Preaching about the converted: how meiotic gene conversion influences genomic diversity

Meiotic crossover (CO) recombination involves a reciprocal exchange between homologous chromosomes. COs are often associated with gene conversion at the exchange site where genetic information is unidirectionally transferred from one chromosome to the other. COs and independent assortment of homologous chromosomes contribute significantly to the promotion of genomic diversity. What has not been appreciated is the contribution of another product of meiotic recombination, noncrossovers (NCOs), which result in gene conversion without exchange of flanking markers. Here, we review our comprehensive analysis of recombination at a highly polymorphic mouse hotspot. We found that NCOs make up ∼90% of recombination events. Preferential recombination initiation on one chromosome allowed us to estimate the contribution of CO and NCO gene conversion to transmission distortion, a deviation from Mendelian inheritance in the population. While NCO gene conversion tracts are shorter, and thus have a more punctate effect, their higher frequency translates into an approximately two-fold greater contribution than COs to gene conversion-based allelic shuffling and transmission distortion. We discuss the potential impact of mammalian NCO characteristics on evolution and genomic diversity.

The joy of six: how to control your crossovers

Meiotic cells tightly regulate the number and distribution of crossovers to promote accurate chromosome segregation. Yokoo and colleagues uncover a metazoan-specific, cyclin-like protein that is crucial for crossover formation. They utilize this protein's unique properties to explore a remarkable example of biological numerology, whereby nearly every meiotic cell in C. elegans makes precisely six crossovers, one for each of its six chromosome pairs.

Scale matters: the spatial correlation of yeast meiotic DNA breaks with histone H3 trimethylation is driven largely by independent colocalization at promoters

During meiosis in many organisms, homologous chromosomes engage in numerous recombination events initiated by DNA double-strand breaks (DSBs) formed by the Spo11 protein. DSBs are distributed nonrandomly, which governs how recombination influences inheritance and genome evolution. The chromosomal features that shape DSB distribution are not well understood. In the budding yeast Saccharomyces cerevisiae, trimethylation of lysine 4 of histone H3 (H3K4me3) has been suggested to play a causal role in targeting Spo11 activity to small regions of preferred DSB formation called hotspots. The link between H3K4me3 and DSBs is supported in part by a genome-wide spatial correlation between the two. However, this correlation has only been evaluated using relatively low-resolution maps of DSBs, H3K4me3 or both. These maps illuminate chromosomal features that influence DSB distributions on a large scale (several kb and greater) but do not adequately resolve features, such as chromatin structure, that act on finer scales (kb and shorter). Using recent nucleotide-resolution maps of DSBs and meiotic chromatin structure, we find that the previously described spatial correlation between H3K4me3 and DSB hotspots is principally attributable to coincident localization of both to gene promoters. Once proximity to the nucleosome-depleted regions in promoters is accounted for, H3K4me3 status has only modest predictive power for determining DSB frequency or location. This analysis provides a cautionary tale about the importance of scale in genome-wide analyses of DSB and recombination patterns.

Homeostatic control of recombination is implemented progressively in mouse meiosis

Humans suffer from high rates of fetal aneuploidy, often arising from the absence of meiotic crossover recombination between homologous chromosomes. Meiotic recombination is initiated by double-strand breaks (DSBs) generated by the SPO11 transesterase. In yeast and worms, at least one buffering mechanism, crossover homeostasis, maintains crossover numbers despite variation in DSB numbers. We show here that mammals exhibit progressive homeostatic control of recombination. In wild-type mouse spermatocytes, focus numbers for early recombination proteins (RAD51, DMC1) were highly variable from cell to cell, whereas foci of the crossover marker MLH1 showed little variability. Furthermore, mice with greater or fewer copies of the Spo11 gene--with correspondingly greater or fewer numbers of early recombination foci--exhibited relatively invariant crossover numbers. Homeostatic control is enforced during at least two stages, after the formation of early recombination intermediates and later while these intermediates mature towards crossovers. Thus, variability within the mammalian meiotic program is robustly managed by homeostatic mechanisms to control crossover formation, probably to suppress aneuploidy. Meiotic recombination exemplifies how order can be progressively implemented in a self-organizing system despite natural cell-to-cell disparities in the underlying biochemical processes.

Expression of arf tumor suppressor in spermatogonia facilitates meiotic progression in male germ cells

The mammalian Cdkn2a (Ink4a-Arf) locus encodes two tumor suppressor proteins (p16^Ink4a and p19^Arf) that respectively enforce the anti-proliferative functions of the retinoblastoma protein (Rb) and the p53 transcription factor in response to oncogenic stress. Although p19^Arf is not normally detected in tissues of young adult mice, a notable exception occurs in the male germ line, where Arf is expressed in spermatogonia, but not in meiotic spermatocytes arising from them. Unlike other contexts in which the induction of Arf potently inhibits cell proliferation, expression of p19^Arf in spermatogonia does not interfere with mitotic cell division. Instead, inactivation of Arf triggers germ cell–autonomous, p53-dependent apoptosis of primary spermatocytes in late meiotic prophase, resulting in reduced sperm production. Arf deficiency also causes premature, elevated, and persistent accumulation of the phosphorylated histone variant H2AX, reduces numbers of chromosome-associated complexes of Rad51 and Dmc1 recombinases during meiotic prophase, and yields incompletely synapsed autosomes during pachynema. Inactivation of Ink4a increases the fraction of spermatogonia in S-phase and restores sperm numbers in Ink4a-Arf doubly deficient mice but does not abrogate γ-H2AX accumulation in spermatocytes or p53-dependent apoptosis resulting from Arf inactivation. Thus, as opposed to its canonical role as a tumor suppressor in inducing p53-dependent senescence or apoptosis, Arf expression in spermatogonia instead initiates a salutary feed-forward program that prevents p53-dependent apoptosis, contributing to the survival of meiotic male germ cells.

The intimately linked Arf and Ink4a genes, encoded in part by overlapping reading frames within the Cdkn2a locus, are induced by oncogenic stress, activating the p53 and Rb tumor suppressors, respectively, to inhibit proliferation of incipient cancer cells. As such, expression of the p19^Arf and p16^Ink4a proteins is undetected in most normal mouse tissues. However, p19^Arf is physiologically expressed in mitotically dividing spermatogonia, the progenitor cells that differentiate to form meiotic spermatocytes in which Arf expression is extinguished. We show that, instead of provoking cell cycle arrest or death, Arf expression in spermatogonia facilitates survival of their meiotic progeny, ensuring production of normal numbers of mature sperm. When Arf is ablated, meiotic defects ensue, along with p53-dependent cell death of spermatocytes, indicating an unexpected role of p53 in monitoring meiotic progression. Surprisingly, it is the absence of p19^Arf rather than its induction that enforces p53 expression in this setting. Co-inactivation of Ink4a compensates for Arf loss by fueling proliferation of spermatogonial progenitors, but does not correct meiotic defects triggered by Arf loss. Although the Arf and Ink4a tumor suppressors are expected to restrain cellular self-renewal, Arf plays an unexpected role in male germ cells by facilitating their proper meiotic progression.

The E3 ubiquitin ligase Cullin 4A regulates meiotic progression in mouse spermatogenesis

The Cullin-RING ubiquitin-ligase CRL4 controls cell cycle and DNA damage checkpoint response and ensures genomic integrity. Inactivation of the Cul4 component of the CRL4 E3 ligase complex in Caenorhabditis elegans by RNA interference results in massive mitotic DNA re-replication in the blast cells, largely due to failed degradation of the DNA licensing protein, CDT-1, and premature spermatogenesis. Here we show that inactivation of Cul4a by gene-targeting in mice only affected male but not female fertility. This male infertility phenotype resulted from a combination of decreased spermatozoa number, reduced sperm motility and defective acrosome formation. Agenesis of the mutant germ cells was accompanied by increased cell death in pachytene/diplotene cells with markedly elevated levels of phospho-p53 and CDT-1. Despite apparent normal assembly of synaptonemal complexes and DNA double strand break repair, dissociation of MLH1, a component of the late recombination nodule, was delayed in Cul4a -/- diplotene spermatocytes, which potentially led to subsequent disruptions in meiosis II and spermiogenesis. Together, our study revealed an indispensable role for Cul4a during male germ cell meiosis.

Attitudes to cancer and cancer prevention: what do people aged 35-54 years think?

The aim of this study was to explore the knowledge, attitudes and behaviours of people in mid-life towards cancer prevention. The study was undertaken in Northern Ireland between 2003 and 2007. This was a mixed methods study using a sequential exploratory design. The theoretical framework was the Theory of Planned Behaviour and the methodology was based on Sutton's framework. There were three methodological stages in the study using focus groups, a large cross-sectional survey and a volunteer sample survey. This paper focuses on the findings of the cross-sectional survey relating to the attitudes of people in mid-life towards cancer and cancer prevention. Findings are considered in relation to the respondents' level of knowledge, age, gender, level of educational attainment and socio-economic status. Evidence from this study shows that attitudes towards cancer and cancer prevention are associated significantly with level of knowledge about cancer, gender, socio-economic status and level of educational attainment. In conclusion, the evidence from this study shows that men, those with a lower level of education, those with a lower level of knowledge and those in a lower socio-economic group were more likely to hold negative attitudes about cancer and cancer prevention.

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.

A hierarchical combination of factors shapes the genome-wide topography of yeast meiotic recombination initiation

The nonrandom distribution of meiotic recombination influences patterns of inheritance and genome evolution, but chromosomal features governing this distribution are poorly understood. Formation of the DNA double-strand breaks (DSBs) that initiate recombination results in the accumulation of Spo11 protein covalently bound to small DNA fragments. By sequencing these fragments, we uncover a genome-wide DSB map of unprecedented resolution and sensitivity. We use this map to explore how DSB distribution is influenced by large-scale chromosome structures, chromatin, transcription factors, and local sequence composition. Our analysis offers mechanistic insight into DSB formation and early processing steps, supporting the view that the recombination terrain is molded by combinatorial and hierarchical interaction of factors that work on widely different size scales. This map illuminates the occurrence of DSBs in repetitive DNA elements, repair of which can lead to chromosomal rearrangements. We also discuss implications for evolutionary dynamics of recombination hot spots.

Molecular basis for enhancement of the meiotic DMC1 recombinase by RAD51 associated protein 1 (RAD51AP1)

Homologous recombination is needed for meiotic chromosome segregation, genome maintenance, and tumor suppression. RAD51AP1 (RAD51 associated protein 1) has been shown to interact with and enhance the recombinase activity of RAD51. Accordingly, genetic ablation of RAD51AP1 leads to enhanced sensitivity to and also chromosome aberrations upon DNA damage, demonstrating a role for RAD51AP1 in mitotic homologous recombination. Here we show physical association of RAD51AP1 with the meiosis-specific recombinase DMC1 and a stimulatory effect of RAD51AP1 on the DMC1-mediated D-loop reaction. Mechanistic studies have revealed that RAD51AP1 enhances the ability of the DMC1 presynaptic filament to capture the duplex-DNA partner and to assemble the synaptic complex, in which the recombining DNA strands are homologously aligned. We also provide evidence that functional cooperation is dependent on complex formation between DMC1 and RAD51AP1 and that distinct epitopes in RAD51AP1 mediate interactions with RAD51 and DMC1. Finally, we show that RAD51AP1 is expressed in mouse testes, and that RAD51AP1 foci colocalize with a subset of DMC1 foci in spermatocytes. These results suggest that RAD51AP1 also serves an important role in meiotic homologous recombination.

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.

Evolutionary conservation of meiotic DSB proteins: more than just Spo11

Meiotic recombination is initiated by programmed DNA double-strand breaks (DSBs) generated by the Spo11 protein. In budding yeast, five other meiotic-specific proteins are also required for DSB formation, but, with rare exception, orthologs had not been identified in other species. In this issue of Genes & Development, Kumar and colleagues (pp. 1266-1280) used a phylogenomic approach to identify two of these proteins across multiple clades, and confirmed that one of these, MEI4, is a functional ortholog in mouse.

DNA polymerase beta is critical for mouse meiotic synapsis

We have shown earlier that DNA polymerase beta (Pol beta) localizes to the synaptonemal complex (SC) during Prophase I of meiosis in mice. Pol beta localizes to synapsed axes during zygonema and pachynema, and it associates with the ends of bivalents during late pachynema and diplonema. To test whether these localization patterns reflect a function for Pol beta in recombination and/or synapsis, we used conditional gene targeting to delete the PolB gene from germ cells. We find that Pol beta-deficient spermatocytes are defective in meiotic chromosome synapsis and undergo apoptosis during Prophase I. We also find that SPO11-dependent gammaH2AX persists on meiotic chromatin, indicating that Pol beta is critical for the repair of SPO11-induced double-strand breaks (DSBs). Pol beta-deficient spermatocytes yielded reduced steady-state levels of the SPO11-oligonucleotide complexes that are formed when SPO11 is removed from the ends of DSBs, and cytological experiments revealed that chromosome-associated foci of replication protein A (RPA), RAD51 and DMC1 are less abundant in Pol beta-deficient spermatocyte nuclei. Localization of Pol beta to meiotic chromosomes requires the formation of SPO11-dependent DSBs. Taken together, these findings strongly indicate that Pol beta is required at a very early step in the processing of meiotic DSBs, at or before the removal of SPO11 from DSB ends and the generation of the 3' single-stranded tails necessary for subsequent strand exchange. The chromosome synapsis defects and Prophase I apoptosis of Pol beta-deficient spermatocytes are likely a direct consequence of these recombination defects.

Mouse HORMAD1 and HORMAD2, two conserved meiotic chromosomal proteins, are depleted from synapsed chromosome axes with the help of TRIP13 AAA-ATPase

Meiotic crossovers are produced when programmed double-strand breaks (DSBs) are repaired by recombination from homologous chromosomes (homologues). In a wide variety of organisms, meiotic HORMA-domain proteins are required to direct DSB repair towards homologues. This inter-homologue bias is required for efficient homology search, homologue alignment, and crossover formation. HORMA-domain proteins are also implicated in other processes related to crossover formation, including DSB formation, inhibition of promiscuous formation of the synaptonemal complex (SC), and the meiotic prophase checkpoint that monitors both DSB processing and SCs. We examined the behavior of two previously uncharacterized meiosis-specific mouse HORMA-domain proteins—HORMAD1 and HORMAD2—in wild-type mice and in mutants defective in DSB processing or SC formation. HORMADs are preferentially associated with unsynapsed chromosome axes throughout meiotic prophase. We observe a strong negative correlation between SC formation and presence of HORMADs on axes, and a positive correlation between the presumptive sites of high checkpoint-kinase ATR activity and hyper-accumulation of HORMADs on axes. HORMADs are not depleted from chromosomes in mutants that lack SCs. In contrast, DSB formation and DSB repair are not absolutely required for depletion of HORMADs from synapsed axes. A simple interpretation of these findings is that SC formation directly or indirectly promotes depletion of HORMADs from chromosome axes. We also find that TRIP13 protein is required for reciprocal distribution of HORMADs and the SYCP1/SC-component along chromosome axes. Similarities in mouse and budding yeast meiosis suggest that TRIP13/Pch2 proteins have a conserved role in establishing mutually exclusive HORMAD-rich and synapsed chromatin domains in both mouse and yeast. Taken together, our observations raise the possibility that involvement of meiotic HORMA-domain proteins in the regulation of homologue interactions is conserved in mammals.

Generation of haploid gametes in most organisms requires that homologues become connected via crossovers during meiosis. Efficient formation of crossovers depends on HORMA-domain proteins in diverse taxa. These proteins ensure that programmed meiotic DSBs are preferentially repaired from homologues, rather than from sister chromatids. This inter-homologue bias is crucial for homology search and crossovers formation. HORMA-domain proteins have been also implicated in DSB formation, in suppression of synaptonemal complex formation between non-homologous chromosomes, and in the meiotic prophase checkpoint that monitors DSB repair. Despite the importance of HORMA-domain proteins in various organisms, a role for these proteins in mammalian meiosis hasn't been reported. We examined the behaviour of meiotic mouse HORMA-domain proteins—HORMAD1 and HORMAD2—in wild-type and meiotic mutants. HORMAD1/2 preferentially accumulate on unsynapsed chromosome axes. Our data suggest that HORMAD1/2 depletion from chromosomes is a response to synaptonemal complex formation and it that is a conserved process supported by TRIP13/Pch2 AAA-ATPase. Assuming that HORMA-domain functions are conserved in mammals, we speculate that depletion of HORMADs from axes might contribute to the down-regulation of inter-homologue bias and the prophase checkpoint once homology search is completed and synaptonemal complexes form between aligned homologues.

Gel electrophoresis assays for analyzing DNA double-strand breaks in Saccharomyces cerevisiae at various spatial resolutions

Meiotic recombination is triggered by programmed DNA double-strand breaks (DSBs), which are catalyzed by Spo11 protein in a type II topoisomerase-like manner. Meiotic DSBs can be detected directly using physical assays (gel electrophoresis, Southern blotting, and indirect end-labeling) applied to samples of genomic DNA from sporulating cultures of budding and fission yeast. Such assays are extremely useful for quantifying and characterizing many aspects of the initiation of meiotic recombination, including the timing of DSB formation relative to other events, the distribution of DSBs across the genome, and the influence on DSB formation of mutations in recombination factors and other gene products. By varying the type of gel electrophoresis and other parameters, the spatial resolution of DSB analysis can range from single nucleotides up to whole yeast chromosomes.

Probing meiotic recombination decisions

Meiotic recombination promotes genetic variation by mixing parental alleles. Two recent studies, one in this issue of Developmental Cell, have applied microarray-based methods that allow analysis of nearly all of the recombination events occurring in a single meiosis. These data provide insights into the molecular "decisions" that control the outcome of the recombination process.

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.

C1584 in von Willebrand factor is necessary for enhanced proteolysis by ADAMTS13 in vitro

The cysteine variant of the amino acid change tyrosine/cysteine 1584 (Y/C1584) in von Willebrand factor (VWF) has previously been shown to cosegregate with increased susceptibility of VWF to proteolysis by ADAMTS13. It is not known whether C1584 itself confers increased proteolysis or is linked to a causative change elsewhere in VWF. To address whether C1584 underlies enhanced susceptibility of VWF to ADAMTS13-mediated proteolysis, a single family comprising two heterozygous Y/C1584 individuals and four homozygous Y/Y1584 individuals was investigated. The essential regions of the VWF gene were sequenced in all six individuals and ADAMTS13-mediated proteolysis of plasma VWF was assessed for each individual. Comparison of the VWF coding sequences for the Y/C1584 individuals with those for the Y/Y1584 individuals revealed that two amino acid variants were unique to the heterozygotes: R484 and C1584. The plasma VWF of the two heterozygotes showed increased susceptibility to proteolysis in vitro compared with that of the four homozygotes. In the present study we demonstrate that R484, in the absence of C1584, does not influence VWF proteolysis. Enhanced proteolysis occurred only in the presence of Cys1584. Thus, Cys1584 is necessary for increased susceptibility of VWF to proteolysis by ADAMTS13.

Spo11 and the Formation of DNA Double-Strand Breaks in Meiosis

Meiotic recombination is carried out through a specialized pathway for the formation and repair of DNA double-strand breaks made by the Spo11 protein, a relative of archaeal topoisomerase VI. This review summarizes recent studies that provide insight to the mechanism of DNA cleavage by Spo11, functional interactions of Spo11 with other proteins required for break formation, mechanisms that control the timing of recombination initiation, and evolutionary conservation and divergence of these processes.

Meiotic crossover hotspots contained in haplotype block boundaries of the mouse genome

Fertility requires successful chromosome segregation in meiosis, which in most sexual organisms depends on the formation of appropriately placed crossovers. The nonrandom genome-wide distributions of meiotic recombination events have been examined at the molecular level experimentally in yeast and by inference from linkage disequilibrium patterns in humans. Thus far, no method has existed for pinpointing sites of crossing-over on a genome-wide scale in an experimentally tractable animal whose genome size and complexity models that of humans. Here, we present a genomic approach to identify mouse crossover hotspots, based on targeting haplotype block boundaries. This represents a previously undescribed method potentially applicable to large-scale mouse hotspot identification. Using this method, we have successfully predicted the location of two previously uncharacterized crossover hotspots in male mice. As increasing amounts of single-nucleotide polymorphism data emerge, this approach will be useful for investigating the recombination landscape of the mouse genome.

Clinical supervision for mental health nurses in Northern Ireland: formulating best practice guidelines

Nurses work in a constantly challenging and changing environment. Within this context, there is a continuing need for support. Such support will help increase morale, decrease strain and burnout, and encourage self-awareness and self-expression. Clinical supervision address all these issues and enhances the quality of care for patients. While clinical supervision is a policy imperative in Northern Ireland, it was clear that there were problems in its implementation in mental health nursing. The aim of this project was to explore ways to make clinical supervision available to all mental health nurses and to improve and evaluate their contribution to patient care. The research team undertook a comprehensive literature review and a baseline survey of relevant stakeholders. Results represent the outcome of the group work. They will assist healthcare providers to develop local policies and procedures on clinical supervision for practising mental health nurses.