An essential role of DmRad51/SpnA in DNA repair and meiotic checkpoint control

Drosophila ATR in double-strand break repair

The ability of a cell to sense and respond to DNA damage is essential for genome stability. An important aspect of the response is arrest of the cell cycle, presumably to allow time for repair. Ataxia telangiectasia mutated (ATM) and ATR are essential for such cell-cycle control, but some observations suggest that they also play a direct role in DNA repair. The Drosophila ortholog of ATR, MEI-41, mediates the DNA damage-dependent G2-M checkpoint. We examined the role of MEI-41 in repair of double-strand breaks (DSBs) induced by P-element excision. We found that mei-41 mutants are defective in completing the later steps of homologous recombination repair, but have no defects in end-joining repair. We hypothesized that these repair defects are the result of loss of checkpoint control. To test this, we genetically reduced mitotic cyclin levels and also examined repair in grp (DmChk1) and lok (DmChk2) mutants. Our results suggest that a significant component of the repair defects is due to loss of MEI-41-dependent cell cycle regulation. However, this does not account for all of the defects we observed. We propose a novel role for MEI-41 in DSB repair, independent of the Chk1/Chk2-mediated checkpoint response.

Temporal analysis of meiotic DNA double-strand break formation and repair in Drosophila females

Using an antibody against the phosphorylated form of His2Av (γ-His2Av), we have described the time course for the series of events leading from the formation of a double-strand break (DSB) to a crossover in Drosophila female meiotic prophase. MEI-P22 is required for DSB formation and localizes to chromosomes prior to γ-His2Av foci. Drosophila females, however, are among the group of organisms where synaptonemal complex (SC) formation is not dependent on DSBs. In the absence of two SC proteins, C(3)G and C(2)M, the number of DSBs in oocytes is significantly reduced. This is consistent with the appearance of SC protein staining prior to γ-His2Av foci. However, SC formation is incomplete or absent in the neighboring nurse cells, and γ-His2Av foci appear with the same kinetics as in oocytes and do not depend on SC proteins. Thus, competence for DSB formation in nurse cells occurs with a specific timing that is independent of the SC, whereas in the oocytes, some SC proteins may have a regulatory role to counteract the effects of a negative regulator of DSB formation. The SC is not sufficient for DSB formation, however, since DSBs were absent from the heterochromatin even though SC formation occurs in these regions. All γ-His2Av foci disappear before the end of prophase, presumably as repair is completed and crossovers are formed. However, oocytes in early prophase exhibit a slower response to X-ray–induced DSBs compared to those in the late pachytene stage. Assuming all DSBs appear as γ-His2Av foci, there is at least a 3:1 ratio of noncrossover to crossover products. From a comparison of the frequency of γ-His2Av foci and crossovers, it appears that Drosophila females have only a weak mechanism to ensure a crossover in the presence of a low number of DSBs.

Meiosis is a specialized pair of cell divisions that creates haploid gametes by separating homologous chromosomes. Unlike most any other cell type, cells in meiotic prophase generate double-strand DNA breaks (DSBs) that are repaired using the homolog as a template. While there are several DSBs per chromosome, usually only one is repaired as a crossover, which is when the two homologs have exchanged large segments of genetic information. Each crossover is important because it creates a linkage that holds the homologs together during the first meiotic division. To learn more about how the meiotic cell regulates the formation of crossovers, the authors performed a temporal analysis of the events from break formation through repair into a crossover in Drosophila females. These results indicate that timing is a critical factor in both the formation and repair of DSBs. DSB formation occurs only during the earliest stages of meiotic prophase and initiates at a specific time after premeiotic DNA replication. Surprisingly, the response to DSBs is slower in the middle of meiotic prophase than at later time points. It is only during this time, mid–meiotic prophase, when the repair process is competent to produce crossovers.

Double-stranded DNA breaks and gene functions in recombination and meiosis

Meiotic prophase I is a long and complex phase. Homologous recombination is an important process that occurs between homologous chromosomes during meiotic prophase I. Formation of chiasmata, which hold homologous chromosomes together until the metaphase I to anaphase I transition, is critical for proper chromosome segregation. Recent studies have suggested that the SPO11 proteins have conserved functions in a number of organisms in generating sites of double-stranded DNA breaks (DSBs) that are thought to be the starting points of homologous recombination. Processing of these sites of DSBs requires the function of RecA homologs, such as RAD51, DMC1, and others, as suggested by mutant studies; thus the failure to repair these meiotic DSBs results in abnormal chromosomal alternations, leading to disrupted meiosis. Recent discoveries on the functions of these RecA homologs have improved the understanding of the mechanisms underlying meiotic homologous recombination.

Characterization of Drosophila Rad51/SpnA protein in DNA binding and embryonic development

The Rad51 is a highly conserved protein throughout the eukaryotic kingdom and an essential enzyme in DNA repair and recombination. It possesses DNA binding activity and ATPase activity, and interacts with meiotic chromosomes during prophase I of meiosis. Drosophila Rad51, Spindle-A (SpnA) protein has been shown to be involved in repair of DNA damage in somatic cells and meiotic recombination in female germ cells. In this study, DNA binding activity of SpnA is demonstrated by both agarose gel mobility shift assay and restriction enzyme protection assay. SpnA is also shown to interact with meiotic chromosomes during prophase I in the primary spermatocytes of hsp26-spnA transgenic flies. In addition, SpnA is highly expressed in embryos, and the depletion of SpnA by RNA interference (RNAi) leads to embryonic lethality implying that SpnA is involved in early embryonic development. Therefore, these results suggest that Drosophila SpnA protein possesses properties similar to mammalian Rad51 homologs.

Drosophila mus301/spindle-C encodes a helicase with an essential role in double-strand DNA break repair and meiotic progression

mus301 was identified independently in two genetic screens, one for mutants hypersensitive to chemical mutagens and another for maternal mutants with eggshell defects. mus301 is required for the proper specification of the oocyte and for progression through meiosis in the Drosophila ovary. We have cloned mus301 and show that it is a member of the Mus308 subfamily of ATP-dependent helicases and the closest homolog of human and mouse HEL308. Functional analyses demonstrate that Mus301 is involved in chromosome segregation in meiosis and in the repair of double-strand-DNA breaks in both meiotic and mitotic cells. Most of the oogenesis defects of mus301 mutants are suppressed by mutants in the checkpoint kinase Mei41 and in MeiW68, the Spo11 homolog that is thought to generate the dsDNA breaks that initiate recombination, indicating that these phenotypes are caused by activation of the DNA damage checkpoint in response to unrepaired Mei-W68-induced dsDNA breaks. However, neither mei-W68 nor mei-41 rescue the defects in oocyte specification of mus301 mutants, suggesting that this helicase has another function in oocyte selection that is independent from its role in meiotic recombination.

A novel mutant phenotype implicates dicephalic in cyst formation in the Drosophila ovary

The establishment of polarity in Drosophila requires the correct specification of the oocyte in early stages of oogenesis, its positioning at the posterior of the egg chamber, and signalling events between the oocyte and the adjacent posterior follicle cells. As a consequence, the anterior-posterior and the dorsal-ventral axes are fixed. The posterior localisation of the oocyte depends on cadherin-mediated adhesion between the oocyte and the follicle cells. Here we show that dicephalic mutants affect the posterior positioning of the oocyte without interfering with oocyte specification in the germarium. Unlike other mutants that also affect the posterior placement of the oocyte, dicephalic mutants affect neither gurken expression nor karyosome formation during meiosis. By analysing in detail the mutant phenotypes of dicephalic, we find that cyst formation in mutant germaria is defective and that it shares some similarities with cysts that lack DE-cadherin in the germline cells. We propose a model in which dicephalic is involved in the proper adhesion between the oocyte and the somatic follicle cells.

REC, Drosophila MCM8, drives formation of meiotic crossovers

Crossovers ensure the accurate segregation of homologous chromosomes from one another during meiosis. Here, we describe the identity and function of the Drosophila melanogaster gene recombination defective (rec), which is required for most meiotic crossing over. We show that rec encodes a member of the mini-chromosome maintenance (MCM) protein family. Six MCM proteins (MCM2–7) are essential for DNA replication and are found in all eukaryotes. REC is the Drosophila ortholog of the recently identified seventh member of this family, MCM8. Our phylogenetic analysis reveals the existence of yet another family member, MCM9, and shows that MCM8 and MCM9 arose early in eukaryotic evolution, though one or both have been lost in multiple eukaryotic lineages. Drosophila has lost MCM9 but retained MCM8, represented by REC. We used genetic and molecular methods to study the function of REC in meiotic recombination. Epistasis experiments suggest that REC acts after the Rad51 ortholog SPN-A but before the endonuclease MEI-9. Although crossovers are reduced by 95% in rec mutants, the frequency of noncrossover gene conversion is significantly increased. Interestingly, gene conversion tracts in rec mutants are about half the length of tracts in wild-type flies. To account for these phenotypes, we propose that REC facilitates repair synthesis during meiotic recombination. In the absence of REC, synthesis does not proceed far enough to allow formation of an intermediate that can give rise to crossovers, and recombination proceeds via synthesis-dependent strand annealing to generate only noncrossover products.

Most of our cells have two copies of each chromosome. For sexual reproduction, these must separate from one another to produce sperm or eggs with one copy of each chromosome. This occurs during meiosis, when chromosomes pair and exchange DNA segments. This exchange— meiotic recombination—creates physical linkages between chromosome pairs and is also a source of genetic diversity. To learn more about the process of meiotic recombination, the authors characterized the gene recombination defective (rec) from the fruit fly Drosophila melanogaster. Molecular analysis revealed that rec is related to a large family of genes found in all animals, plants, and protists. These genes are thought to be important in DNA replication, but rec appears to have a novel function. The authors found that mutants lacking rec are unable to copy enough DNA during meiotic recombination to form linkages between chromosomes. This results in chromosomes segregating randomly during meiosis, so that most eggs have an incorrect number or composition of chromosomes.

spn-F encodes a novel protein that affects oocyte patterning and bristle morphology in Drosophila

The anteroposterior and dorsoventral axes of the Drosophila embryo are established during oogenesis through the activities of Gurken (Grk), a Tgfalpha-like protein, and the Epidermal growth factor receptor (Egfr). spn-F mutant females produce ventralized eggs similar to the phenotype produced by mutations in the grk-Egfr pathway. We found that the ventralization of the eggshell in spn-F mutants is due to defects in the localization and translation of grk mRNA during mid-oogenesis. Analysis of the microtubule network revealed defects in the organization of the microtubules around the oocyte nucleus. In addition, spn-F mutants have defective bristles. We cloned spn-F and found that it encodes a novel coiled-coil protein that localizes to the minus end of microtubules in the oocyte, and this localization requires the microtubule network and a Dynein heavy chain gene. We also show that Spn-F interacts directly with the Dynein light chain Ddlc-1. Our results show that we have identified a novel protein that affects oocyte axis determination and the organization of microtubules during Drosophila oogenesis.

Checking Your Breaks: Surveillance Mechanisms of Meiotic Recombination

Numerous DNA double-strand breaks (DSBs) are introduced into the genome in the course of meiotic recombination. This poses a significant hazard to the genomic integrity of the cell. Studies in a number of organisms have unveiled the existence of surveillance mechanisms or checkpoints that couple the formation and repair of DSBs to cell cycle progression. Through these mechanisms, aberrant meiocytes are delayed in their meiotic progression, thereby facilitating repair of meiotic DSBs, or are culled through programmed cell death, thereby protecting the germline from aneuploidies that could lead to spontaneous abortions, birth defects and cancer predisposition in the offspring. Here we summarize recent progress in our understanding of these checkpoints. This review focuses on the surveillance mechanisms of the budding yeast S. cerevisiae, where the molecular details are best understood, but will frequently compare and contrast these mechanisms with observations in other organisms.

Expression of meiotic genes in the germline progenitors of Drosophila embryos

Meiosis is one of the fundamental characteristics of germ cells. In Drosophila, genetic screens have identified many genes required for meiotic division. However, it remains elusive as to when and how these meiotic genes are activated during germline development. To obtain insights into their regulatory mechanisms, we examined the expression of 38 meiotic genes in the germline progenitors, pole cells, during embryogenesis. We found that the transcripts of 12 meiotic genes were enriched in pole cells within the embryonic gonads. Among them, bag of marbles (bam), benign gonial cell neoplasia (bgcn), deadhead (dhd), matotopetli (topi) and twine (twe) were activated only in pole cells within the gonads, whereas the transcripts from grapes (grp), Kinesin-like protein at 3A (Klp3A), pavarotti (pav), lesswright (lwr), mei-P26, Topoisomerase 2 (Top2) and out at first (oaf) were distributed ubiquitously in early embryos and then became restricted to pole cells and to a subset of somatic tissues at later embryonic stages. The remaining meiotic genes were either expressed ubiquitously in the embryos (15 genes) or were undetectable in pole cells within the gonads (11 genes). These observations suggest that pole cells have already acquired the potential to express several meiotic genes. Our data will thus provide a useful basis for analyzing how the germline acquires a potential to execute meiosis.

Trypanosoma brucei DMC1 does not act in DNA recombination, repair or antigenic variation in bloodstream stage cells

Homologous recombination acts in the repair of cellular DNA damage and can generate genetic variation. Some of this variation provides a discrete purpose in the cell, although it can also be genome-wide and contribute to longer-term natural selection. In Trypanosoma brucei, a eukaryotic parasite responsible for sleeping sickness disease in sub-Saharan Africa, homologous recombination acts to catalyse antigenic variation, an immune evasion strategy involving switches in variant surface glycoprotein. In addition, T. brucei can undergo genetic exchange by homologous recombination in the tsetse vector, and some evidence suggests that this occurs by meiosis. Here, we show that T. brucei, Trypanosoma cruzi and Leishmania major each contain a single copy gene whose product is highly related to the eukaryotic meiosis-specific protein Dmc1, which is structurally and functionally related to Rad51. We show that T. brucei DMC1 is transcribed in the bloodstream stage of the parasite, where the gene can be mutated by reverse genetic disruption. DMC1 mutation does not, however, result in detectable alterations in DNA repair, recombination or antigenic variation efficiency in this life cycle stage.

Distinct roles for two RAD51-related genes in Trypanosoma brucei antigenic variation

In Trypanosoma brucei, DNA recombination is crucial in antigenic variation, a strategy for evading the mammalian host immune system found in a wide variety of pathogens. T.brucei has the capacity to encode >1000 antigenically distinct variant surface glycoproteins (VSGs). By ensuring that only one VSG is expressed on the cell surface at one time, and by periodically switching the VSG gene that is expressed, T.brucei can evade immune killing for prolonged periods. Much of VSG switching appears to rely on a widely conserved DNA repair pathway called homologous recombination, driven by RAD51. Here, we demonstrate that T.brucei encodes a further five RAD51-related proteins, more than has been identified in other single-celled eukaryotes to date. We have investigated the roles of two of the RAD51-related proteins in T.brucei, and show that they contribute to DNA repair, homologous recombination and RAD51 function in the cell. Surprisingly, however, only one of the two proteins contributes to VSG switching, suggesting that the family of diverged RAD51 proteins present in T.brucei have assumed specialized functions in homologous recombination, analogous to related proteins in metazoan eukaryotes.

Stage specific expression of histone deacetylase 4 (HDAC4) during oogenesis and early preimplantation development in mice

Oogenesis is a critical event in the formation of gametes, which transfer genomic information to the next generation. During this process, the gene expression pattern changes dramatically concomitant with genome remodeling, while the genomic information is stably maintained. Histone acetylation, the level of which is dramatically changed during oogenesis, has been implicated in the regulation of genome remodeling. In order to identify genes that are involved in the dynamic changes in histone acetylation levels during oogenesis, we performed suppressive subtraction hybridization (SSH) using unfertilized versus fertilized oocytes. Among the genes identified by SSH, we found histone deacetylase 4 (HDAC4), whose expression has been detected in only a few types of adult tissues. RT-PCR analysis revealed that the expression of HDAC4 was specific for full-grown oocytes. The HDAC4 transcript was barely detected in the growing oocytes, whereas it was readily detectable in fully grown oocytes. The expression was maintained at a high level until the MII stage, but decreased prominently after fertilization. Then, the expression level remained low until the morula stage. Immunocytochemistry localized the HDAC4 protein in the chromosome of fully grown oocytes. Although HDACs are most closely linked with transcriptional regulation owing to deacetylation of the core histones of the chromatin, previous studies have shown that all of the histone residues examined were highly acetylated in full-grown oocytes. Therefore, our results suggest that HDAC4 acts on a non-histone protein and plays a role in some other cellular functions of full-grown oocytes. To address this possibility, we examined the expression of p53BP1, which is associated with HDAC4 and plays a critical role in the DNA damage response. RT-PCR analysis revealed that p53BP1 was expressed specifically in fully grown oocytes, as was HDAC4. These results suggest that HDAC4 plays a role in stabilizing the genomes of fully grown oocytes.

Recent advances in understanding of the DNA double-strand break repair machinery of plants

Living cells suffer numerous and varied alterations of their genetic material. Of these, the DNA double-strand break (DSB) is both particularly threatening and common. Double-strand breaks arise from exposure to DNA damaging agents, but also from cell metabolism-in a fortuitous manner during DNA replication or repair of other kinds of lesions and in a programmed manner, for example during meiosis or V(D)J gene rearrangement. Cells possess several overlapping repair pathways to deal with these breaks, generally designated as genetic recombination. Genetic and biochemical studies have provided considerable amounts of data about the proteins involved in recombination processes and their functions within these processes. Although they have long played a key role in building understanding of genetics, relatively little is known at the molecular level of the genetic recombination processes in plants. The use of reverse genetic approaches and the public availability of sequence tagged mutants in Arabidopsis thaliana have led to increasingly rapid progress in this field over recent years. The rapid progress of studies of recombination in plants is obviously not limited to the DSB repair machinery as such and we ask readers to understand that in order to maintain the focus and to rest within a reasonable length, we present only limited discussion of the exciting advances in the of plant meiosis field, which require a full review in their own right . We thus present here an update on recent advances in understanding of the DSB repair machinery of plants, focussing on Arabidopsis and making a particular effort to place these in the context of more general of understanding of these processes.

End-joining repair of double-strand breaks in Drosophila melanogaster is largely DNA ligase IV independent

Repair of DNA double-strand breaks can occur by either nonhomologous end joining or homologous recombination. Most nonhomologous end joining requires a specialized ligase, DNA ligase IV (Lig4). In Drosophila melanogaster, double-strand breaks created by excision of a P element are usually repaired by a homologous recombination pathway called synthesis-dependent strand annealing (SDSA). SDSA requires strand invasion mediated by DmRad51, the product of the spn-A gene. In spn-A mutants, repair proceeds through a nonconservative pathway involving the annealing of microhomologies found within the 17-nt overhangs produced by P excision. We report here that end joining of P-element breaks in the absence of DmRad51 does not require Drosophila LIG4. In wild-type flies, SDSA is sometimes incomplete, and repair is finished by an end-joining pathway that also appears to be independent of LIG4. Loss of LIG4 does not increase sensitivity to ionizing radiation in late-stage larvae, but lig4 spn-A double mutants do show heightened sensitivity relative to spn-A single mutants. Together, our results suggest that a LIG4-independent end-joining pathway is responsible for the majority of double-strand break repair in the absence of homologous recombination in flies.

The AtRAD51C gene is required for normal meiotic chromosome synapsis and double-stranded break repair in Arabidopsis

Meiotic prophase I is a complex process involving homologous chromosome (homolog) pairing, synapsis, and recombination. The budding yeast (Saccharomyces cerevisiae) RAD51 gene is known to be important for recombination and DNA repair in the mitotic cell cycle. In addition, RAD51 is required for meiosis and its Arabidopsis (Arabidopsis thaliana) ortholog is important for normal meiotic homolog pairing, synapsis, and repair of double-stranded breaks. In vertebrate cell cultures, the RAD51 paralog RAD51C is also important for mitotic homologous recombination and maintenance of genome integrity. However, the function of RAD51C in meiosis is not well understood. Here we describe the identification and analysis of a mutation in the Arabidopsis RAD51C ortholog, AtRAD51C. Although the atrad51c-1 mutant has normal vegetative and flower development and has no detectable abnormality in mitosis, it is completely male and female sterile. During early meiosis, homologous chromosomes in atrad51c-1 fail to undergo synapsis and become severely fragmented. In addition, analysis of the atrad51c-1 atspo11-1 double mutant showed that fragmentation was nearly completely suppressed by the atspo11-1 mutation, indicating that the fragmentation largely represents a defect in processing double-stranded breaks generated by AtSPO11-1. Fluorescence in situ hybridization experiments suggest that homolog juxtaposition might also be abnormal in atrad51c-1 meiocytes. These results demonstrate that AtRAD51C is essential for normal meiosis and is probably required for homologous synapsis.

Functional analysis of the Drosophila Rad51 gene (spn-A) in repair of DNA damage and meiotic chromosome segregation

Rad51 is a crucial enzyme in DNA repair, mediating the strand invasion and strand exchange steps of homologous recombination (HR). Mutations in the Drosophila Rad51 gene (spn-A) disrupt somatic as well as meiotic double-strand break (DSB) repair, similar to fungal Rad51 genes. However, the sterility of spn-A mutant females prevented a thorough analysis of the role of Rad51 in meiosis. In this study, we generated transgenic animals that express spn-A dsRNA under control of an inducible promoter, and examined the effects of inhibiting expression of spn-A on DNA repair, meiotic recombination and meiotic chromosome pairing and segregation. We found that depletion of spn-A mRNA had no effect on the viability of non-mutagen-treated transgenic animals but greatly reduced the survival of larvae that were exposed to the radiomimetic drug MMS, in agreement with the MMS and X-ray sensitivity of spn-A mutant animals. We also found that increases in dose of spn-A gene enhanced larval resistance to MMS exposure, suggesting that at high damage levels, Rad51 protein levels may be limiting for DNA repair. spn-A RNAi strongly stimulated X-X nondisjunction and decreased recombination along the X in female meiosis, consistent with a requirement of Rad51 in meiotic recombination. However, neither RNAi directed against the spn-A mRNA nor homozygosity for a spn-A null mutation had any effect on male fertility or on X-Y segregation in male meiosis, indicating that Rad51 likely plays no role in male meiotic chromosome pairing. Our results support a central role for Rad51 in HR in both somatic and meiotic DSB repair, but indicate that Rad51 in Drosophila is dispensable for meiotic chromosome pairing. Our results also provide the first demonstration that RNAi can be used to inhibit the functions of meiotic genes in Drosophila.

Evidence for multiple cycles of strand invasion during repair of double-strand gaps in Drosophila

DNA double-strand breaks (DSBs), a major source of genome instability, are often repaired through homologous recombination pathways. Models for these pathways have been proposed, but the precise mechanisms and the rules governing their use remain unclear. In Drosophila, the synthesis-dependent strand annealing (SDSA) model can explain most DSB repair. To investigate SDSA, we induced DSBs by excision of a P element from the male X chromosome, which produces a 14-kb gap relative to the sister chromatid. In wild-type males, repair synthesis tracts are usually long, resulting in frequent restoration of the P element. However, repair synthesis is often incomplete, resulting in internally deleted P elements. We examined the effects of mutations in spn-A, which encodes the Drosophila Rad51 ortholog. As expected, there is little or no repair synthesis in homozygous spn-A mutants after P excision. However, heterozygosity for spn-A mutations also resulted in dramatic reductions in the lengths of repair synthesis tracts. These findings support a model in which repair DNA synthesis is not highly processive. We discuss a model wherein repair of a double-strand gap requires multiple cycles of strand invasion, synthesis, and dissociation of the nascent strand. After dissociation, the nascent strand may anneal to a complementary single strand, reinvade a template to be extended by additional synthesis, or undergo end joining. This model can explain aborted SDSA repair events and the prevalence of internally deleted transposable elements in genomes.

Unique invasions and resolutions: DNA repair proteins in meiotic recombination in Drosophila melanogaster

To ensure the accurate disjunction of homologous chromosomes during meiosis, most eukaryotes rely on physical connections called chiasmata, which form at sites of crossing over. In the absence of crossing over, homologs may segregate randomly, resulting in high frequencies of aneuploid gametes. The process of meiotic recombination poses unique problems for the cell that must be overcome to ensure normal disjunction of homologous chromosomes. How is it ensured that crossovers occur between homologous chromosomes, rather than between sister chromatids? What determines the number and location of crossovers? The functions of DNA repair proteins hold some of the answers to these questions. In this review, we discuss DNA repair proteins that function in meiotic recombination in Drosophila melanogaster. We emphasize the processes of strand invasion and Holliday junction resolution in order to shed light on the questions raised above. Also, we compare the variety of ways several eukaryotes perform these processes and the different proteins they require.

Differing requirements for the Arabidopsis Rad51 paralogs in meiosis and DNA repair

In addition to the recombinase Rad51, vertebrates have five paralogs of Rad51, all members of the Rad51-dependent recombination pathway. These paralogs form two complexes (Rad51C/Xrcc3 and Rad51B/C/D/Xrcc2), which play roles in somatic recombination, DNA repair and chromosome stability. However, little is known of their possible involvement in meiosis, due to the inviability of the corresponding knockout mice. We have recently reported that the Arabidopsis homolog of one of these Rad51 paralogs (AtXrcc3) is involved in DNA repair and meiotic recombination and present here Arabidopsis lines carrying mutations in three other Rad51 paralogs (AtRad51B, AtRad51C and AtXrcc2). Disruption of any one of these paralogs confers hypersensitivity to the DNA cross-linking agent Mitomycin C, but not to gamma-irradiation. Moreover, the atrad51c-1 mutant is the only one of these to show meiotic defects similar to those of the atxrcc3 mutant, and thus only the Rad51C/Xrcc3 complex is required to achieve meiosis. These results support conservation of functions of the Rad51 paralogs between vertebrates and plants and differing requirements for the Rad51 paralogs in meiosis and DNA repair.

A large-scale screen for mutagen-sensitive loci in Drosophila

In a screen for new DNA repair mutants, we tested 6275 Drosophila strains bearing homozygous mutagenized autosomes (obtained from C. Zuker) for hypersensitivity to methyl methanesulfonate (MMS) and nitrogen mustard (HN2). Testing of 2585 second-chromosome lines resulted in the recovery of 18 mutants, 8 of which were alleles of known genes. The remaining 10 second-chromosome mutants were solely sensitive to MMS and define 8 new mutagen-sensitive genes (mus212-mus219). Testing of 3690 third chromosomes led to the identification of 60 third-chromosome mutants, 44 of which were alleles of known genes. The remaining 16 mutants define 14 new mutagen-sensitive genes (mus314-mus327). We have initiated efforts to identify these genes at the molecular level and report here the first two identified. The HN2-sensitive mus322 mutant defines the Drosophila ortholog of the yeast snm1 gene, and the MMS- and HN2-sensitive mus301 mutant defines the Drosophila ortholog of the human HEL308 gene. We have also identified a second-chromosome mutant, mus215(ZIII-2059), that uniformly reduces the frequency of meiotic recombination to <3% of that observed in wild type and thus defines a function required for both DNA repair and meiotic recombination. At least one allele of each new gene identified in this study is available at the Bloomington Stock Center.

Distinct DNA-damage-dependent and -independent responses drive the loss of oocytes in recombination-defective mouse mutants

Defects in meiotic recombination in many organisms result in arrest because of activation of a meiotic checkpoint(s). The proximal defect that triggers this checkpoint in mammalian germ cells is not understood, but it has been suggested to involve either the presence of DNA damage in the form of unrepaired recombination intermediates or defects in homologous chromosome pairing and synapsis independent of DNA damage per se. To distinguish between these possibilities in the female germ line, we compared mouse oocyte development in a mutant that fails to form the double-strand breaks (DSBs) that initiate meiotic recombination (Spo11-/-) to mutants with defects in processing DSBs when they are formed (Dmc1-/- and Msh5-/-), and we examined the epistasis relationships between these mutations. Absence of DSB formation caused a partial defect in follicle formation, whereas defects in DSB repair caused earlier and more severe meiotic arrest, which could be suppressed by eliminating DSB formation. Therefore, our analysis reveals that there are both DNA-damage-dependent and -independent responses to recombination errors in mammalian oocytes. By using these findings as a paradigm, we also examined oocyte loss in mutants lacking the DNA-damage checkpoint kinase ATM. The absence of ATM caused defects in folliculogenesis that were similar to those in Dmc1 mutants and that could be suppressed by Spo11 mutation, implying that oocyte death in Atm-deficient animals is a response to defective DSB repair.

Disruption of Drosophila Rad50 causes pupal lethality, the accumulation of DNA double-strand breaks and the induction of apoptosis in third instar larvae

The Rad50/Mre11/Nbs1 protein complex has a crucial role in DNA metabolism, in particular in double-strand break (DSB) repair through homologous recombination (HR). To elucidate the role of the Rad50 protein complex in DSB repair in a multicellular eukaryote, we generated a Rad50 deficient Drosophila strain by P-element mediated mutagenesis. Disruption of Rad50 causes retarded development and pupal lethality. To investigate the mechanism of pupal death, brains and wing imaginal discs from third instar larvae were studied in more detail. Wing imaginal discs from Rad50 mutant larvae displayed a 3.5-fold increase in the induction of spontaneous apoptotic cells in comparison to their heterozygous siblings. This finding correlates with increased levels of phosphorylated histone H2Av, indicating an accumulation of DSBs in Rad50 mutant larvae. A 45-fold increase in the frequency of anaphase bridges was detected in the brains of Rad50 deficient larvae, consistent with a role for Rad50 in telomere maintenance and/or replication of DNA. The induction of DSBs and defects in chromosome segregation are in agreement with a role of Drosophila Rad50 in repairing the DSBs that arise during replication.

Formation of deletions during double-strand break repair in Drosophila DmBlm mutants occurs after strand invasion

Bloom syndrome is a rare disorder associated with cancer predisposition and genomic instability and is caused by loss of the RecQ helicase BLM. The Drosophila ortholog of BLM (DmBlm) is required for accurate repair of DNA double-strand gaps by homologous recombination. Repair products from DmBlm mutants have shorter repair synthesis tract lengths compared to wild type and are frequently associated with deletions flanking the break site. To determine the mechanisms responsible for deletion formation in the absence of DmBlm, we characterized repair after excision of the P[w(a)] element in various genetic backgrounds. Flies lacking DmRad51 do not have an elevated deletion frequency. Moreover, loss of DmRad51 suppresses deletion formation in DmBlm mutants. These data support a model in which DmBlm acts downstream of strand invasion to unwind a D-loop intermediate to free the newly synthesized strand. In the absence of DmBlm, alternative pathways of D-loop disassembly result in short repair synthesis tracts or flanking deletions. This model explains how RecQ helicases can promote homologous recombination while preventing illegitimate recombination.

The origin of asymmetry: early polarisation of the Drosophila germline cyst and oocyte

The anterior-posterior axis of Drosophila is established before fertilisation when the oocyte becomes polarised to direct the localisation of bicoid and oskar mRNAs to opposite poles of the egg. Here we review recent results that reveal that the oocyte acquires polarity much earlier than previously thought, at the time when it acquires its fate. The oocyte arises from a 16-cell germline cyst, and its selection and the initial cue for its polarisation are controlled by the asymmetric segregation of a germline specific organelle called the fusome. Several different downstream pathways then interpret this asymmetry to restrict distinct aspects of oocyte identity to this cell. Mutations in any of the six conserved Par proteins disrupt the early polarisation of the oocyte and lead to a failure to maintain its identity. Surprisingly, mutations affecting the control of the mitotic or meiotic cell cycle also lead to a failure to maintain the oocyte fate, indicating crosstalk between the nuclear and cytoplasmic events of oocyte differentiation. The early polarity of the oocyte initiates a series of reciprocal signaling events between the oocyte and the somatic follicle cells that leads to a reversal of oocyte polarity later in oogenesis, which defines the anterior-posterior axis of the embryo.

The atspo11-1 mutation rescues atxrcc3 meiotic chromosome fragmentation

Homologous recombination events occurring during meiotic prophase I ensure the proper segregation of homologous chromosomes at the first meiotic division. These events are initiated by programmed double-strand breaks produced by the Spo11 protein and repair of such breaks by homologous recombination requires a strand exchange activity provided by the Rad51 protein. We have recently reported that the absence of AtXrcc3, an Arabidopsis Rad51 paralogue, leads to extensive chromosome fragmentation during meiosis, first visible in diplotene of meiotic prophase I. The present study clearly shows that this fragmentation results from un- or mis-repaired AtSpo11-1 induced double-strand breaks and is thus due to a specific defect in the meiotic recombination process.

Meiotic defects in the Arabidopsis rad50 mutant point to conservation of the MRX complex function in early stages of meiotic recombination

The Rad50, Mre11 and Xrs2/Nbs1 proteins, which form the highly conserved MRX complex, perform a wide range of functions concerning the maintenance and function of DNA in eukaryotes. These include recombination, DNA repair, replication, telomere homeostasis and meiosis. Notwithstanding the attention paid to this complex, the inviability of vertebrate rad50 and mre11 mutants has led to a relative lack of information concerning the role of these proteins in meiosis in higher eukaryotes. We have previously reported that Arabidopsis atrad50 mutant plants are viable and that atrad50 mutant plants are sterile. The present study reports an analysis of the causes of this sterility and the implication of the AtRad50 protein in meiosis. Both male and female gametogenesis are defective in the Arabidopsis atrad50 mutant and cytological observation of male meiosis indicates that in the absence of the AtRad50 protein, homologous chromosomes are unable to synapse. Finally, the atrad50 mutation leads to the destruction of chromosomes during meiosis. These phenotypes support a role for the Arabidopsis MRX complex in early stages of meiotic recombination.

The Arabidopsis AtRAD51 gene is dispensable for vegetative development but required for meiosis

The maintenance of genome integrity and the generation of biological diversity are important biological processes, and both involve homologous recombination. In yeast and animals, homologous recombination requires the function of the RAD51 recombinase. In vertebrates, RAD51 seems to have acquired additional functions in the maintenance of genome integrity, and rad51 mutations cause lethality, but it is not clear how widely these functions are conserved among eukaryotes. We report here a loss-of-function mutant in the Arabidopsis homolog of RAD51, AtRAD51. The atrad51-1 mutant exhibits normal vegetative and flower development and has no detectable abnormality in mitosis. Therefore, AtRAD51 is not necessary under normal conditions for genome integrity. In contrast, atrad51-1 is completely sterile and defective in male and female meioses. During mutant prophase I, chromosomes fail to synapse and become extensively fragmented. Chromosome fragmentation is suppressed by atspo11-1, indicating that AtRAD51 functions downstream of AtSPO11-1. Therefore, AtRAD51 likely plays a crucial role in the repair of DNA double-stranded breaks generated by AtSPO11-1. These results suggest that RAD51 function is essential for chromosome pairing and synapsis at early stages in meiosis in Arabidopsis. Furthermore, major aspects of meiotic recombination seem to be conserved between yeast and plants, especially the fact that chromosome pairing and synapsis depend on the function of SPO11 and RAD51.