those inheriting heteromorphic dyads. These results demonstrate that meiotic drive occurs at MII and that preferential death of one genotypic class of embryo does not play a large role in the TRD.

Transmission-ratio distortion and allele sharing in affected sib pairs: a new linkage statistic with reduced bias, with application to chromosome 6q25.3

We studied the effect of transmission-ratio distortion (TRD) on tests of linkage based on allele sharing in affected sib pairs. We developed and implemented a discrete-trait allele-sharing test statistic, Sad, analogous to the Spairs test statistic of Whittemore and Halpern, that evaluates an excess sharing of alleles at autosomal loci in pairs of affected siblings, as well as a lack of sharing in phenotypically discordant relative pairs, where available. Under the null hypothesis of no linkage, nuclear families with at least two affected siblings and one unaffected sibling have a contribution to Sad that is unbiased, with respect to the effects of TRD independent of the disease under study. If more distantly related unaffected individuals are studied, the bias of Sad is generally reduced compared with that of Spairs, but not completely. Moreover, Sad has higher power, in some circumstances, because of the availability of unaffected relatives, who are ignored in affected-only analyses. We discuss situations in which it may be an efficient use of resources to genotype unaffected relatives, which would give insights for promising study designs. The method is applied to a sample of pedigrees ascertained for asthma in a chromosomal region in which TRD has been reported. Results are consistent with the presence of transmission distortion in that region.

How mammalian sex chromosomes acquired their peculiar gene content

It has become increasingly evident that gene content of the sex chromosomes is markedly different from that of the autosomes. Both sex chromosomes appear enriched for genes related to sexual differentiation and reproduction; but curiously, the human X chromosome also seems to bear a preponderance of genes linked to brain and muscle functions. In this review, we will synthesize several evolutionary theories that may account for this nonrandom assortment of genes on the sex chromosomes, including 1) asexual degeneration, 2) sexual antagonism, 3) constant selection, and 4) hemizygous exposure. Additionally, we will speculate on how the evolution of sex-chromosome gene content might have impacted on the phenotypic evolution of mammals and particularly humans. Our discussion will focus on the mammalian sex chromosomes, but will cross reference other species where appropriate.

Comparative recombination rates in the rat, mouse, and human genomes

Levels of recombination vary among species, among chromosomes within species, and among regions within chromosomes in mammals. This heterogeneity may affect levels of diversity, efficiency of selection, and genome composition, as well as have practical consequences for the genetic mapping of traits. We compared the genetic maps to the genome sequence assemblies of rat, mouse, and human to estimate local recombination rates across these genomes. Humans have greater overall levels of recombination, as well as greater variance. In rat and mouse, the size of the chromosome and proximity to telomere have less effect on local recombination rate than in human. At the chromosome level, rat and mouse X chromosomes have the lowest recombination rates, whereas human chromosome X does not show the same pattern. In all species, local recombination rate is significantly correlated with several sequence variables, including GC%, CpG density, repetitive elements, and the neutral mutation rate, with some pronounced differences between species. Recombination rate in one species is not strongly correlated with the rate in another, when comparing homologous syntenic blocks of the genome. This comparative approach provides additional insight into the causes and consequences of genomic heterogeneity in recombination.