Male mouse recombination maps for each autosome identified by chromosome painting

A comparative study of meiotic recombination in cattle (Bos taurus) and three wildebeest species (Connochaetes gnou, C. taurinus taurinus and C. t. albojubatus)

The karyotypic evolution in the family Bovidae is based on centric fusions of ancestral acrocentric chromosomes. Here, the frequency and distribution of meiotic recombination was analyzed in pachytene spermatocytes from Bos taurus (2n = 60) and 3 wildebeest species (Connochaetes gnou, C. taurinus taurinus and C. t. albojubatus) (2n = 58) using immunofluorescence and fluorescence in situ hybridization. Significant differences in mean numbers of recombination events per cell were observed between B. taurus and members of the genus Connochaetes (47.2 vs. 43.7, p < 0.001). The number of MLH1 foci was significantly correlated with the length of the autosomal synaptonemal complexes. The average interfocus distance was influenced by interference. The male recombination maps of bovine chromosomes 2 and 25 and of their fused homologues in wildebeests were constructed. A significant reduction of recombination in the fused chromosome BTA25 was observed in wildebeests (p = 0.005). This was probably caused by interference acting across the centromere, which was significantly stronger than the intra-arm interference. This comparative meiotic study showed significant differences among the species from the family Bovidae with the same fundamental number of autosomal arms (FNa = 29) which differ by a single centric fusion.

Molecular cytogenetic identification and characterization of Robertsonian chromosomes in the large Japanese field mouse (Apodemus speciosus) using FISH

Robertsonian (Rb) karyotypic polymorphism in Apodemus speciosus has interested many researchers with particular referece to the genetic divergence between Rb and non-Rb populations. Failure to find morphologic, biochemical, or genetic differences in previous studies reveals the necessity of focusing on loci on Rb chromosomes, which can be characterized by FISH mapping with DNA probes. In an Rb heterozygote, DNA probes from laboratory mouse chromosomes (MMUs) 1 and 10 were simultaneously hybridized to the long arm of a metacentric and a medium-sized acrocentric chromosome and to the short arm of the metacentric and a small acrocentric chromosome, respectively. Four additional probes derived from each of MMUs 1 and 10 were mapped to the long and short arms, respectively, of the Rb chromosome identified by the above markers. Homologies between the long arm of the Rb chromosome and MMU 1 and between the short arm and MMU 10 were supported by all ten markers, which were dispersed along nearly the entire lengths of the Rb chromosomes. These results indicate that the long and short arms of the Rb chromosomes are homologous to Apodemus speciosus chromosomes 12 and 19 (defined in a previous study), respectively. This ten-marker series can be useful for detecting chromosome-specific divergence between the two karyotypic populations at the gene level.

Fine scale analysis of crossover and non-crossover and detection of recombination sequence motifs in the honeybee (Apis mellifera)

BACKGROUND

Meiotic exchanges are non-uniformly distributed across the genome of most studied organisms. This uneven distribution suggests that recombination is initiated by specific signals and/or regulations. Some of these signals were recently identified in humans and mice. However, it is unclear whether or not sequence signals are also involved in chromosomal recombination of insects.

METHODOLOGY

We analyzed recombination frequencies in the honeybee, in which genome sequencing provided a large amount of SNPs spread over the entire set of chromosomes. As the genome sequences were obtained from a pool of haploid males, which were the progeny of a single queen, an oocyte method (study of recombination on haploid males that develop from unfertilized eggs and hence are the direct reflect of female gametes haplotypes) was developed to detect recombined pairs of SNP sites. Sequences were further compared between recombinant and non-recombinant fragments to detect recombination-specific motifs.

CONCLUSIONS

Recombination events between adjacent SNP sites were detected at an average distance of 92 bp and revealed the existence of high rates of recombination events. This study also shows the presence of conversion without crossover (i. e. non-crossover) events, the number of which largely outnumbers that of crossover events. Furthermore the comparison of sequences that have undergone recombination with sequences that have not, led to the discovery of sequence motifs (CGCA, GCCGC, CCGCA), which may correspond to recombination signals.

Multiple independent evolutionary losses of XY pairing at meiosis in the grey voles

In many eutherian mammals, X-Y chromosome pairing and recombination is required for meiotic progression and correct sex chromosome disjunction. Arvicoline rodents present a notable exception to this meiotic rule, with multiple species possessing asynaptic sex chromosomes. Most asynaptic vole species belong to the genus Microtus sensu lato. However, many of the species both inside and outside the genus Microtus display normal X-Y synapsis at meiosis. These observations suggest that the synaptic condition was present in the common ancestor of all voles, but gaps in current taxonomic sampling across the arvicoline phylogeny prevent identification of the lineage(s) along which the asynaptic state arose. In this study, we use electron and immunofluorescent microscopy to assess heterogametic sex chromosome pairing in 12 additional arvicoline species. Our sample includes ten species of the tribe Microtini and two species of the tribe Lagurini. This increased breadth of sampling allowed us to identify asynaptic species in each major Microtine lineage. Evidently, the ability of the sex chromosomes to pair and recombine in male meiosis has been independently lost at least three times during the evolution of Microtine rodents. These results suggest a lack of evolutionary constraint on X-Y synapsis in Microtini, hinting at the presence of alternative molecular mechanisms for sex chromosome segregation in this large mammalian tribe.

Genome-wide crossover distribution in Arabidopsis thaliana meiosis reveals sex-specific patterns along chromosomes

In most species, crossovers (COs) are essential for the accurate segregation of homologous chromosomes at the first meiotic division. Their number and location are tightly regulated. Here, we report a detailed, genome-wide characterization of the rate and localization of COs in Arabidopsis thaliana, in male and female meiosis. We observed dramatic differences between male and female meiosis which included: (i) genetic map length; 575 cM versus 332 cM respectively; (ii) CO distribution patterns: male CO rates were very high at both ends of each chromosome, whereas female CO rates were very low; (iii) correlations between CO rates and various chromosome features: female CO rates correlated strongly and negatively with GC content and gene density but positively with transposable elements (TEs) density, whereas male CO rates correlated positively with the CpG ratio. However, except for CpG, the correlations could be explained by the unequal repartition of these sequences along the Arabidopsis chromosome. For both male and female meiosis, the number of COs per chromosome correlates with chromosome size expressed either in base pairs or as synaptonemal complex length. Finally, we show that interference modulates the CO distribution both in male and female meiosis.

Gamma-irradiation increased meiotic crossovers in mouse spermatocytes

In mice, the occurrence of immunofluorescent foci for mismatch repair protein MLH1 correlates closely with the occurrence of crossovers, as detected genetically, and MLH1 foci represent virtually all prospective crossover positions. To examine the effects of γ-irradiation on meiotic crossovers in mouse spermatocytes, male mice were subjected to whole-body γ-irradiation at different sub-stages of meiotic prophase and crossovers on synaptonemal complexes (SCs) were analysed by visualising and quantifying the immunofluorescent MLH1 foci. At both 24 and 48 h after exposure, significant dose-dependent increases in the number of total MLH1 foci per spermatocyte were observed at late zygotene-early pachytene with the gradient increase of radiation dose from 0, 1.5, 3-6 Gy. Furthermore, irradiation at preleptotene-leptotene still led to significant dose-dependent increased meiotic crossovers in the spermatocytes analysed 120 h after exposure. In further analysis, these dose-dependent increases in the number of total MLH1 foci per cell were attributed to significant dose-dependent decreases in autosomal SCs with 0 MLH1 focus, and the dose-dependent increases in autosomal SCs with 2 MLH1 foci and the percentage of cells with MLH1 focus on XY bivalent. The increased number of cells with an MLH1 focus on the pseudoautosomal regions (PARs) may indicate that there is a delay in meiotic progression in the irradiated cells. Although significant dose-dependent increases in the number of total MLH1 foci per cell were examined 24, 48 or 120 h after exposure with the gradient increase of radiation doses, these increases were mild compared to the control groups. This suggests that there is tight control of crossover formation (at least with respect to MLH1 foci number). The mechanisms underlying irradiation-induced DNA lesion repair, cellular responses independent of DNA damage and meiotic crossover homeostasis in mammals will be the subjects of future study.

A comparative study of the recombination pattern in three species of Platyrrhini monkeys (primates)

Homologous chromosomes exchange genetic information through recombination during meiotic synapsis, a process that increases genetic diversity and is fundamental to sexual reproduction. Meiotic studies in mammalian species are scarce and mainly focused on human and mouse. Here, the meiotic recombination events were determined in three species of Platyrrhini monkeys (Cebus libidinosus, Cebus nigritus and Alouatta caraya) by analysing the distribution of MLH1 foci at the stage of pachytene. Moreover, the combination of immunofluorescence and fluorescent in situ hybridisation has enabled us to construct recombination maps of primate chromosomes that are homologous to human chromosomes 13 and 21. Our results show that (a) the overall number of MLH1 foci varies among all three species, (b) the presence of heterochromatin blocks does not have a major influence on the distribution of MLH1 foci and (c) the distribution of crossovers in the homologous chromosomes to human chromosomes 13 and 21 are conserved between species of the same genus (C. libidinosus and C. nigritus) but are significantly different between Cebus and Alouatta. This heterogeneity in recombination behaviour among Ceboidea species may reflect differences in genetic diversity and genome composition.

Chromatin configuration and epigenetic landscape at the sex chromosome bivalent during equine spermatogenesis

Pairing of the sex chromosomes during mammalian meiosis is characterized by the formation of a unique heterochromatin structure at the XY body. The mechanisms underlying the formation of this nuclear domain are reportedly highly conserved from marsupials to mammals. In this study, we demonstrate that in contrast to all eutherian species studied to date, partial synapsis of the heterologous sex chromosomes during pachytene stage in the horse is not associated with the formation of a typical macrochromatin domain at the XY body. While phosphorylated histone H2AX (γH2AX) and macroH2A1.2 are present as a diffuse signal over the entire macrochromatin domain in mouse pachytene spermatocytes, γH2AX, macroH2A1.2, and the cohesin subunit SMC3 are preferentially enriched at meiotic sex chromosome cores in equine spermatocytes. Moreover, although several histone modifications associated with this nuclear domain in the mouse such as H3K4me2 and ubH2A are conspicuously absent in the equine XY body, prominent RNA polymerase II foci persist at the sex chromosomes. Thus, the localization of key marker proteins and histone modifications associated with the XY body in the horse differs significantly from all other mammalian systems described. These results demonstrate that the epigenetic landscape and heterochromatinization of the equine XY body might be regulated by alternative mechanisms and that some features of XY body formation may be evolutionary divergent in the domestic horse. We propose equine spermatogenesis as a unique model system for the study of the regulatory networks leading to the epigenetic control of gene expression during XY body formation.

Synapsis and meiotic recombination in male Chinese muntjac (Muntiacus reevesi)

The muntjacs (Muntiacus, Cervidae) have been extensively studied in terms of chromosomal and karyotypic evolution. However, little is known about their meiotic chromosomes particularly the recombination patterns of homologous chromosomes. We used immunostained surface spreads to visualise synaptonemal complexes (SCs), recombination foci and kinetochores with antibodies against marker proteins. As in other mammals pachytene was the longest stage of meiotic prophase. 39.4% of XY bivalents lacked MLH1 foci compared to less than 0.5% of autosomes. The average number of MLH1 foci per pachytene cell in M. reevesi was 29.8. The distribution of MLH1 foci differed from other mammals. On SCs with one focus, the distribution was more even in M. reevesi than in other mammals; for SCs that have two or more MLH1 foci, usually there was a larger peak in the sub-centromere region than other regions on SC in M. reevesi. Additionally, there was a lower level of interference between foci in M. reevesi than in mouse or human. These observations may suggest that the regulation of homologous recombination in M. reevesi is slightly different from other mammals and will improve our understanding of the regulation of meiotic recombination, with respect to recombination frequency and position.

On the origin of crossover interference: A chromosome oscillatory movement (COM) model

BACKGROUND

It is now nearly a century since it was first discovered that crossovers between homologous parental chromosomes, originating at the Prophase stage of Meiosis I, are not randomly placed. In fact, the number and distribution of crossovers are strictly regulated with crossovers/chiasmata formed in optimal positions along the length of individual chromosomes, facilitating regular chromosome segregation at the first meiotic division. In spite of much research addressing this question, the underlying mechanism(s) for the phenomenon called crossover/chiasma interference is/are still unknown; and this constitutes an outstanding biological enigma.

RESULTS

The Chromosome Oscillatory Movement (COM) model for crossover/chiasma interference implies that, during Prophase of Meiosis I, oscillatory movements of the telomeres (attached to the nuclear membrane) and the kinetochores (within the centromeres) create waves along the length of chromosome pairs (bivalents) so that crossing-over and chiasma formation is facilitated by the proximity of parental homologs induced at the nodal regions of the waves thus created. This model adequately explains the salient features of crossover/chiasma interference, where (1) there is normally at least one crossover/chiasma per bivalent, (2) the number is correlated to bivalent length, (3) the positions are dependent on the number per bivalent, (4) interference distances are on average longer over the centromere than along chromosome arms, and (5) there are significant changes in carriers of structural chromosome rearrangements.

CONCLUSIONS

The crossover/chiasma frequency distribution in humans and mice with normal karyotypes as well as in carriers of structural chromosome rearrangements are those expected on the COM model. Further studies are underway to analyze mechanical/mathematical aspects of this model for the origin of crossover/chiasma interference, using string replicas of the homologous chromosomes at the Prophase stage of Meiosis I. The parameters to vary in this type of experiment will include: (1) the mitotic karyotype, i.e. ranked length and centromere index of the chromosomes involved, (2) the specific bivalent/multivalent length and flexibility, dependent on the way this structure is positioned within the nucleus and the size of the respective meiocyte nuclei, (3) the frequency characteristics of the oscillatory movements at respectively the telomeres and the kinetochores.

Distinct properties of the XY pseudoautosomal region crucial for male meiosis

Meiosis requires that each chromosome find its homologous partner and undergo at least one crossover. X-Y chromosome segregation hinges on efficient crossing-over in a very small region of homology, the pseudoautosomal region (PAR). We find that mouse PAR DNA occupies unusually long chromosome axes, potentially as shorter chromatin loops, predicted to promote double-strand break (DSB) formation. Most PARs show delayed appearance of RAD51/DMC1 foci, which mark DSB ends, and all PARs undergo delayed DSB-mediated homologous pairing. Analysis of Spo11β isoform-specific transgenic mice revealed that late RAD51/DMC1 foci in the PAR are genetically distinct from both early PAR foci and global foci and that late PAR foci promote efficient X-Y pairing, recombination, and male fertility. Our findings uncover specific mechanisms that surmount the unique challenges of X-Y recombination.

Synaptonemal complex length variation in wild-type male mice

Meiosis yields haploid gametes following two successive divisions of a germ cell in the absence of intervening DNA replication. Balanced segregation of homologous chromosomes in Meiosis I is aided by a proteinaceous structure, the synaptonemal complex (SC). The objective of this study was to determine total average autosomal SC lengths in spermatocytes in three commonly used mouse strains (129S4/SvJae, C57BL/6J, and BALB/c). Our experiments revealed that the total autosomal SC length in BALB/c spermatocytes is 9% shorter than in the two other strains. Shorter SCs are also observed in spermatocytes of (BALB/c × 129S4/SvJae) and (C57BL/6J × BALB/c) F1 hybrids suggesting a genetic basis of SC length regulation. Along these lines, we studied expression of a selected group of genes implicated in meiotic chromosome architecture. We found that BALB/c testes express up to 6-fold less of Rec8 mRNA and 4-fold less of REC8 protein. These results suggest that the mechanism that defines the SC length operates via a REC8‑dependent process. Finally, our results demonstrate that genetic background can have an effect on meiotic studies in mice.

Patterns of recombination activity on mouse chromosome 11 revealed by high resolution mapping

The success of high resolution genetic mapping of disease predisposition and quantitative trait loci in humans and experimental animals depends on the positions of key crossover events around the gene of interest. In mammals, the majority of recombination occurs at highly delimited 1-2 kb long sites known as recombination hotspots, whose locations and activities are distributed unevenly along the chromosomes and are tightly regulated in a sex specific manner. The factors determining the location of hotspots started to emerge with the finding of PRDM9 as a major hotspot regulator in mammals, however, additional factors modulating hotspot activity and sex specificity are yet to be defined. To address this limitation, we have collected and mapped the locations of 4829 crossover events occurring on mouse chromosome 11 in 5858 meioses of male and female reciprocal F1 hybrids of C57BL/6J and CAST/EiJ mice. This chromosome was chosen for its medium size and high gene density and provided a comparison with our previous analysis of recombination on the longest mouse chromosome 1. Crossovers were mapped to an average resolution of 127 kb, and thirteen hotspots were mapped to <8 kb. Most crossovers occurred in a small number of the most active hotspots. Females had higher recombination rate than males as a consequence of differences in crossover interference and regional variation of sex specific rates along the chromosome. Comparison with chromosome 1 showed that recombination events tend to be positioned in similar fashion along the centromere-telomere axis but independently of the local gene density. It appears that mammalian recombination is regulated on at least three levels, chromosome-wide, regional, and at individual hotspots, and these regulation levels are influenced by sex and genetic background but not by gene content.

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.

Mouse TRIP13/PCH2 is required for recombination and normal higher-order chromosome structure during meiosis

Accurate chromosome segregation during meiosis requires that homologous chromosomes pair and become physically connected so that they can orient properly on the meiosis I spindle. These connections are formed by homologous recombination closely integrated with the development of meiosis-specific, higher-order chromosome structures. The yeast Pch2 protein has emerged as an important factor with roles in both recombination and chromosome structure formation, but recent analysis suggested that TRIP13, the mouse Pch2 ortholog, is not required for the same processes. Using distinct Trip13 alleles with moderate and severe impairment of TRIP13 function, we report here that TRIP13 is required for proper synaptonemal complex formation, such that autosomal bivalents in Trip13-deficient meiocytes frequently displayed pericentric synaptic forks and other defects. In males, TRIP13 is required for efficient synapsis of the sex chromosomes and for sex body formation. Furthermore, the numbers of crossovers and chiasmata are reduced in the absence of TRIP13, and their distribution along the chromosomes is altered, suggesting a role for TRIP13 in aspects of crossover formation and/or control. Recombination defects are evident very early in meiotic prophase, soon after DSB formation. These findings provide evidence for evolutionarily conserved functions for TRIP13/Pch2 in both recombination and formation of higher order chromosome structures, and they support the hypothesis that TRIP13/Pch2 participates in coordinating these key aspects of meiotic chromosome behavior.

Meiosis is the specialized cell division that gives rise to reproductive cells such as sperm and eggs. During meiosis in most organisms, genetic information is exchanged between homologous maternal and paternal chromosomes through the process of homologous recombination. This recombination forms connections between homologous chromosomes that allow them to segregate accurately when the meiotic cell divides. Recombination defects can result in reproductive cells with abnormal chromosome numbers, which are a major cause of developmental disorders and spontaneous abortions in humans. Meiotic recombination is tightly controlled such that each pair of chromosomes undergoes at least one crossover recombination event despite a low average number of crossovers per chromosome. Recombination is coordinated with the development of specialized, meiosis-specific chromosome structures that stabilize pairing interactions between homologous maternal and paternal chromosomes. We show here that the mouse TRIP13 protein is required for normal execution of many aspects of meiotic recombination and chromosome structure development that it was not previously known to influence. Intriguingly, many of these new roles appear to parallel known functions of a homologous protein from budding yeast, called Pch2. These findings thus indicate that TRIP13/Pch2 functions are more widely conserved throughout evolution than thought before.

A- and B-chromosome pairing and recombination in male meiosis of the silver fox (Vulpes vulpes L., 1758, Carnivora, Canidae)

We examined A- and B-chromosome pairing and recombination in 12 males from the farm-bred population of the silver fox (2n = 34 + 0-10 Bs) by means of electron and immunofluorescent microscopy. To detect recombination at A and B chromosomes, we used immunolocalisation of MLH1, a mismatch repair protein of mature recombination nodules, at synaptonemal complexes. The mean total number of MLH1 foci at A-autosomes was 29.6 foci per cell. The XY bivalent had one MLH1 focus at the pairing region. Total recombination length of the male fox genome map was estimated as 1,530 centimorgans. We detected single MLH1 foci at 61% of linear synaptic configurations involving B chromosomes. The distribution of the foci along B- and A-bivalents was the same. This may be considered as a first molecular evidence that meiotic recombination does occur in mammalian B chromosomes. There was no correlation between the number of synaptic configurations involving B chromosomes per cell and the recombination rate of the A-genome.

FISH and DAPI staining of the synaptonemal complex of the Nile tilapia (Oreochromis niloticus) allow orientation of the unpaired region of bivalent 1 observed during early pachytene

Bivalent 1 of the synaptonemal complex (SC) in XY male Oreochromis niloticus shows an unpaired terminal region in early pachytene. This appears to be related to recombination suppression around a sex determination locus. To allow more detailed analysis of this, and unpaired regions in the karyotype of other Oreochromis species, we developed techniques for FISH on SC preparations, combined with DAPI staining. DAPI staining identified presumptive centromeres in SC bivalents, which appeared to correspond to the positions observed in the mitotic karyotype (the kinetochores could be identified only sporadically in silver-stained EM SC images). Furthermore, two BAC clones containing Dmo (dmrt4) and OniY227 markers that hybridize to known positions in chromosome pair 1 in mitotic spreads (near the centromere, Flpter 0.25, and the putative sex-determination locus, Flpter 0.57, respectively) were used as FISH probes on SCs to verify that the presumptive centromere identified by DAPI staining was located in the expected position. Visualization of both the centromere and FISH signals on bivalent 1 allowed the unpaired region to be positioned at Flpter 0.80 to 1.00, demonstrating that the unpaired region is located in the distal part of the long arm(s). Finally, differences between mitotic and meiotic measurements are discussed.

Two types of meiotic crossovers coexist in maize

We apply modeling approaches to investigate the distribution of late recombination nodules in maize (Zea mays). Such nodules indicate crossover positions along the synaptonemal complex. High-quality nodule data were analyzed using two different interference models: the "statistical" gamma model and the "mechanical" beam film model. For each chromosome, we exclude at a 98% significance level the hypothesis that a single pathway underlies the formation of all crossovers, pointing to the coexistence of two types of crossing-over in maize, as was previously demonstrated in other organisms. We estimate the proportion of crossovers coming from the noninterfering pathway to range from 6 to 23% depending on the chromosome, with a cell average of approximately 15%. The mean number of noninterfering crossovers per chromosome is significantly correlated with the length of the synaptonemal complex. We also quantify the intensity of interference. Finally, we develop inference tools that allow one to tackle, without much loss of power, complex crossover interference models such as the beam film. The lack of a likelihood function in such models had prevented their use for parameter estimation. This advance will allow more realistic mechanisms of crossover formation to be modeled in the future.

Incidence of chromosomal mosaicism in morphologically normal nonhuman primate preimplantation embryos

OBJECTIVE

To establish the exact rates of chromosomal mosaicism in morphologically normal rhesus macaque embryos by determining the chromosomal complement of all blastomeres.

DESIGN

Retrospective rhesus monkey IVF study.

SETTING

Academic laboratory and primate research center.

PATIENT(S)

Young fertile rhesus macaque females.

INTERVENTION(S)

Morphologically normal in vitro-produced rhesus macaque embryos were dissociated and cytogenetically assessed using a five-color fluorescent in situ hybridization assay developed for rhesus macaque chromosomes homologous to human chromosomes 13, 16, 18, X, and Y.

MAIN OUTCOME MEASURE(S)

The incidence and extent of chromosomal mosaicism in rhesus macaque preimplantation embryos.

RESULT(S)

Seventy-seven preimplantation embryos, displaying normal morphology and development, from 17 young rhesus macaque females were analyzed. Overall, 39 embryos (50.6%) were normal, 14 embryos (18.2%) were completely abnormal, and 24 embryos (31.2%) were mosaic. Of the 226 blastomeres analyzed in the mosaic group, 110 blastomeres (48.7%) were normal.

CONCLUSION(S)

The observed rate of mosaicism in good-quality rhesus embryos resembles previously documented frequencies in poor-quality human preimplantation embryos. A high incidence of mosaicism may limit the diagnostic accuracy of preimplantation genetic diagnosis.

Molecular cloning and characterization of the germline-restricted chromosome sequence in the zebra finch

The zebra finch (Taeniopygia guttata) germline-restricted chromosome (GRC) is the largest chromosome and has a unique system of transmission in germ cells. In the male, the GRC exists as a single heterochromatic chromosome in the germline and is eliminated from nuclei in late spermatogenesis. In the female, the GRC is bivalent and euchromatic and experiences recombination. These characteristics suggest a female-specific or female-beneficial function of the GRC. To shed light on the function of GRC, we cloned a portion of the GRC using random amplified polymorphic DNA-polymerase chain reaction and analyzed it using molecular genetic and cytogenetic methods. The GRC clone hybridized strongly to testis but not blood DNA in genomic Southern blots. In fluorescent in situ hybridization analysis on meiotic chromosomes from synaptonemal complex spreads, the probe showed hybridization across a large area of the GRC, suggesting that it contains repetitive sequences. We isolated a sequence homologous to the GRC from zebra finch chromosome 3 and a region of chicken chromosome 1 that is homologous to zebra finch chromosome 3; the phylogenetic analysis of these three sequences suggested that the GRC sequence and the zebra finch chromosome 3 sequence are most closely related. Thus, the GRC sequences likely originated from autosomal DNA and have evolved after the galliform-passeriform split. The present study provides a foundation for further study of the intriguing GRC.

Genome-wide control of the distribution of meiotic recombination

Meiotic recombination events are not randomly distributed in the genome but occur in specific regions called recombination hotspots. Hotspots are predicted to be preferred sites for the initiation of meiotic recombination and their positions and activities are regulated by yet-unknown controls. The activity of the Psmb9 hotspot on mouse Chromosome 17 (Chr 17) varies according to genetic background. It is active in strains carrying a recombinant Chr 17 where the proximal third is derived from Mus musculus molossinus. We have identified the genetic locus required for Psmb9 activity, named Dsbc1 for Double-strand break control 1, and mapped this locus within a 6.7-Mb region on Chr 17. Based on cytological analysis of meiotic DNA double-strand breaks (DSB) and crossovers (COs), we show that Dsbc1 influences DSB and CO, not only at Psmb9, but in several other regions of Chr 17. We further show that CO distribution is also influenced by Dsbc1 on Chrs 15 and 18. Finally, we provide direct molecular evidence for the regulation in trans mediated by Dsbc1, by showing that it controls the CO activity at the Hlx1 hotspot on Chr 1. We thus propose that Dsbc1 encodes for a trans-acting factor involved in the specification of initiation sites of meiotic recombination genome wide in mice.

In many organisms, an essential feature of meiosis is genetic recombination, which creates diversity in the gametes by mixing the genetic information from each parent into new combinations. Reciprocal recombination, or crossovers, also play a mechanical role during meiosis and are required for the proper segregation of homologous chromosomes to the daughter cells. Crossovers do not occur randomly in the genome but rather are clustered in small regions called hotspots. The factors that determine hotspot locations are poorly understood. We have analyzed a particular recombination hotspot in the mouse genome, called Psmb9, and showed that its activity is induced by a specific allele of a locus that we have mapped and named Dsbc1, for Double-strand break control 1. We have analyzed the properties of Dsbc1 both by the direct detection of recombinant DNA molecules in specific regions and by chromosome-wide cytological detection of proteins involved in recombination. Our results show that Dsbc1 acts genome wide and regulates the distribution of crossovers in several regions on different chromosomes, at least in part by regulating the initiation step of meiotic recombination characterized by the formation of DNA double-strand breaks.

Dsbc1 is a novel locus involved in controlling the localization of meiotic recombination events in the mouse genome.

Mutation of the mouse Syce1 gene disrupts synapsis and suggests a link between synaptonemal complex structural components and DNA repair

In mammals, the synaptonemal complex is a structure required to complete crossover recombination. Although suggested by cytological work, in vivo links between the structural proteins of the synaptonemal complex and the proteins of the recombination process have not previously been made. The central element of the synaptonemal complex is traversed by DNA at sites of recombination and presents a logical place to look for interactions between these components. There are four known central element proteins, three of which have previously been mutated. Here, we complete the set by creating a null mutation in the Syce1 gene in mouse. The resulting disruption of synapsis in these animals has allowed us to demonstrate a biochemical interaction between the structural protein SYCE2 and the repair protein RAD51. In normal meiosis, this interaction may be responsible for promoting homologous synapsis from sites of recombination.

Detection of meiotic DNA breaks in mouse testicular germ cells

The study of location and intensity of double-strand breaks (DSBs) in mammalian systems is more challenging than in yeast because, unlike yeast, the progression through meiosis is not synchronous and only a small fraction of all testis cells are actually at the stage where DSB formation is initiated. We devised a quantitative approach that is sensitive enough to detect the position of rare DNA strand breaks in mouse germ cell-enriched testicular cell populations. The method can detect DNA breaks at any desired location in the genome but is not specific for DSBs-overhangs, nicks, or gaps with a free 3' OH group are also detected. The method was successfully used to compare testicular cells from mouse strains that possess or lack an active recombination hot spot at the H2-Ea gene. Breaks that were due to meiotic hot spot activity could be distinguished from the background of DNA breaks. This highly sensitive approach could be used to study other biological processes where rare DNA breaks are generated.

Cytological analysis of interference in mouse meiosis

In most eukaryotes, meiotic crossovers (COs) are non-randomly placed along the bivalents, such that the presence of a CO reduces the probability of additional COs nearby. This phenomenon, named CO interference, was originally defined genetically, but can also be analyzed cytologically by studying the chromosomal positions of protein complexes that are involved in CO formation, or by studying the positions of chiasmata. Here we focus on the cytological analysis of interference among protein complexes involved in meiotic recombination and CO formation in the mouse. During the pachytene stage of meiosis, these protein complexes can be visualized as immunofluorescent foci along synaptonemal complexes (SCs), which are linear protein structures that are formed along homologous chromosome pairs (bivalents) during meiotic prophase. We describe how to make cytological preparations that are suitable for the analysis of interference among these foci, and how to estimate the strength of interference among foci, using the gamma distribution as a mathematical model for focus/CO positioning.

Rhesus macaque embryos derived from MI oocytes maturing after retrieval display high rates of chromosomal anomalies

BACKGROUND

Rhesus macaque and human preimplantation embryos display similar rates of chromosomal abnormalities. The aim of this study was to determine whether embryos developing from MI oocytes that mature post-retrieval display more chromosomal anomalies than those embryos that are generated from oocytes that are at MII at the time of retrieval.

METHODS

Rhesus macaque oocytes were obtained after hormonal ovarian stimulation. Immediately after retrieval, the oocytes were classified according to their maturational status. Following in vitro fertilization, Day 3 embryos with good morphology and development derived from oocytes maturing post-retrieval and those from oocytes that were mature at the time of retrieval were cytogenetically assessed using a five-color fluorescent in situ fluorescent hybridization assay developed for rhesus macaque chromosomes homologous to human chromosomes 13, 16, 18, X and Y.

RESULTS

Blastomeres from 53 embryos were analyzed. Of the 27 embryos that developed from oocytes that were mature at collection, 18 embryos were chromosomally normal (66.7%), while from the 26 embryos that developed from oocytes that matured post-retrieval, only 9 embryos were chromosomally normal (34.6%).

CONCLUSIONS

These results indicate that embryos developing from oocytes maturing post-retrieval display high rates of chromosomal abnormalities and have therefore a reduced developmental competence. As a result, the clinical relevance of using immature oocytes that are retrieved after stimulated cycles in human IVF warrants further investigation.

MUS81 generates a subset of MLH1-MLH3-independent crossovers in mammalian meiosis

Two eukaryotic pathways for processing double-strand breaks (DSBs) as crossovers have been described, one dependent on the MutL homologs Mlh1 and Mlh3, and the other on the structure-specific endonuclease Mus81. Mammalian MUS81 has been implicated in maintenance of genomic stability in somatic cells; however, little is known about its role during meiosis. Mus81-deficient mice were originally reported as being viable and fertile, with normal meiotic progression; however, a more detailed examination of meiotic progression in Mus81-null animals and WT controls reveals significant meiotic defects in the mutants. These include smaller testis size, a depletion of mature epididymal sperm, significantly upregulated accumulation of MLH1 on chromosomes from pachytene meiocytes in an interference-independent fashion, and a subset of meiotic DSBs that fail to be repaired. Interestingly, chiasmata numbers in spermatocytes from Mus81^−/− animals are normal, suggesting additional integrated mechanisms controlling the two distinct crossover pathways. This study is the first in-depth analysis of meiotic progression in Mus81-nullizygous mice, and our results implicate the MUS81 pathway as a regulator of crossover frequency and placement in mammals.

Failure to undergo faithful meiotic chromosome segregation during mammalian meiosis can result in aneuploidy in the offspring and is a major cause of pregnancy loss and birth defects in humans. One essential component of meiotic prophase I is the exchange of genetic information between maternal and paternal chromosomes, known as recombination or crossing over, and is mediated, at least in part, by the mismatch repair proteins MSH4–MSH5 and MLH1–MLH3. A distinct subset of crossovers in lower organisms is generated by an alternate pathway involving Mus81 endonuclease. Previous studies into the impact of Mus81 mutations in mice revealed no adverse effect on the fertility of these animals. In this study, we report subtle, yet significant, defects in meiotic progression in male and female Mus81 mice, coupled with intriguing results showing that MUS81 protein is essential for crossover control in mammals. MUS81 appears to be required for correct localization of MLH1–MLH3 complexes to paired homologous chromosomes, however, not for the maintenance of physical crossovers, visualized as chiasmata. These results show a complex interplay between the MUS81 and MLH1–MLH3 pathways for generation of crossovers and, as such, are critical to the further understanding of the intricacies of crossover control with a view to reducing meiotic error rate in humans.

Relationship between physical and genetic distances along the zebra finch Z chromosome

Nine bacterial artificial chromosomes containing genes linked to the Z chromosome of the zebra finch (Taeniopygia guttata) were localized using FISH on synaptonemal complex spreads. Their positions were correlated with those previously reported on the mitotic Z chromosome, showing a linear relationship between positions along the mitotic chromosome and its synaptonemal complex. Distances in cM between the genes were calculated using a cytological map of the crossing-over based on the distribution of MLH1 foci along the ZZ synaptonemal complex (MLH1-cM map). It is shown that physical and genetic distances lack a linear relationship along most of the chromosome length, due to clustering of crossover events around the telomeres. This relationship departs strongly from that observed in the chicken Z chromosome and reflects the existence of different recombination rates and patterns among birds in spite of wide genomic conservation.

A high-resolution radiation hybrid map of rhesus macaque chromosome 5 identifies rearrangements in the genome assembly

A 10,000-rad radiation hybrid (RH) cell panel of the rhesus macaque was generated to construct a comprehensive RH map of chromosome 5. The map represents 218 markers typed in 185 RH clones. The 4846-cR map has an average marker spacing of 798 kb. Alignments of the RH map to macaque and human genome sequences confirm a large inversion and reveal a previously unreported telomeric inversion. The macaque genome sequence indicates small translocations from the ancestral homolog of macaque chromosome 5 to macaque chromosomes 1 and 6. The RH map suggests that these are probably assembly artifacts. Unlike the genome sequence, the RH mapping data indicate the conservation of synteny between macaque chromosome 5 and human chromosome 4. This study shows that the 10,000-rad panel is appropriate for the generation of a high-resolution whole-genome RH map suitable for the verification of the rhesus genome assembly.

Meiotic recombination at the ends of chromosomes in Saccharomyces cerevisiae

Meiotic reciprocal recombination (crossing over) was examined in the outermost 60-80 kb of almost all Saccharomyces cerevisiae chromosomes. These sequences included both repetitive gene-poor subtelomeric heterochromatin-like regions and their adjacent unique gene-rich euchromatin-like regions. Subtelomeric sequences underwent very little crossing over, exhibiting approximately two- to threefold fewer crossovers per kilobase of DNA than the genomic average. Surprisingly, the adjacent euchromatic regions underwent crossing over at twice the average genomic rate and contained at least nine new recombination "hot spots." These results prompted an analysis of existing genetic mapping data, which showed that meiotic reciprocal recombination rates were on average greater near chromosome ends exclusive of the subtelomeres. Thus, the distribution of crossovers in S. cerevisiae appears to resemble that found in several higher eukaryotes where the outermost chromosomal regions show increased crossing over.

General pattern of meiotic recombination in male dogs estimated by MLH1 and RAD51 immunolocalization

The aim of this study was to estimate a general pattern of meiotic recombination in the domestic dog (Canis familiaris) using immunolocalization of MLH1, a mismatch repair protein of mature recombination nodules. We prepared synaptonemal complex (SC) spreads from 124 spermatocytes of three male dogs and mapped 4959 MLH1 foci along 4712 autosomes. The mean number of MLH1 foci for all autosomes was 40.0 foci per cell. Total recombination length of the male dog autosomal genome map was estimated as 2000 cM. A global pattern of MLH1 foci distribution along the autosomal bivalents was rather similar to that found in the mammals studied: a high frequency near the telomeres and a low frequency near the centromeres. An obligate MLH1 focus in the X-Y pairing region was usually located very close to Xp-Yq telomeres. The distances between MLH1 foci at autosomal bivalents were consistent with crossover interference. A comparison of the interference estimates coming from the distribution of MLH1 interfocus distances and RAD51/MLH1 focus ratio indicated a substantial variation between species in the strength of interference.

ATM promotes the obligate XY crossover and both crossover control and chromosome axis integrity on autosomes

During meiosis in most sexually reproducing organisms, recombination forms crossovers between homologous maternal and paternal chromosomes and thereby promotes proper chromosome segregation at the first meiotic division. The number and distribution of crossovers are tightly controlled, but the factors that contribute to this control are poorly understood in most organisms, including mammals. Here we provide evidence that the ATM kinase or protein is essential for proper crossover formation in mouse spermatocytes. ATM deficiency causes multiple phenotypes in humans and mice, including gonadal atrophy. Mouse Atm^−/− spermatocytes undergo apoptosis at mid-prophase of meiosis I, but Atm^−/− meiotic phenotypes are partially rescued by Spo11 heterozygosity, such that ATM-deficient spermatocytes progress to meiotic metaphase I. Strikingly, Spo11^+/−Atm^−/− spermatocytes are defective in forming the obligate crossover on the sex chromosomes, even though the XY pair is usually incorporated in a sex body and is transcriptionally inactivated as in normal spermatocytes. The XY crossover defect correlates with the appearance of lagging chromosomes at metaphase I, which may trigger the extensive metaphase apoptosis that is observed in these cells. In addition, control of the number and distribution of crossovers on autosomes appears to be defective in the absence of ATM because there is an increase in the total number of MLH1 foci, which mark the sites of eventual crossover formation, and because interference between MLH1 foci is perturbed. The axes of autosomes exhibit structural defects that correlate with the positions of ongoing recombination. Together, these findings indicate that ATM plays a role in both crossover control and chromosome axis integrity and further suggests that ATM is important for coordinating these features of meiotic chromosome dynamics.

Meiosis is the specialized cell division that gives rise to reproductive cells such as sperm and eggs. During meiosis in most organisms, genetic information is exchanged between homologous maternal and paternal chromosomes through the process of homologous recombination. This recombination forms connections between homologous chromosomes that allow them to segregate accurately when the meiotic cell divides. Recombination defects can result in reproductive cells with abnormal chromosome numbers, which are a major cause of developmental disorders and spontaneous abortions in humans. Meiotic recombination is tightly controlled such that each pair of chromosomes undergoes at least one crossover recombination event despite a low average number of crossovers per chromosome. Moreover, multiple crossovers on the same chromosome tend to be evenly and widely spaced. Mechanisms of this control are not well understood, but here we provide evidence that ATM protein is required for normal operation of this process(es) in male mice. ATM has long been known to be involved in cellular responses to DNA damage. Our studies reveal a new function for this protein and also provide new insight into the mechanisms by which meiotic cells ensure accurate transmission of genetic material from one generation to the next.

Chromosomal abnormalities, meiotic behavior and fertility in domestic animals

Since the advent of the surface microspreading technique for synaptonemal complex analysis, increasing interest in describing the synapsis patterns of chromosome abnormalities associated with fertility of domestic animals has been noticed during the past three decades. In spite of the number of scientific reports describing the occurrence of structural chromosome abnormalities, their meiotic behavior and gametic products, little is known in domestic animal species about the functional effects of such chromosome aberrations in the germ cell line of carriers. However, some interesting facts gained from recent and previous studies on the meiotic behavior of chromosome abnormalities of domestic animals permit us to discuss, in the frame of recent knowledge emerging from mouse and human investigations, the possible mechanism implicated in the well known association between meiotic disruption and chromosome pairing failure. New cytogenetic techniques, based on molecular and immunofluorescent analyses, are allowing a better description of meiotic processes, including gamete production. The present communication reviews the knowledge of the meiotic consequences of chromosome abnormalities in domestic animals.

Chromosomal instability in rhesus macaque preimplantation embryos

OBJECTIVE

To establish a relevant animal model to systematically investigate chromosomal instability in human oocytes and preimplantation embryos.

DESIGN

Prospective rhesus monkey IVF study.

SETTING

Academic laboratory, Oregon National Primate Research Center and Caribbean Primate Research Center.

ANIMAL(S)

Young rhesus macaque females.

INTERVENTION(S)

In vitro produced entire rhesus macaque preimplantation embryos were cytogenetically assessed using a five-color fluorescent in situ hybridization assay developed for rhesus macaque chromosomes homologous to human chromosomes 13, 16, 18, X, and Y, using human bacterial artificial chromosome probes.

MAIN OUTCOME MEASURE(S)

Chromosomal abnormality rates in preimplantation embryos from young rhesus macaque females were established.

RESULT(S)

Fifty preimplantation embryos, displaying good morphology and normal development, were analyzed from 11 young rhesus macaque females. Overall, 27 embryos (54%) were normal, 11 embryos (22%) mosaic, 3 embryos (6%) chaotic, 2 embryos (4%) aneuploid, 3 embryos (6%) haploid, and 4 embryos (8%) triploid.

CONCLUSION(S)

These data indicate that in vitro produced rhesus macaque and human preimplantation embryos exhibit similar numerical chromosomal aberrations. Rhesus macaques appear to be a suitable animal model for investigating the origin of chromosomal instability observed in human preimplantation embryos.

Recombination map of the common shrew, Sorex araneus (Eulipotyphla, Mammalia)

The Eurasian common shrew (Sorex araneus L.) is characterized by spectacular chromosomal variation, both autosomal variation of the Robertsonian type and an XX/XY(1)Y(2) system of sex determination. It is an important mammalian model of chromosomal and genome evolution as it is one of the few species with a complete genome sequence. Here we generate a high-precision cytological recombination map for the species, the third such map produced in mammals, following those for humans and house mice. We prepared synaptonemal complex (SC) spreads of meiotic chromosomes from 638 spermatocytes of 22 males of nine different Robertsonian karyotypes, identifying each autosome arm by differential DAPI staining. Altogether we mapped 13,983 recombination sites along 7095 individual autosomes, using immunolocalization of MLH1, a mismatch repair protein marking recombination sites. We estimated the total recombination length of the shrew genome as 1145 cM. The majority of bivalents showed a high recombination frequency near the telomeres and a low frequency near the centromeres. The distances between MLH1 foci were consistent with crossover interference both within chromosome arms and across the centromere in metacentric bivalents. The pattern of recombination along a chromosome arm was a function of its length, interference, and centromere and telomere effects. The specific DNA sequence must also be important because chromosome arms of the same length differed substantially in their recombination pattern. These features of recombination show great similarity with humans and mice and suggest generality among mammals. However, contrary to a widespread perception, the metacentric bivalent tu usually lacked an MLH1 focus on one of its chromosome arms, arguing against a minimum requirement of one chiasma per chromosome arm for correct segregation. With regard to autosomal chromosomal variation, the chromosomes showing Robertsonian polymorphism display MLH1 foci that become increasingly distal when comparing acrocentric homozygotes, heterozygotes, and metacentric homozygotes. Within the sex trivalent XY(1)Y(2), the autosomal part of the complex behaves similarly to other autosomes.

Localization of single-copy sequences on chicken synaptonemal complex spreads using fluorescence in situ hybridization (FISH)

Synaptonemal complex (SC) spreads from bird oocytes and spermatocytes show the complete chromosome complement and can be observed at the light microscope using immunostaining of the proteins that compose the lateral elements. To investigate the use of avian SC spreads as substrates for fluorescent in situ hybridization (FISH) in combination with immunostaining, we applied two single-copy sequences to chicken oocyte spreads. Signals for both target sequences were consistently observed on the short arm of bivalent 1 in a large number of nuclei. Based on previous data about the size of chromosome 1 and from measurements on probed SC spreads, an estimate of the physical distance in Mb between each sequence and the telomere was calculated. The crossover frequencies along SC 1 obtained by immunolocalization of MLH1 foci during pachytene were used to calculate the distances in cM to the target sequences and to compare this cytogenetic SC map with the consensus linkage map for GGA1. The combination of SC-FISH and immunostaining could be generally applied to obtain high-resolution mapping of single-copy sequences in birds and, coupled with MLH1 crossover maps, it could be a reliable approach to obtain genetic distances between markers to test the genetic linkage maps generated from molecular markers.

Initiation and resolution of interhomolog connections: crossover and non-crossover sites along mouse synaptonemal complexes

Programmed double-strand breaks at prophase of meiosis acquire immunologically detectable RAD51-DMC1 foci or early nodules (ENs) that are associated with developing chromosome core segments; each focus is surrounded by a gammaH2AX-modified chromosome domain. The 250-300 ENs per nucleus decline in numbers during the development of full-length cores and the remaining foci are relatively evenly distributed along the mature cores (gamma distribution of nu=2.97). The ENs become transformed nodules (TNs) by the acquisition of RPA, BLM, MSH4 and topoisomerases that function in repair and Holliday junction resolution. At the leptotene-zygotene transition, TNs orient to positions between the aligned cores where they initiate structural interhomolog contacts prior to synaptonemal complex (SC) formation, possibly future crossover sites. Subsequently, TNs are associated with SC extension at the synaptic forks. Dephosphorylation of TN-associated histone gammaH2AX chromatin suggests annealing of single strands or repair of double-strand breaks DSBs at this time. Some 200 TNs per pachytene nucleus are distributed proportional to SC length and are evenly distributed along the SCs (nu= approximately 4). At this stage, gammaH2AX-modified chromatin domains are associated with transcriptionally silenced sex chromosomes and autosomal sites. Immunogold electron microscope evidence shows that one or two TNs of the 10-15 TNs per SC acquire MLH1 protein, the hallmark of reciprocal recombination, whereas the TNs that do not acquire MLH1 protein relocate from their positions along the midline of the SCs to the periphery of the SCs. Relocation of TNs may be associated with the conversion of potential crossovers into non-crossovers.

Regulating double-stranded DNA break repair towards crossover or non-crossover during mammalian meiosis

During meiosis the programmed induction of DNA double-stranded breaks (DSB) leads to crossover (CO) and non-crossover products (NCO). One key role of CO is to connect homologs before metaphase I and thus to ensure the proper reductional segregation. This role implies an accurate regulation of CO frequency with the establishment of at least one CO per chromosome arm. Current major challenges are to understand how CO and NCO formation are regulated and what is the role of NCO. We present here the current knowledge about CO and NCO and their regulation in mammals. CO density varies widely along chromosomes and their distribution is not random as they are subject to positive interference. As documented in the mouse and human, a significant excess of DSB are generated relative to the number of CO. In fact, evidence has been obtained for the formation of NCO products, for regulation of the choice of DSB repair towards CO or NCO and for a CO specific pathway. We discuss the roles of Msh4, Msh5 and Sycp1 which affect DSB repair and probably not only the CO pathway. We suggest that, in mammals, the regulation of NCO differs from that described in Saccharomyces cerevisiae.

Sex chromosomes of basal placental mammals

Placental (eutherian) mammals are currently classified into four superordinal clades (Afrotheria, Xenarthra, Laurasiatheria and Supraprimates) of which one, the Afrotheria (a unique lineage of African origin), is generally considered to be basal. Therefore, Afrotheria provide a pivotal evolutionary link for studying fundamental differences between the sex chromosomes of human/mouse (both representatives of Supraprimates and the index species for studies of sex chromosomes) and those of the distantly related marsupials. In this study, we use female fibroblasts to investigate classical features of X chromosome inactivation including replication timing of the X chromosomes and Barr body formation. We also examine LINE-1 accumulation on the X chromosomes of representative afrotherians and look for evidence of a pseudoautosomal region (PAR). Our results demonstrate that asynchronous replication of the X chromosomes is common to Afrotheria, as with other mammals, and Barr body formation is observed across all Placentalia, suggesting that mechanisms controlling this evolved before their radiation. Finally, we provide evidence of a PAR (which marsupials lack) and demonstrate that LINE1 is accumulated on the afrotherian and xenarthran X, although this is probably not due to transposition events in a common ancestor, but rather ongoing selection to retain recently inserted LINE1 on the X.

Predicting and testing physical locations of genetically mapped loci on tomato pachytene chromosome 1

Predicting the chromosomal location of mapped markers has been difficult because linkage maps do not reveal differences in crossover frequencies along the physical structure of chromosomes. Here we combine a physical crossover map based on the distribution of recombination nodules (RNs) on Solanum lycopersicum (tomato) synaptonemal complex 1 with a molecular genetic linkage map from the interspecific hybrid S. lycopersicum x S. pennellii to predict the physical locations of 17 mapped loci on tomato pachytene chromosome 1. Except for one marker located in heterochromatin, the predicted locations agree well with the observed locations determined by fluorescence in situ hybridization. One advantage of this approach is that once the RN distribution has been determined, the chromosomal location of any mapped locus (current or future) can be predicted with a high level of confidence.

Sex-specific crossover distributions and variations in interference level along Arabidopsis thaliana chromosome 4

In many species, sex-related differences in crossover (CO) rates have been described at chromosomal and regional levels. In this study, we determined the CO distribution along the entire Arabidopsis thaliana Chromosome 4 (18 Mb) in male and female meiosis, using high density genetic maps built on large backcross populations (44 markers, >1,300 plants). We observed dramatic differences between male and female map lengths that were calculated as 88 cM and 52 cM, respectively. This difference is remarkably parallel to that between the total synaptonemal complex lengths measured in male and female meiocytes by immunolabeling of ZYP1 (a component of the synaptonemal complex). Moreover, CO landscapes were clearly different: in particular, at both ends of the map, male CO rates were higher (up to 4-fold the mean value), whereas female CO rates were equal or even below the chromosomal average. This unique material gave us the opportunity to perform a detailed analysis of CO interference on Chromosome 4 in male and female meiosis. The number of COs per chromosome and the distances between them clearly departs from randomness. Strikingly, the interference level (measured by coincidence) varied significantly along the chromosome in male meiosis and was correlated to the physical distance between COs. The significance of this finding on the relevance of current CO interference models is discussed.

Meiotic crossovers between homologous chromosomes ensure their proper segregation to generate ultimately gametes. They also create new allelic combinations which contribute to the diversity of traits among individuals. In all eukaryotes, the number and the localization of crossovers along chromosomes are not random. In addition, crossovers are not independent of each other: the occurrence of a crossover lowers the probability that another crossover arises in its vicinity. The mechanism of this phenomenon, called “crossover interference,” is one of the most challenging puzzles that geneticists have been faced with in the last century. In this paper, we precisely described the distribution of crossovers along Chromosome 4 of the model plant species Arabidopsis thaliana, separately in male and female meiosis. Interestingly, we observed that crossovers are 1.7 more numerous in male than in female meiosis, and this increase is especially marked at the ends of the chromosome. Moreover, our results provide the first evidence that the level of interference along a chromosome is not a constant and is correlated with the physical distance between crossovers. These results shed new light on the determinism of crossover localization and could have important outcomes on the relevance of current models of crossover interference.

Patterns of recombination and MLH1 foci density along mouse chromosomes: modeling effects of interference and obligate chiasma

Crossover interference in meiosis is often modeled via stationary renewal processes. Here we consider a new model to incorporate the known biological feature of "obligate chiasma" whereby in most organisms each bivalent almost always has at least one crossover. The initial crossover is modeled as uniformly distributed along the chromosome, and starting from its position, subsequent crossovers are placed with forward and backward stationary renewal processes using a chi-square distribution of intercrossover distances. We used our model as well as the standard chi-square model to simulate the patterns of crossover densities along bivalents or chromatids for those having zero, one, two, or three or more crossovers; indeed, such patterns depend on the number of crossovers. With both models, simulated patterns compare very well to those found experimentally in mice, both for MLH1 foci on bivalents and for crossovers on genetic maps. However, our model provides a better fit to experimental data as compared to the standard chi-square model, particularly regarding the distribution of numbers of crossovers per chromosome. Finally, our model predicts an enhancement of the recombination rate near the extremities, which, however, explains only a part of the pattern observed in mouse.

Development of a non-human primate sperm aneuploidy assay tested in the rhesus macaque (Macaca mulatta)

Numerical chromosome aberrations in germ cells are important factors contributing to abnormal reproductive outcomes. Fluorescence in situ hybridization onto spermatozoa (sperm-FISH) has allowed the study of the influence of a wide range of biological factors and chemical exposure on aneuploidy incidences in human sperm as well as in mouse and rat animal models. The assay presented here extends the applicability of the sperm-FISH method to non-human primates and was tested in the prevalent model species, the rhesus macaque. The assay provides probes for macaque chromosomes 17, 18, 19, 20, X and Y, the homologues of human chromosomes 13, 18, 19, 16, X and Y, respectively. The analysis of 11 000 spermatozoa each from five individuals revealed spontaneous sex chromosomal disomy frequencies (X: 0.08%; Y: 0.09%) and an average autosomal disomy frequency (0.03%) coinciding with some of the lowest incidences scored in human studies. The non-human primate sperm-FISH assay provides a fast and efficient tool complementing the available analysis methods in non-human primate exposure studies. Since the assay employs large locus-specific FISH probes representing evolutionary conserved DNA sequences, it can be expected that the assay is also applicable to other cercopithecoid and hominoid non-human primate species.

Gender effects on the incidence of aneuploidy in mammalian germ cells

Aneuploidy occurs in 0.3% of newborns, 4% of stillbirths, and more than 35% of all human spontaneous abortions. Human gametogenesis is uniquely and gender-specific susceptible to errors in chromosome segregation. Overall, between 1% and 4% of sperm and as many as 20% of human oocytes have been estimated by molecular cytogenetic analysis to be aneuploid. Maternal age remains the paramount aetiological factor associated with human aneuploidy. The majority of extra chromosomes in trisomic offspring appears to be of maternal origin resulting from nondisjunction of homologous chromosomes during the first meiotic division. Differences in the recombination patterns between male and female meiosis may partly account for the striking gender- and chromosome-specific differences in the genesis of human aneuploidy, especially in aged oocytes. Nondisjunction of entire chromosomes during meiosis I as well as premature separation of sister chromatids or homologues prior to meiotic anaphase can contribute to aneuploidy. During meiosis, checkpoints at meiotic prophase and the spindle checkpoint at M-phase can induce meiotic arrest and/or cell death in case of disturbances in pairing/recombination or spindle attachment of chromosomes. It has been suggested that gender differences in aneuploidy may result from more permissive checkpoints in females than males. Furthermore, age-related loss of chromosome cohesion in oocytes as a cause of aneuploidy may be female-specific. Comparative data about the susceptibility of human male and female germ cells to aneuploidy-causing chemicals is lacking. Increases of aneuploidy frequency in sperm have been shown after exposure to therapeutic drugs, occupational agents and lifestyle factors. Conversely, data on oocyte aneuploidy caused by exogenous agents is limited because of the small numbers of oocytes available for analysis combined with potential maternal age effects. The vast majority of animal studies on aneuploidy induction in germ cells represent cause and effect data. Specific studies designed to evaluate possible gender differences in induction of germ cell aneuploidy have not been found. However, the comparison of rodent data available from different laboratories suggests that oocytes are more sensitive than male germ cells when exposed to chemicals that effect the meiotic spindle. Only recently, in vitro experiments, analyses of transgenic animals and knockdown of expression of meiotic genes have started to address the molecular mechanisms underlying chromosome missegregation in mammalian germ cells whereby striking differences between genders could be shown. Such information is needed to clarify the extent and the mechanisms of gender effects, including possible differential susceptibility to environmental agents.