InteractiveFly: GeneBrief

crossover suppressor on 3 of Gowen: Biological Overview | References

The synaptonemal complex (SC) is a conserved meiotic structure that regulates the repair of double-strand breaks (DSBs) into crossovers or gene conversions. The removal of any central-region SC component, such as the Drosophila melanogaster transverse filament protein C(3)G, causes a complete loss of SC structure and crossovers. To better understand the role of the SC in meiosis, CRISPR/Cas9 was used to construct 3 in-frame deletions within the predicted coiled-coil region of the C(3)G protein. Since these 3 deletion mutations disrupt SC maintenance at different times during pachytene and exhibit distinct defects in key meiotic processes, they allowed definition the stages of pachytene when the SC is necessary for homolog pairing and recombination during pachytene. These studies demonstrate that the X chromosome and the autosomes display substantially different defects in pairing and recombination when SC structure is disrupted, suggesting that the X chromosome is potentially regulated differently from the autosomes (Billmyre, 2019).

Several facets of meiosis ensure the faithful inheritance of chromosomes from parents to offspring. During the creation of eggs and sperm the genome must be reduced to a haploid state containing a single set of chromosomes. The failure to properly segregate chromosomes results in gametes with an incorrect number of chromosomes. Indeed, errors in meiotic chromosome segregation are the leading cause of miscarriage and aneuploidy in humans, which can result in chromosomal disorders such as Down syndrome and Turner syndrome (Billmyre, 2019).

Proper segregation of chromosomes during meiosis relies on the formation of programmed double-strand breaks (DSBs), which are initiated by the evolutionarily conserved type II DNA topoisomerase-like protein Spo11 (Mei-W68 in Drosophila). These DSBs are then repaired as crossover or gene conversion events. Crossovers mature into chiasmata, which physically hold homologous chromosomes together from nuclear envelope breakdown until homolog separation at anaphase I, thus ensuring proper segregation of chromosomes. The placement of crossover events is highly nonrandom and is strictly regulated by multiple processes. First, crossover interference prevents 2 crossovers from occurring in close proximity to each other. Second, crossovers are excluded from the heterochromatin. Third, as a result of the centromere effect, crossing over is also reduced in those euchromatic regions that lie in proximity to the centromeres. Finally, even within the medial and distal euchromatin, crossing over is substantially higher toward the middle of the chromosome arms. These constraints do not affect the frequency or distribution of gene conversion events, which appear to be randomly distributed throughout the euchromatin. Thus, the control of crossover distribution may act at the level of DSB fate choice, rather than in determining the position of DSBs (Billmyre, 2019).

Previous studies have demonstrated that the synaptonemal complex (SC), a large protein structure that forms between homologous chromosomes, plays a role in controlling crossover distribution. The SC is a highly conserved tripartite structure, with 2 lateral elements and a central region. The central region is composed of transverse filament and central element proteins, while the lateral element proteins connect the central region to the chromosome axes. The known proteins that make up the Drosophila central region include the main transverse filament protein C(3)G, the transverse filament-like protein Corolla, and the central element protein Corona (CONA) (Billmyre, 2019).

Work in Caenorhabditis elegans has shown that the SC functions to monitor crossover placement by preventing additional crossover designation in a region adjacent to an existing crossover precursor. Furthermore, there is evidence in Saccharomyces cerevisiae that Zip1, a transverse filament protein, has 2 separable functions-one in building the SC and the other in recombination. Lastly, in rice, there is evidence that a partial loss of the SC results in increased crossing over and crossover proximity similar to what was reported in C. elegans. Based on what is known in other model systems, it is likely that the Drosophila SC is also playing a role in regulating the fate of DSBs and monitoring crossover placement (Billmyre, 2019).

In Drosophila females, ~24 DSBs are formed in early pachytene. Unlike in many other organisms where DSBs occur prior to SC formation, in Drosophila DSBs are formed in the context of fully formed SC. In the absence of the central region of the SC, DSB formation is substantially reduced, but not eliminated. Nonetheless, even in the presence of a substantial number of residual DSBs (37% of wild type), the loss of these SC proteins results in a complete loss of crossover formation. The abolishment of the central region of the SC also results in a high frequency of unpaired homologs during pachytene. In addition to disrupting meiotic pairing, the loss of any of the known central-region components in the (premeiotic) mitotic region of the ovaries also impairs mitotic pairing of the second and third chromosomes (Billmyre, 2019).

Since the vast majority of SC mutants in Drosophila are null mutants and therefore fail to form any SC structure, it is difficult to investigate the interactions of the wild-type versions of these proteins at the protein level or discover how the SC is involved in DSB repair and fate choice. In Drosophila, the study of transgenes carrying in-frame deletions of either the N- or C-terminal globular domains of C(3)G has shown that both of these regions are required for proper SC assembly and crossover formation. However, these defects were too severe to allow investigation of the function of the SC in crossover placement and formation. One domain which has not been tested is the large predicted coiled-coil domain in C(3)G. Coiled-coil domains are a key conserved feature of transverse filament proteins across many organisms and are known to be important for protein-protein interactions (Billmyre, 2019).

This study has characterize 3 in-frame deletion mutations in the coiled-coil domain of the Drosophila melanogaster c(3)G, all of which cause a partial loss of SC function at different stages in early meiosis. Advantage was taken of the different stages of SC loss to examine when the SC is necessary for multiple meiotic events such as pairing and recombination. Unlike any previously characterized Drosophila meiotic mutants, the effects of these mutants on X chromosome recombination is different from their effects on autosomal recombination. It is inferred from this observation that chromosomes can respond differently to a failure in SC maintenance. It was also shown that the SC in early pachytene is important for the maintenance of euchromatic pairing, especially in the distal euchromatin (in relation to the centromere) regions of the chromosome arms. The maintenance of X chromosome pairing is more sensitive to SC defects than is pairing maintenance on the autosomes, suggesting there may be additional chromosome-specific processes that mediate pairing. These mutants allowed us to examine the temporal requirement for the synaptonemal complex in crossover placement and maintenance of pairing (Billmyre, 2019).

Both the regulation of SC assembly and disassembly, and its maintenance after assembly, is poorly understood. Work in other organisms has shown that posttranslational modifications are important in SC structure and function. It is known that SUMOylation and N-terminal acetylation promote assembly of the SC while phosphorylation or dephosphorylation promote disassembly of the SC with modifications occurring on multiple SC proteins. Thus far, no posttranslationally modified sites have been identified on C(3)G. However, it is likely that these sites do exist, and it is speculated that sites promoting SC assembly, maintenance, and disassembly may be disrupted in these mutants (Billmyre, 2019).

Another possibility is that the deletions described in this study could destabilize protein-protein interaction sites between C(3)G and other central-region proteins, resulting in an unstable SC that is difficult to maintain. It is noted that the mutant with the smallest deletion, c(3)GccΔ3, exhibited the strongest SC defect. While this deletion was predicted to only disrupt a single coil, the best explanation for the more severe phenotype is that it actually disrupts the coiled coil. This may have caused a large disruption in the rest of the coiled-coil structure. In the future, it will be important to further dissect these domains to better understand the regulation of SC assembly and disassembly (Billmyre, 2019).

A surprising result from these studies was the ability of these deletions to allow the progressive loss of homologous euchromatic pairing through pachytene. The mechanism behind establishing and maintaining homolog pairing is a long-standing, unanswered question in the meiosis field. Previous work in Drosophila has shown that in the complete absence of the central-region proteins C(3)G and CONA, euchromatic pairing is significantly reduced in early to mid and mid pachytene (Billmyre, 2019).

Partial loss-of-function mutations have allowed testing of the importance of C(3)G in maintaining pairing throughout pachytene when the SC is present in early pachytene (unlike previous studies of null mutants in which the SC is always absent). From these mutants, a timeline is now available of when the SC is necessary to maintain pairing and recombination on the X chromosome and the autosomes. By comparing these mutants, it is hypothesized that the X chromosome needs a full-length SC earlier in pachytene for proper maintenance of pairing and recombination while the autosomes are likely capable of placing crossovers as late as mid pachytene, resulting in a proximal euchromatin shift in crossovers where pairing is maintained (Billmyre, 2019).

In both c(3)GccΔ1 and c(3)GccΔ3 mutants, distal pairing of the X chromosome and the autosomes was most strongly reduced. One likely explanation for this stronger effect on distal regions of the chromosome arms is that normally the disassembly of the SC is initiated on the euchromatic chromosome arms with the centromeric region being removed last. Since the loss of the SC in c(3)GccΔ1 and c(3)GccΔ3 mutants occurs in a manner similar to wild-type SC disassembly, the distal regions of the chromosome may be affected earlier and more strongly than the proximal euchromatin regions. The proximal euchromatin region contains a large amount of heterochromatin that could be mediating pairing interactions and stabilizing pairing in the absence of the SC. Furthermore, examination of centromere pairing suggests that the centromeres are still paired and could be facilitating the proximal euchromatic pairing. This idea is supported by the higher levels of proximal euchromatic pairing compared with distal pairing in c(3)Gnull (Billmyre, 2019).

Finally, it is speculated that the ability of the c(3)GccΔ1 mutants to exhibit a distal euchromatic pairing defect that is more severe than the defect seen in c(3)G null mutants results from the residual proximal crossovers that do form in c(3)GccΔ1 mutants. Previous work has shown that crossovers can preserve synapsis but only in their vicinity. Perhaps the stresses that provoke separation become more concentrated on the distal regions that lack crossovers. For example, it is possible that the untethered distal regions could experience a higher mechanical stress due to nuclear movements than the pericentric regions containing a crossover. The lack of a strong pairing defect in c(3)GccΔ2 mutants is probably due to the persistence of a full-length SC until mid pachytene. Together, these data support a role for the SC in maintaining euchromatic pairing during early to mid prophase (Billmyre, 2019).

The autosomal increase in proximal euchromatin crossovers displayed in these mutants mimics the interchromosomal effect. The interchromosomal effect has been reported in flies that are heterozygous for chromosome aberrations that suppress exchange in trans to a wild-type chromosome. Thus, the absence of crossover formation on one chromosome promotes increased recombination on the other chromosomes, with more crossovers placed in the proximal euchromatin regions. The mechanism that controls the interchromosomal effect in balancer heterozygotes is poorly understood. Additionally, the interchromosomal effect has been reported in C. elegans mutants with defective synapsis, further supporting this possibility (56). It is possible that the interchromosomal effect is partially responsible for the increase in proximal euchromatin crossovers in c(3)GccΔ1 and c(3)GccΔ3 mutants due to the loss of X chromosome recombination (Billmyre, 2019).

However, the interchromosomal effect cannot explain the increase in proximal euchromatin recombination in c(3)GccΔ2 mutants since X recombination appears normal. In theory, this phenotype could be explained by crossover homeostasis, which functions to control the number of crossovers so the appropriate number is placed. In many organisms, when there is a deficit of crossovers by the end of early to mid pachytene, the cell will continue to place crossovers in alternative locations to maintain an appropriate number. Such a process could result in crossovers being placed later than normal, which could be an issue when the SC is breaking down prematurely and homolog pairing is lost. However, Mehrotra and McKim (2006) provide evidence that crossover homeostasis is unlikely to occur in Drosophila females. It is unknown how much of a role the SC plays in the repair of DSBs into crossover versus non-crossover events. It is possible the SC must be present to interact with factors necessary for regulating the placement of crossovers. For example, Vilya, a pro-crossover factor, localizes to the SC and DSBs prior to being recruited to recombination nodules. If DSB repair on the autosomes does not occur until early to mid pachytene and the SC is necessary for the determination of a crossover fate, it follows that loss of the SC in the euchromatin would result in a shift of crossover formation toward proximal euchromatin regions where the SC may still be present. This mechanism could also be increasing proximal euchromatin recombination in c(3)GccΔ1 and c(3)GccΔ3 flies. Alternatively, SC-independent heterochromatic pairing may be holding the proximal euchromatin region in close proximity, allowing for crossing over in that region. In addition to interacting with pro-crossover factors, the SC may be interacting with a currently unknown protein which regulates crossover placement differently on the X chromosome versus the autosomes (Billmyre, 2019).

This set of mutants represents a unique tool to investigate not only the temporal requirements of the SC but also the differences in crossover placement between the X chromosome and the autosomes. Since c(3)GccΔ2 mutants do not display defects in X chromosome recombination, it is concluded that a full-length SC throughout early to mid pachytene is sufficient for X chromosome crossover placement but not for normal distribution of autosomal crossovers. Examining autosomal recombination in all 3 mutants suggests that a full-length SC is necessary in mid pachytene for proper crossover distribution on the autosomes. There are multiple explanations for the recombination differences between the X chromosome and the autosomes (Billmyre, 2019).

The first of these hypotheses is that there might exist a timing difference in either synapsis or crossover placement between the X chromosome and the autosomes. Work in C. elegans has provided evidence for timing differences between the sex chromosomes and the autosomes. For example, the X chromosome initiates premeiotic DNA replication later than the autosomes. This could be significant, as replication timing has been shown to impact crossover designation in barley. Additionally, in C. elegans, the X chromosome and the autosomes pair at the same time, but synapsis of the X chromosome is delayed and the X chromosome has lower levels of DSB formation compared with the autosomes. Thus, the timing of when each chromosome is fully synapsed could be critical to ensure normal crossover placement, and the premature disruption of synapsis may affect the activity of pro-crossover factors. For example, in C. elegans, the XND-1 protein is required for genome-wide crossover placement and is important for normal rates of DSBs on the X chromosome. Currently, it is unknown in Drosophila if there are differences in the timing of DSB repair or synapsis of the X chromosome as compared with the autosomes, and the data suggest this as a possibility (Billmyre, 2019).

A second, but not mutually exclusive, explanation for the differences between the chromosomes may be a structural one. The X chromosome is acrocentric (the centromere is near the end of the chromosome), while the autosomes are both metacentric (the centromere is near the center of the chromosome) and, perhaps, these structural differences mean that the X chromosome is more sensitive to loss of the SC. The data suggest that loss of SC maintenance disrupts the maintenance of euchromatic homolog pairing more severely on the X chromosome than on the autosomes. It is unknown if metacentric chromosomes are different in terms of synapsis and recombination as compared with acrocentric chromosomes, and further investigation is needed to determine if structural differences affect these processes (Billmyre, 2019).

It is clear from decades of research that the regulation of recombination requires many factors and precise timing. This study shows that the SC plays a vital role in maintaining homolog pairing and proper crossover distribution in Drosophila female meiosis. Many differences between sex chromosomes and autosomes have been documented in a multitude of organisms, and our data are consistent with these differences extending into the processes that control chromosome pairing and recombination. With this set of mutants, a system has been established to examine X chromosome and autosome biology in Drosophila meiosis that will allow future work to unravel the mechanism behind meiotic chromosomal differences (Billmyre, 2019).

Sister centromere fusion during meiosis I depends on maintaining cohesins and destabilizing microtubule attachments

Sister centromere fusion is a process unique to meiosis that promotes co-orientation of the sister kinetochores, ensuring they attach to microtubules from the same pole during metaphase I. This study found that the kinetochore protein SPC105R/KNL1 and Protein Phosphatase 1 (PP1-87B) regulate sister centromere fusion in Drosophila oocytes. The analysis of these two proteins, however, has shown that two independent mechanisms maintain sister centromere fusion. Maintenance of sister centromere fusion by SPC105R depends on Separase, suggesting cohesin proteins must be maintained at the core centromeres. In contrast, maintenance of sister centromere fusion by PP1-87B does not depend on either Separase or WAPL. Instead, PP1-87B maintains sister centromeres fusion by regulating microtubule dynamics. This study has demonstrated that this regulation is through antagonizing Polo kinase and BubR1, two proteins known to promote stability of kinetochore-microtubule (KT-MT) attachments, suggesting that PP1-87B maintains sister centromere fusion by inhibiting stable KT-MT attachments. Surprisingly, C(3)G, the transverse element of the synaptonemal complex (SC), is also required for centromere separation in Pp1-87B RNAi oocytes. This is evidence for a functional role of centromeric SC in the meiotic divisions, that might involve regulating microtubule dynamics. Together, this study proposes that two mechanisms maintain co-orientation in Drosophila oocytes: one involves SPC105R to protect cohesins at sister centromeres and another involves PP1-87B to regulate spindle forces at end-on attachments (Wang, 2019).

The necessity of sister kinetochores to co-orient toward the same pole for co-segregation at anaphase I differentiates the first meiotic division from the second division. A meiosis-specific mechanism exists that ensures sister chromatid co-segregation by rearranging sister kinetochores, aligning them next to each other and facilitating microtubule attachments to the same pole]. This process is referred to as co-orientation, in contrast to mono-orientation, when homologous kinetochores orient to the same pole. Given the importance of co-orientation in meiosis the mechanism underlying this process is still poorly understood, maybe because many of the essential proteins are not conserved across phyla (Wang, 2019).

Most studies of co-orientation have focused on how fusion of the centromeres and kinetochores is established. In budding yeast, centromere fusion occurs independently of cohesins: Spo13 and the Polo kinase homolog Cdc5 recruit a meiosis-specific protein complex, monopolin (Csm1, Lrs4, Mam1, CK1) to the kinetochore. Lrs4 and Csm1 form a V-shaped structure that interacts with the N-terminal domain of Dsn1 in the Mis12 complex to fuse sister kinetochores. While the monopolin complex is not widely conserved, cohesin-independent mechanisms may exist in other organisms. A bridge between the kinetochore proteins MIS12 and NDC80 is required for co-orientation in maize. In contrast, cohesins are required for co-orientation in several organisms. The meiosis-specific cohesin Rec8 is indispensable for sister centromere fusion in fission yeast and Arabidopsis. Cohesin is localized to the core-centromere in fission yeast and mice. In Drosophila melanogaster oocytes, cohesins (SMC1/SMC3/SOLO/SUNN) establish cohesion in meiotic S-phase and show an enrichment that colocalizes with centromere protein CID/CENP-A. Like fission yeast and mouse, Drosophila may require high concentrations of cohesins to fuse sister centromeres together for co-orientation during meiosis (Wang, 2019).

In mice, a novel kinetochore protein, Meikin, recruits Plk1 to protect Rec8 at centromeres. Although poorly conserved, Meikin is proposed to be a functional homolog of Spo13 in budding yeast and Moa1 in fission yeast. They all contain Polo-box domains that recruit Polo kinase to centromeres. Loss of Polo in both fission yeast (Plo1) and mice results in kinetochore separation, suggesting a conserved role for Polo in co-orientation. In fission yeast, Moa1-Plo1 phosphorylates Spc7 (KNL1) to recruit Bub1 and Sgo1 for the protection of centromere cohesion in meiosis I. These results suggest the mechanism for maintaining sister centromere fusion involves kinetochore proteins recruiting proteins that protect cohesion. However, how centromere cohesion is established prior to metaphase I, and how sister centromere fusion is released during meiosis II, still needs to be investigated (Wang, 2019).

Previous work has found that depletion of the kinetochore protein SPC105R (KNL1) in Drosophila oocytes results in separated centromeres at metaphase I, suggesting a defect in sister centromere fusion. Thus, Drosophila SPC105R and fission yeast Spc7 may have conserved functions in co-orientation (Radford, 2015). This study has identified a second Drosophila protein required for sister-centromere fusion, Protein Phosphatase 1 isoform 87B (PP1-87B). However, sister centromere separation in SPC105R and PP1-87B depleted Drosophila oocytes occurs by different mechanisms, the former is Separase dependent and the latter is Separase independent. Based on these results, a model is proposed for the establishment, protection and release of co-orientation. Sister centromere fusion necessary for co-orientation is established through cohesins that are protected by SPC105R. Subsequently, PP1-87B maintains co-orientation in a cohesin-independent manner by antagonizing stable kinetochore-microtubule (KT-MT) interactions. The implication is that the release of co-orientation during meiosis II is cohesin-independent and MT dependent. A surprising interaction was found between PP1-87B and C(3)G, the transverse element of the synaptonemal complex (SC), in regulating sister centromere separation. Overall, these results suggest a new mechanism where KT-MT interactions and centromeric SC regulate sister kinetochore co-orientation during female meiosis (Wang, 2019).

Pleiotropic functions of the chromodomain-containing protein Hat-trick during oogenesis in Drosophila melanogaster

Chromatin-remodeling proteins play a profound role in the transcriptional regulation of gene expression during development. This study shows that the chromodomain-containing protein Hat-trick is predominantly expressed within the oocyte nucleus, specifically within the heterochromatinized karyosome and that a mild expression is observed in follicle cells. Co-localization of Hat-trick with Heterochromatin Protein 1 and a Synaptonemal Complex component- C(3)G along with the diffused karyosome after hat-trick down-regulation shows the role of this protein in heterochromatin clustering and karyosome maintenance. Germline mosaic analysis reveals that hat-trick is required for maintaining the dorso-ventral patterning of eggs by regulating the expression of Gurken. The increased incidence of Double strand breaks (DSBs), delayed DSB repair, defects in karyosome formation, altered Vasa mobility and consequently misexpression and altered localization of Gurken in hat-trick mutant egg chambers, clearly suggest a putative involvement of Hat-trick in the early stages of oogenesis. In addition, based on phenotypic observations in hat-trick mutant egg chambers, it is speculated that hat-trick plays a substantial role in cystoblast proliferation, oocyte determination, nurse cell endoreplication, germ cell positioning, cyst encapsulation, and nurse cell migration. These results demonstrate that hat-trick has profound pleiotropic functions during oogenesis in Drosophila melanogaster (Singh, 2018).

Holding it together: rapid evolution and positive selection in the synaptonemal complex of Drosophila

The synaptonemal complex (SC) is a highly conserved meiotic structure that functions to pair homologs and facilitate meiotic recombination in most eukaryotes. Five Drosophila SC proteins have been identified and localized within the complex: C(3)G, C(2)M, CONA, ORD, and the newly identified Corolla. The SC is required for meiotic recombination in Drosophila and absence of these proteins leads to reduced crossing over and chromosomal nondisjunction. Despite the conserved nature of the SC and the key role that these five proteins have in meiosis in D. melanogaster, they display little apparent sequence conservation outside the genus. To identify factors that explain this lack of apparent conservation, a molecular evolutionary analysis of these genes was performed across the Drosophila genus. For the five SC components, gene sequence similarity declines rapidly with increasing phylogenetic distance and only ORD and C(2)M are identifiable outside of the Drosophila genus. SC gene sequences have a higher dN/dS (omega) rate ratio than the genome wide average and this can in part be explained by the action of positive selection in almost every SC component. Across the genus, there is significant variation in omega for each protein. It further appears that omega estimates for the five SC components are in accordance with their physical position within the SC. Components interacting with chromatin evolve slowest and components comprising the central elements evolve the most rapidly. Finally, using population genetic approaches, it was demonstrated that positive selection on SC components is ongoing. It is concluded that SC components within Drosophila show little apparent sequence homology to those identified in other model organisms due to their rapid evolution. It is proposed that the Drosophila SC is evolving rapidly due to two combined effects. First, it is proposed that a high rate of evolution can be partly explained by low purifying selection on protein components whose function is to simply hold chromosomes together. It is also proposed that positive selection in the SC is driven by its sex-specificity combined with its role in facilitating both recombination and centromere clustering in the face of recurrent bouts of drive in female meiosis (Hemmer, 2016).

The cohesion protein SOLO associates with SMC1 and is required for synapsis, recombination, homolog bias and cohesion and pairing of centromeres in Drosophila meiosis

Cohesion between sister chromatids is mediated by cohesin and is essential for proper meiotic segregation of both sister chromatids and homologs. solo encodes a Drosophila meiosis-specific cohesion protein with no apparent sequence homology to cohesins that is required in male meiosis for centromere cohesion, proper orientation of sister centromeres and centromere enrichment of the cohesin subunit SMC1. This study shows that solo is involved in multiple aspects of meiosis in female Drosophila. Null mutations in solo caused the following phenotypes: 1) high frequencies of homolog and sister chromatid nondisjunction (NDJ) and sharply reduced frequencies of homolog exchange; 2) reduced transmission of a ring-X chromosome, an indicator of elevated frequencies of sister chromatid exchange (SCE); 3) premature loss of centromere pairing and cohesion during prophase I, as indicated by elevated foci counts of the centromere protein CID; 4) instability of the lateral elements (LE)s and central regions of synaptonemal complexes (SCs), as indicated by fragmented and spotty staining of the chromosome core/LE component SMC1 and the transverse filament protein C(3)G, respectively, at all stages of pachytene. SOLO and SMC1 are both enriched on centromeres throughout prophase I, co-align along the lateral elements of SCs and reciprocally co-immunoprecipitate from ovarian protein extracts. These studies demonstrate that SOLO is closely associated with meiotic cohesin and required both for enrichment of cohesin on centromeres and stable assembly of cohesin into chromosome cores. These events underlie and are required for stable cohesion of centromeres, synapsis of homologous chromosomes, and a recombination mechanism that suppresses SCE to preferentially generate homolog crossovers (homolog bias). It is proposes that SOLO is a subunit of a specialized meiotic cohesin complex that mediates both centromeric and axial arm cohesion and promotes homolog bias as a component of chromosome cores (Yan, 2013).

A pathway for synapsis initiation during zygotene in Drosophila oocytes

Formation of the synaptonemal complex (SC), or synapsis, between homologs in meiosis is essential for crossing over and chromosome segregation. How SC assembly initiates is poorly understood but may have a critical role in ensuring synapsis between homologs and regulating double-strand break (DSB) and crossover formation. This study investigated the genetic requirements for synapsis in Drosophila and found that there are three temporally and genetically distinct stages of synapsis initiation. In meiotic prophase 1 'early zygotene' oocytes, synapsis is only observed at the centromeres. It was also found that nonhomologous centromeres are clustered during this process. In 'mid-zygotene' oocytes, SC initiates at several euchromatic sites. The centromeric and first euchromatic SC initiation sites depend on the cohesion protein ORD. In 'late zygotene' oocytes, SC initiates at many more sites that depend on the Kleisin-like protein C(2)M. Surprisingly, late zygotene synapsis initiation events are independent of the earlier mid-zygotene events, whereas both mid and late synapsis initiation events depend on the cohesin subunits SMC1 and SMC3. It is proposed that the enrichment of cohesion proteins at specific sites promotes homolog interactions and the initiation of euchromatic SC assembly independent of DSBs. Furthermore, the early euchromatic SC initiation events at mid-zygotene may be required for DSBs to be repaired as crossovers (Tanneti, 2011).

Drosophila pro-oocytes develop within 16-cell cysts that are arranged in temporal order within the ovary. Each ovary contains several germaria, where pairs of pro-oocytes begin their development and enter prophase in region 2a and a single oocyte is selected by region 3. Oocytes are defined by the presence of the synaptonemal complex (SC), which is detected by antibodies to the transverse element C(3)G (Page, 2001), a coiled-coil protein similar to proteins in budding yeast (ZIP1), C. elegans (SYP-1, SYP-2), and mammals (SYCP1) (Page, 2004; Watts, 2011). Zygotene pro-oocytes were identified by their patchy C(3)G staining, as opposed to the thread-like staining typical of pachytene. Furthermore, by comparing the amount of synapsis to the relative positions of the pro-oocytes in the wild-type germarium, three stages of zygotene were defined (Tanneti, 2011).

First, early zygotene pro-oocytes have one or two patches of C(3)G that colocalize with CID, a centromere-specific histone H3. These pro-oocytes reside in the earliest (most anterior) part of region 2a, indicating that synapsis initiates at the centromeres before any other sites. These results were confirmed by comparing CID localization to histone modifications specific for the heterochromatin or euchromatin. Because there are four pairs of centromeres, the observation that most wild-type pro-oocytes have one or two CID foci indicates that nonhomologous centromeres cluster in meiotic prophase, confirming previous observations using electron microscopy (Tanneti, 2011).

Second, mid-zygotene pro-oocytes have the centromeric C(3)G staining plus approximately six additional sites in the euchromatin. Finally, late zygotene pro-oocytes contain many C(3)G foci but lack the continuous threadlike pattern of pachytene. Surprisingly, the mid-zygotene patches do not appear to get longer. Instead, there are more patches in late zygotene, suggesting that the progression from mid- to late zygotene involves the establishment of new SC initiation sites rather than polymerization from the small number of sites in mid-zygotene. It is suggested that the noncentromeric C(3)G sites in mid-zygotene represent the first euchromatic sites to initiate synapsis. This study provides evidence that the mid-zygotene sites have features in common with centromere synapsis sites but are mechanistically distinct and genetically separable from the additional synapsis initiation sites observed in late zygotene (Tanneti, 2011).

C(2)M is a lateral element component and is a member of the Kleisen family that includes Rec8 and Rad21 homologs (Schleiffer, 2003). In wild-type, C(2)M colocalizes with C(3)G in most locations except at the centromeres. In females lacking C(2)M, the first two stages of zygotene appear to occur normally. Early zygotene pro-oocytes exhibit one or two foci of CID that colocalize with C(3)G, showing that C(2)M is not required for centromere clustering or centromere synapsis. These results confirm previous observations (Khetani, 2007) that C(2)M is not required for centromere clustering in pachytene oocytes and are consistent with the observation that C(2)M does not localize to the centromeric regions. Early zygotene in c(2)M mutants is followed by cysts with several patches of euchromatic C(3)G staining that resemble wildtype cells in mid-zygotene. Synapsis in a c(2)M mutant does not, however, progress beyond this point. Examination of histone modifications in c(2)M mutants confirmed that synapsis is blocked in mid-zygotene with a small number of euchromatin initiation sites. Based on the similarities between wild-type mid-zygotene and c(2)M mutants, it is suggested that synapsis initiates in a c(2)M-independent manner at a small number of specialized sites on the chromosomes, which include approximately six euchromatic sites and the centromeres, and that C(2)M is required for additional initiation sites typical of late zygotene (Tanneti, 2011).

There is a striking similarity between the number of euchromatic synapsis initiation sites (~6) during mid-zygotene and the number of crossovers in Drosophila females. In order to determine the relationship between SC initiation sites and double-strand break (DSB) formation, c(2)M mutant oocytes were stained for C(3)G and γ-H2AV. DSBs in a c(2)M mutant are usually associated with a patch of C(3)G staining (55/56 γ-H2AV foci were touching or overlapped a patch of C(3)G). This experiment was also performed in an okr mutant background (okr encodes the Drosophila homolog of Rad54) where the DSBs are not repaired and γ-H2AV staining accumulates, allowing all DSBs to be counted. Most of the γ-H2AV foci in okr c(2)M mutant germaria colocalized with a patch of C(3)G, suggesting that the initiation of SC and recombination usually occur within the same region in c(2)M mutants. Indeed, MEI-P22, a protein required for DSB formation, also colocalizes with the SC in c(2)M mutant oocytes. It should be noted that previous observations showed that DSB formation is partially dependent on the SC. Indeed, the number of γ-H2AV foci in the okr c(2)M double mutant in region 3 oocytes was reduced compared to a okr single mutant. Overall, these results suggest that the SC, or a factor which stimulates SC formation, promotes recruitment of proteins required for DSB formation (Tanneti, 2011).

To investigate whether there is a connection between early SC initiation events and meiotic recombination, double mutants with c(2)M were constructed. Unlike wild-type, where γ-H2AV foci are not observed until pachytene, the block in synapsis observed in c(2)M mutants allowed examination of the relationship between SC initiation and DSB formation. By double staining with CID, it was found that eliminating meiotic DSBs with a mei-W68 mutation did not prevent formation of either the centromere and euchromatic SC in a c(2)M mutant. The small decrease in the number of euchromatic SC sites in the c(2)M mei-W68 double mutant may indicate that the number of initiation sites is sensitive to DSB formation. Furthermore, SC initiation is not grossly affected by a reduction in crossing over (mei-218), an increase in crossing over (TM6), or a defect in DSB repair (okr). DSBs do not occur in the heterochromatin; thus, it is not surprising that centromere SC is independent of DSB formation. However, these results show that the initiation of euchromatic synapsis at mid zygotene does not depend on DSBs or crossovers (Tanneti, 2011).

Because DSBs or recombination are not required for synapsis in wild-type or c(2)M mutants, tests were performed to see whether structural components of the meiotic chromosomes regulate SC initiation. ORD is a meiosis-specific protein required for cohesion and crossover formation that may be a component of the SC lateral elements. Although previous studies have shown that ord mutant oocytes generate threads of C(3)G staining that resemble pachytene, the effect of ord on zygotene progression has not been previously examined (Tanneti, 2011).

Consistent with previous results, this study found that centromere clustering is defective and the association of SC proteins with the centromeres is disrupted in ord mutant oocytes. Furthermore, zygotene appeared abnormal; rather than observing centromeric and euchromatic SC initiation sites typical of mid-zygotene in early region 2a, it was found that many ord mutant pro-oocytes with C(3)G staining only around the nuclear DNA. Of the 108 pro-oocytes examined in five germaria, 36 (33%) had no nuclear C(3)G. The remaining pro-oocytes [72, (67%)] either had a number of C(3)G patches that was more typical of late zygotene, usually in region 2a, or were in pachytene. It is concluded that the centromeric and euchromatic synapsis sites typical of early and mid zygotene are absent in ord mutants, suggesting that, in the absence of ORD, synapsis does not initiate normally (Tanneti, 2011).

Because ord mutants do eventually form threads of SC, it was difficult to be sure that SC initiation was defective. To test whether ord has a role in mid-zygotene synapsis, tests were performed to see whether the euchromatic patches of C(3)G in a c(2)M mutant depend on ord. Even though both single mutants exhibit at least some SC formation, most of the C(3)G staining in the c(2)M ord double mutant surrounded the DNA and within the nucleus. This nonchromosomal C(3)G localization in the c(2)M ord double mutant was much more pronounced than in the ord single mutant. In addition, C(3)G-staining ring-like structures were observed similar to what has been reported in some c(3)G missense mutants. All the nonchromosomal C(3)G staining may be due to polycomplex formation. c(2)M ord double mutant pro-oocytes were identified by the prominent C(3)G around the DNA, and the number of C(3)G patches on the chromosomes was found to be drastically reduced compared to wild-type zygotene or either single mutant (Tanneti, 2011).

These results demonstrate that ord is required for the centromeric and euchromatic synapsis sites observed in c(2)M mutants. Conversely, C(2)M is required for the threadlike synapsis observed in ord mutants. The synergistic phenotype of the double mutant suggests that there are two types of synapsis initiation - one depends on ORD (early and mid-zygotene) and the other depends on C(2)M (late zygotene) - and that these are independent events. In the absence of both types of synapsis initiation, C(3)G cannot load onto the chromosomes and accumulates in polycomplexes (Tanneti, 2011).

Like other Kleisin family members, C(2)M has been shown to physically interact with the cohesin subunit SMC3 (Heidmann, 2004). To determine whether C(2)M localization depends on an interaction with cohesin, oocytes lacking SMC1 and SMC3 were examined. To examine oocytes lacking SMC3 (encoded by cap), the recently developed short hairpin RNA (shRNA) resource, which allows RNA interference (RNAi) knockdown of gene expression in the Drosophila female germline, was used. Both the chromosomal localization of C(3)G and C(2)M were absent when cap shRNA was expressed in the germline. Furthermore, SMC1 staining was eliminated, suggesting that the RNAi was effective at knocking out SMC3 function. Like the c(2)M ord double mutant, most C(3)G staining accumulated around the periphery of the DNA, suggesting that the function of SMC3 in synapsis occurs through at least two independent interactions with C(2)M and ORD. Unlike the c(2)M ord double mutant, however, it was not possible to distinguish the pro-oocytes from the nurse cells because C(3)G staining was evenly distributed among the cells in each germarium cyst. Importantly, oocyte selection was not perturbed because one cell in each cyst accumulated ORB protein, a cytoplasmic marker for the oocyte. Thus, the loss of SMC3 may have a more severe phenotype than the c(2)M ord double mutant (Tanneti, 2011).

These results were confirmed with the analysis of SMC1 mutant germline clones. As with cap RNAi, there was an absence of nuclear C(2)M and C(3)G threads in oocytes lacking SMC1, indicating a complete block in synapsis. Also similar to cap RNAi, the accumulation of ORB in one cell indicated that an oocyte was established. The only difference compared to cap RNAi was that there was much less C(3)G staining around the periphery of the DNA. It is not known whether this minor difference is due to the different methods (RNAi versus germline clone) or distinct functions of the two SMC proteins. Nevertheless, the results of these two experiments demonstrate that SMC1 and SMC3 are required for synapsis (Tanneti, 2011).

It is concluded that synapsis initiation during zygotene in Drosophila females occurs in three stages. In early zygotene, the centromeres are the first sites to accumulate the transverse filament protein C(3)G. Indeed, cohesion proteins SMC1, SMC3, and ORD are detected at the centromeres before meiotic prophase (prior to or during premeiotic S phase), which could explain why synapsis is first observed at the centromeres. Interestingly, the SC also forms first at the centromeres in budding yeast and depends on cohesion proteins. In mid-zygotene, synapsis initiates at a small number of euchromatic sites. These first two steps depend on the ORD protein. Finally, in late zygotene, synapsis initiates at a larger number of euchromatic sites. This stage requires C(2)M and appears to occur through a new set of initiation events rather than extending synapsis, or 'zipping up,' from the mid-zygotene initiation sites. Indeed, the synapsis initiation events in mid and late zygotene are independent and genetically separable, supporting a model where synapsis occurs through two independent waves of initiation events. In the absence of ORD, early and mid-zygotene synapsis events are skipped and the late zygotene initiation events occur with normal kinetics. This is not without consequence, however, because at the electron microscopy level, this synapsis is abnormal and tripartite SC is not visible. Both waves of synapsis initiation depend on the cohesin proteins SMC1 and SMC3, which may interact independently with C(2)M and ORD (Tanneti, 2011).

In addition to its role in centromere synapsis, ORD and the SMC proteins are required for the pairing and clustering of centromeres, whereas the SC components C(2)M or C(3)G are not. Thus, cohesion proteins may be able to function in a pairing role independent of DSBs, as Rec8 does in budding yeast for centromere coupling. It is suggested that the first euchromatic sites to initiate SC assembly in Drosophila are in regions where cohesion proteins are most abundant. This model is attractive because it provides a mechanism for SC initiation in the absence of DSBs. Interestingly, the number of euchromatic initiation sites in mid-zygotene or in c(2)M mutants approximates the number of crossovers in the genome. Not only do these mid-zygotene sites depend on ORD, but in ord mutants, crossing over is reduced to less than 10% of wild-type, even though DSBs occur normally. It is suggested that the reduction in crossing over in ord mutants is due to the absence of the synapsis initiation sites at mid-zygotene. Whether the synapsis initiation sites actually correspond to crossover sites awaits further study (Tanneti, 2011).

ORD may have a function similar to yeast Rec8 because it is required for synapsis at the centromeres and a subset of euchromatic sites. Interestingly, the findings with C(2)M, which is not an ortholog of Rec8, are also probably relevant to other species. Several recent studies have revealed Non-Rec8 Kleisin homologs in mouse and C. elegans (COH-3 and COH-). These parallels between the synapsis pathway in flies and that of organisms that depend on DSBs for synapsis could reflect the existence of a conserved underlying mechanism of synapsis. If synapsis initiation sites can be marked prior to DSB formation in a process involving cohesion proteins, and if proteins like Zip3 can be recruited in the absence of DSBs, as is true in C. elegans and likely in Drosophila, the timing of the DSB then becomes less of a determining factor in the process of synapsis (Tanneti, 2011).

Corona is required for higher-order assembly of transverse filaments into full-length synaptonemal complex in Drosophila oocytes

The synaptonemal complex (SC) is an intricate structure that forms between homologous chromosomes early during the meiotic prophase, where it mediates homolog pairing interactions and promotes the formation of genetic exchanges. In Drosophila melanogaster, C(3)G protein forms the transverse filaments (TFs) of the SC. The N termini of C(3)G homodimers localize to the Central Element (CE) of the SC, while the C-termini of C(3)G connect the TFs to the chromosomes via associations with the axial elements/lateral elements (AEs/LEs) of the SC. This study shows that the Drosophila protein Corona (CONA) co-localizes with C(3)G in a mutually dependent fashion and is required for the polymerization of C(3)G into mature thread-like structures, in the context both of paired homologous chromosomes and of C(3)G polycomplexes that lack AEs/LEs. Although AEs assemble in cona oocytes, they exhibit defects that are characteristic of c(3)G mutant oocytes, including failure of AE alignment and synapsis. These results demonstrate that CONA, which does not contain a coiled coil domain, is required for the stable 'zippering' of TFs to form the central region of the Drosophila SC. It is speculated that CONA's role in SC formation may be similar to that of the mammalian CE proteins SYCE2 and TEX12. However, the observation that AE alignment and pairing occurs in Tex12 and Syce2 mutant meiocytes but not in cona oocytes suggests that the SC plays a more critical role in the stable association of homologs in Drosophila than it does in mammalian cells (Page, 2008).

The formation of the central element of the synaptonemal complex may occur by multiple mechanisms: the roles of the N- and C-terminal domains of the Drosophila C(3)G protein in mediating synapsis and recombination

In Drosophila melanogaster oocytes, the C(3)G protein comprises the transverse filaments (TFs) of the synaptonemal complex (SC). Like other TF proteins, such as Zip1p in yeast and SCP1 in mammals, C(3)G is composed of a central coiled-coil-rich domain flanked by N- and C-terminal globular domains. This study analyze in-frame deletions within the N- and C-terminal regions of C(3)G in Drosophila oocytes. As is the case for Zip1p, a C-terminal deletion of C(3)G fails to attach to the lateral elements of the SC. Instead, this C-terminal deletion protein forms a large cylindrical polycomplex structure. EM analysis of this structure reveals a polycomplex of concentric rings alternating dark and light bands. However, unlike both yeast and mammals, all three proteins deleted for N-terminal regions completely abolished both SC and polycomplex formation. Both the N- and C-terminal deletions significantly reduce or abolish meiotic recombination similarly to c(3)G null homozygotes. To explain these data, it is proposed that in Drosophila the N terminus, but not the C-terminal globular domain, of C(3)G is critical for the formation of antiparallel pairs of C(3)G homodimers that span the central region and thus for assembly of complete TFs, while the C terminus is required to affix these homodimers to the lateral elements (Jeffress, 2007).

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

Using an antibody against the phosphorylated form of His2Av (gamma-His2Av),this study describes 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 gamma-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 gamma-His2Av foci. However, SC formation is incomplete or absent in the neighboring nurse cells, and gamma-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 gamma-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 gamma-His2Av foci, there is at least a 3:1 ratio of noncrossover to crossover products. From a comparison of the frequency of gamma-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 (Mehrotra, 2006).

Juxtaposition of C(2)M and the transverse filament protein C(3)G within the central region of Drosophila synaptonemal complex

The synaptonemal complex (SC) is intimately involved in the process of meiotic recombination in most organisms, but its exact role remains enigmatic. One reason for this uncertainty is that the overall structure of the SC is evolutionarily conserved, but many SC proteins are not. Two putative SC proteins have been identified in Drosophila: C(3)G and C(2)M. Mutations in either gene cause defects in SC structure and meiotic recombination. Although neither gene is well conserved at the amino acid level, the predicted secondary structure of C(3)G is similar to that of transverse filament proteins, and C(2)M is a distantly related member of the alpha-kleisin family that includes Rec8, a meiosis-specific cohesin protein. This study used immunogold labeling of SCs in Drosophila ovaries to localize C(3)G and C(2)M at the EM level. Both C(3)G and C(2)M are shown to be components of the SC, the orientation of C(3)G within the SC is similar to other transverse-filament proteins, and the N terminus of C(2)M is located in the central region adjacent to the lateral elements (LEs). Based on these data and the known phenotypes of C(2)M and C(3)G mutants, a model of SC structure is proposed in which C(2)M links C(3)G to the LEs (Anderson, 2005).

Meiotic recombination in Drosophila females depends on chromosome continuity between genetically defined boundaries

In the pairing-site model, specialized regions on each chromosome function to establish meiotic homolog pairing. Analysis of these sites could provide insights into the mechanism used by Drosophila females to form a synaptonemal complex (SC) in the absence of meiotic recombination. These specialized sites were first established on the X chromosome by noting that there were barriers to crossover suppression caused by translocation heterozygotes. These sites were genetically mapped and proposed to be pairing sites. By comparing the cytological breakpoints of third chromosome translocations to their patterns of crossover suppression, two sites on chromosome 3R were mapped. Experiments were performed to determine if these sites have a role in meiotic homolog pairing and the initiation of recombination. Translocation heterozygotes exhibit reduced gene conversion within the crossover-suppressed region, consistent with an effect on the initiation of meiotic recombination. To determine if homolog pairing is disrupted in translocation heterozygotes, fluorescent in situ hybridization was used to measure the extent of homolog pairing. In wild-type oocytes, homologs are paired along their entire lengths prior to accumulation of the SC protein C(3)G. Surprisingly, translocation heterozygotes exhibited homolog pairing similar to wild type within the crossover-suppressed regions. This result contrasted with observations of c(3)G mutant females, which were found to be defective in pairing. It is proposed that each Drosophila chromosome is divided into several domains by specialized sites. These sites are not required for homolog pairing. Instead, the initiation of meiotic recombination requires continuity of the meiotic chromosome structure within each of these domains (Sherizen, 2005).

Relationship of DNA double-strand breaks to synapsis in Drosophila

The relationship between synaptonemal complex formation (synapsis) and double-strand break formation (recombination initiation) differs between organisms. Although double-strand break creation is required for normal synapsis in Saccharomyces cerevisiae and the mouse, it is not necessary for synapsis in Drosophila and Caenorhabditis elegans. To investigate the timing of and requirements for double-strand break formation during Drosophila meiosis, an antibody was used that recognizes a histone modification at double-strand break sites, phosphorylation of HIS2AV (gamma-HIS2AV). The results support the hypothesis that double-strand break formation occurs after synapsis. Interestingly, a low (10%-25% of wildtype) number of gamma-HIS2AV foci were observed in c(3)G mutants, which fail to assemble synaptonemal complex, suggesting that there may be both synaptonemal complex-dependent and synaptonemal complex-independent mechanisms for generating double-strand breaks. Furthermore, mutations in Drosophila Rad54 (okr) and Rad51 (spnB) homologs cause delayed and prolonged gamma-HIS2AV staining, suggesting that double-strand break repair is delayed but not eliminated in these mutants. There may also be an interaction between the recruitment of repair proteins and phosphorylation (Jang, 2003).

c(3)G encodes a Drosophila synaptonemal complex protein

The meiotic mutant c(3)G (crossover suppressor on 3 of Gowen) abolishes both synaptonemal complex (SC) formation and meiotic recombination, whereas mutations in the mei-W68 and mei-P22 genes prevent recombination but allow normal SC to form. These data, as well as a century of cytogenetic studies, support the argument that meiotic recombination between homologous chromosomes in Drosophila females requires synapsis and SC formation. This study has cloned the c(3)G gene and shown that it encodes a protein that is structurally similar to SC proteins from yeast and mammals. Immunolocalization of the C(3)G protein, as well as the analysis of a C(3)G-eGFP expression construct, reveals that C(3)G is present in a thread-like pattern along the lengths of chromosomes in meiotic prophase, consistent with a role as an SC protein present on meiotic bivalents. The availability of a marker for SC in Drosophila allowed the investigation of the extent of synapsis in exchange-defective mutants. These studies indicate that SC formation is impaired in certain meiotic mutants and that the synaptic defect correlates with the exchange defects. Moreover, the observation of interference among the residual exchanges in these mutant oocytes implies that complete SC formation is not required for crossover interference in Drosophila (Page, 2001).

Search PubMed for articles about Drosophila C(3)G

Anderson, L. K., Royer, S. M., Page, S. L., McKim, K. S., Lai, A., Lilly, M. A. and Hawley, R. S. (2005). Juxtaposition of C(2)M and the transverse filament protein C(3)G within the central region of Drosophila synaptonemal complex. Proc Natl Acad Sci U S A 102(12): 4482-4487. PubMed ID: 15767569

Billmyre, K. K., Cahoon, C. K., Heenan, G. M., Wesley, E. R., Yu, Z., Unruh, J. R., Takeo, S. and Hawley, R. S. (2019). X chromosome and autosomal recombination are differentially sensitive to disruptions in SC maintenance. Proc Natl Acad Sci U S A. PubMed ID: 31570610

Heidmann, D., Horn, S., Heidmann, S., Schleiffer, A., Nasmyth, K. and Lehner, C. F. (2004). The Drosophila meiotic kleisin C(2)M functions before the meiotic divisions. Chromosoma 113(4): 177-187. PubMed ID: 15375666

Hemmer, L. W. and Blumenstiel, J. P. (2016). Holding it together: rapid evolution and positive selection in the synaptonemal complex of Drosophila. BMC Evol Biol 16: 91. PubMed ID: 27150275

Jang, J. K., Sherizen, D. E., Bhagat, R., Manheim, E. A. and McKim, K. S. (2003). Relationship of DNA double-strand breaks to synapsis in Drosophila. J Cell Sci 116(Pt 15): 3069-3077. PubMed ID: 12799415

Jeffress, J. K., Page, S. L., Royer, S. K., Belden, E. D., Blumenstiel, J. P., Anderson, L. K. and Hawley, R. S. (2007). The formation of the central element of the synaptonemal complex may occur by multiple mechanisms: the roles of the N- and C-terminal domains of the Drosophila C(3)G protein in mediating synapsis and recombination. Genetics 177(4): 2445-2456. PubMed ID: 17947423

Mehrotra, S. and McKim, K. S. (2006). Temporal analysis of meiotic DNA double-strand break formation and repair in Drosophila females. PLoS Genet 2(11): e200. PubMed ID: 17166055

Page, S. L. and Hawley, R. S. (2001). c(3)G encodes a Drosophila synaptonemal complex protein. Genes Dev. 15: 3130-3143. PubMed ID: 11731477

Page, S. L., and Hawley, R. S. (2004). The genetics and molecular biology of the synaptonemal complex. Annu. Rev. Cell Dev. Biol. 20: 525-558. PubMed ID: 15473851

Page, S. L., Khetani, R. S., Lake, C. M., Nielsen, R. J., Jeffress, J. K., Warren, W. D., Bickel, S. E. and Hawley, R. S. (2008). Corona is required for higher-order assembly of transverse filaments into full-length synaptonemal complex in Drosophila oocytes. PLoS Genet 4(9): e1000194. PubMed ID: 18802461

Radford, S. J,, Hoang, T. L., Gluszek, A. A., Ohkura, H. and McKim, K. S. (2015). Lateral and end-on kinetochore attachments are coordinated to achieve bi-orientation in Drosophila oocytes. PLoS Genet. 11(10):e1005605. PubMed ID: 26473960

Sherizen, D., Jang, J. K., Bhagat, R., Kato, N. and McKim, K. S. (2005). Meiotic recombination in Drosophila females depends on chromosome continuity between genetically defined boundaries. Genetics 169(2): 767-781. PubMed ID: 15545646

Singh, A., Dutta, D., Paul, M. S., Verma, D., Mutsuddi, M. and Mukherjee, A. (2018). Pleiotropic functions of the chromodomain-containing protein Hat-trick during oogenesis in Drosophila melanogaster. G3 (Bethesda) [Epub ahead of print]. PubMed ID: 29367451

Tanneti, N. S., Landy, K., Joyce, E. F. and McKim, K. S. (2011). A pathway for synapsis initiation during zygotene in Drosophila oocytes. Curr. Biol. 21(21): 1852-7. PubMed ID: 22036181

Wang, L. I., Das, A. and McKim, K. S. (2019). Sister centromere fusion during meiosis I depends on maintaining cohesins and destabilizing microtubule attachments. PLoS Genet 15(5): e1008072. PubMed ID: 31150390

Watts, F. Z., and Hoffmann, E. (2011). SUMO meets meiosis: An encounter at the synaptonemal complex: SUMO chains and sumoylated proteins suggest that heterogeneous and complex interactions lie at the centre of the synaptonemal complex. Bioessays 33: 529-537. PubMed ID: 21590786

Yan, R. and McKee, B. D. (2013). The cohesion protein SOLO associates with SMC1 and is required for synapsis, recombination, homolog bias and cohesion and pairing of centromeres in Drosophila meiosis. PLoS Genet 9(7): e1003637. PubMed ID: 23874232

date revised: 5 November 2019

Home page: The Interactive Fly © 2011 Thomas Brody, Ph.D.