Identification of two proteins required for conjunction and regular segregation of achiasmate homologs in Drosophila male meiosis

Unusual modes of cell and nuclear divisions characterise Drosophila development

The growth and development of metazoan organisms is dependent upon a co-ordinated programme of cellular proliferation and differentiation, from the initial formation of the zygote through to maintenance of mature organs in adult organisms. Early studies of proliferation of ex vivo cultures and unicellular eukaryotes described a cyclic nature of cell division characterised by periods of DNA synthesis (S-phase) and segregation of newly synthesized chromosomes (M-phase) interspersed by seeming inactivity, the gap phases, G1 and G2. We now know that G1 and G2 play critical roles in regulating the cell cycle, including monitoring of favourable environmental conditions to facilitate cell division, and ensuring genomic integrity prior to DNA replication and nuclear division. M-phase is usually followed by the physical separation of nascent daughters, termed cytokinesis. These phases where G1 leads to S phase, followed by G2 prior to M phase and the subsequent cytokinesis to produce two daughters, both identical in genomic composition and cellular morphology are what might be termed an archetypal cell division. Studies of development of many different organs in different species have demonstrated that this stereotypical cell cycle is often subverted to produce specific developmental outcomes, and examples from over 100 years of analysis of the development of Drosophila melanogaster have uncovered many different modes of cell division within this one species.

Uncharted territories: Solving the mysteries of male meiosis in flies

The segregation of homologous chromosomes during meiosis typically requires tight end-to-end chromosome pairing. However, in Drosophila spermatogenesis, male flies segregate their chromosomes without classic synaptonemal complex formation and without recombination, instead compartmentalizing homologs into subnuclear domains known as chromosome territories (CTs). How homologs find each other in the nucleus and are separated into CTs has been one of the biggest riddles in chromosome biology. Here, we discuss our current understanding of pairing and CT formation in flies and review recent data on how homologs are linked and partitioned during meiosis in male flies.

The courtship choreography of homologous chromosomes: timing and mechanisms of DSB-independent pairing

Meiosis involves deep changes in the spatial organisation and interactions of chromosomes enabling the two primary functions of this process: increasing genetic diversity and reducing ploidy level. These two functions are ensured by crucial events such as homologous chromosomal pairing, synapsis, recombination and segregation. In most sexually reproducing eukaryotes, homologous chromosome pairing depends on a set of mechanisms, some of them associated with the repair of DNA double-strand breaks (DSBs) induced at the onset of prophase I, and others that operate before DSBs formation. In this article, we will review various strategies utilised by model organisms for DSB-independent pairing. Specifically, we will focus on mechanisms such as chromosome clustering, nuclear and chromosome movements, as well as the involvement of specific proteins, non-coding RNA, and DNA sequences.

Caenorhabditis elegans spermatocytes can segregate achiasmate homologous chromosomes apart at higher than random frequency during meiosis I

Chromosome segregation errors during meiosis are the leading cause of aneuploidy. Faithful chromosome segregation during meiosis in most eukaryotes requires a crossover which provides a physical attachment holding homologs together in a "bivalent." Crossovers are critical for homologs to be properly aligned and partitioned in the first meiotic division. Without a crossover, individual homologs (univalents) might segregate randomly, resulting in aneuploid progeny. However, Caenorhabditis elegans zim-2 mutants, which have crossover defects on chromosome V, have fewer dead embryos than that expected from random segregation. This deviation from random segregation is more pronounced in zim-2 males than that in females. We found three phenomena that can explain this apparent discrepancy. First, we detected crossovers on chromosome V in both zim-2(tm574) oocytes and spermatocytes, suggesting a redundant mechanism to make up for the ZIM-2 loss. Second, after accounting for the background crossover frequency, spermatocytes produced significantly more euploid gametes than what would be expected from random segregation. Lastly, trisomy of chromosome V is viable and fertile. Together, these three phenomena allow zim-2(tm574) mutants with reduced crossovers on chromosome V to have more viable progeny. Furthermore, live imaging of meiosis in spo-11(me44) oocytes and spermatocytes, which exhibit crossover failure on all 6 chromosomes, showed 12 univalents segregating apart in roughly equal masses in a homology-independent manner, supporting the existence of a mechanism that segregates any 2 chromosomes apart.

Homologous chromosomes are stably conjoined for Drosophila male meiosis I by SUM, a multimerized protein assembly with modules for DNA-binding and for separase-mediated dissociation co-opted from cohesin

For meiosis I, homologous chromosomes must be paired into bivalents. Maintenance of homolog conjunction in bivalents until anaphase I depends on crossovers in canonical meiosis. However, instead of crossovers, an alternative system achieves homolog conjunction during the achiasmate male meiosis of Drosophila melanogaster. The proteins SNM, UNO and MNM are likely constituents of a physical linkage that conjoins homologs in D. melanogaster spermatocytes. Here, we report that SNM binds tightly to the C-terminal region of UNO. This interaction is homologous to that of the cohesin subunits stromalin/Scc3/STAG and α-kleisin, as revealed by sequence similarities, structure modeling and cross-link mass spectrometry. Importantly, purified SU_C, the heterodimeric complex of SNM and the C-terminal region of UNO, displayed DNA-binding in vitro. DNA-binding was severely impaired by mutational elimination of positively charged residues from the C-terminal helix of UNO. Phenotypic analyses in flies fully confirmed the physiological relevance of this basic helix for chromosome-binding and homolog conjunction during male meiosis. Beyond DNA, SU_C also bound MNM, one of many isoforms expressed from the complex mod(mdg4) locus. This binding of MNM to SU_C was mediated by the MNM-specific C-terminal region, while the purified N-terminal part common to all Mod(mdg4) isoforms multimerized into hexamers in vitro. Similarly, the UNO N-terminal domain formed tetramers in vitro. Thus, we suggest that multimerization confers to SUM, the assemblies composed of SNM, UNO and MNM, the capacity to conjoin homologous chromosomes stably by the resultant multivalent DNA-binding. Moreover, to permit homolog separation during anaphase I, SUM is dissociated by separase, since UNO, the α-kleisin-related protein, includes a separase cleavage site. In support of this proposal, we demonstrate that UNO cleavage by tobacco etch virus protease is sufficient to release homolog conjunction in vivo after mutational exchange of the separase cleavage site with that of the bio-orthogonal protease.

Mod(mdg4) variants repress telomeric retrotransposon HeT-A by blocking subtelomeric enhancers

Telomeres in Drosophila are composed of sequential non-LTR retrotransposons HeT-A, TART and TAHRE. Although they are repressed by the PIWI-piRNA pathway or heterochromatin in the germline, the regulation of these retrotransposons in somatic cells is poorly understood. In this study, we demonstrated that specific splice variants of Mod(mdg4) repress HeT-A by blocking subtelomeric enhancers in ovarian somatic cells. Among the variants, we found that the Mod(mdg4)-N variant represses HeT-A expression the most efficiently. Subtelomeric sequences bound by Mod(mdg4)-N block enhancer activity within subtelomeric TAS-R repeats. This enhancer-blocking activity is increased by the tandem association of Mod(mdg4)-N to repetitive subtelomeric sequences. In addition, the association of Mod(mdg4)-N couples with the recruitment of RNA polymerase II to the subtelomeres, which reinforces its enhancer-blocking function. Our findings provide novel insights into how telomeric retrotransposons are regulated by the specific variants of insulator proteins associated with subtelomeric sequences.

Teflon promotes chromosomal recruitment of homolog conjunction proteins during Drosophila male meiosis

Meiosis in males of higher dipterans is achiasmate. In their spermatocytes, pairing of homologs into bivalent chromosomes does not include synaptonemal complex and crossover formation. While crossovers preserve homolog conjunction until anaphase I during canonical meiosis, an alternative system is used in dipteran males. Mutant screening in Drosophila melanogaster has identified teflon (tef) as being required specifically for alternative homolog conjunction (AHC) of autosomal bivalents. The additional known AHC genes, snm, uno and mnm, are needed for the conjunction of autosomal homologs and of sex chromosomes. Here, we have analyzed the pattern of TEF protein expression. TEF is present in early spermatocytes but cannot be detected on bivalents at the onset of the first meiotic division, in contrast to SNM, UNO and MNM (SUM). TEF binds to polytene chromosomes in larval salivary glands, recruits MNM by direct interaction and thereby, indirectly, also SNM and UNO. However, chromosomal SUM association is not entirely dependent on TEF, and residual autosome conjunction occurs in tef null mutant spermatocytes. The higher tef requirement for autosomal conjunction is likely linked to the quantitative difference in the amount of SUM protein that provides conjunction of autosomes and sex chromosomes, respectively. During normal meiosis, SUM proteins are far more abundant on sex chromosomes compared to autosomes. Beyond promoting SUM recruitment, TEF has a stabilizing effect on SUM proteins. Increased SUM causes excess conjunction and consequential chromosome missegregation during meiosis I after co-overexpression. Similarly, expression of SUM without TEF, and even more potently with TEF, interferes with chromosome segregation during anaphase of mitotic divisions in somatic cells, suggesting that the known AHC proteins are sufficient for establishment of ectopic chromosome conjunction. Overall, our findings suggest that TEF promotes alternative homolog conjunction during male meiosis without being part of the final physical linkage between chromosomes.

Sexual reproduction depends on meiosis, a special cell division that generates haploid cells. Haploid cells have only one set of chromosomes in contrast to the diploid precursor cell, which has two sets. Haploid cells can differentiate into gametes. Fusion of two gametes during fertilization recreates the diploid state. Meiosis is distinct in males and females to produce two distinct types of compatible gametes, sperm and egg. In the fly Drosophila melanogaster, sex-specific differences are particularly pronounced. While pairing of homologous chromosomes into bivalents early in meiosis proceeds in a canonical manner in females, males use an alternative system. This system maintains homolog pairing, replacing crossovers that result from homologous recombination during canonical meiosis. Four genes (snm, uno, mnm and tef) are known to be required specifically for alternative homolog conjunction in males. Here, we demonstrate that the TEF protein binds directly to MNM. Thereby, TEF promotes the recruitment of MNM and consequentially SNM and UNO to chromosomes. However, while SNM, UNO and MNM remain on bivalent chromosomes until they are separated apart during the first meiotic division, TEF disappears prematurely, suggesting that it is not part of the final physical linkage between homologous chromosomes.

Dispersive forces and resisting spot welds by alternative homolog conjunction govern chromosome shape in Drosophila spermatocytes during prophase I

The bivalent chromosomes that are generated during prophase of meiosis I comprise a pair of homologous chromosomes. Homolog pairing during prophase I must include mechanisms that avoid or eliminate entanglements between non-homologous chromosomes. In Drosophila spermatocytes, non-homologous associations are disrupted by chromosome territory formation, while linkages between homologous chromosomes are maintained by special conjunction proteins. These proteins function as alternative for crossovers that link homologs during canonical meiosis but are absent during the achiasmate Drosophila male meiosis. How and where within bivalents the alternative homolog conjunction proteins function is still poorly understood. To clarify the rules that govern territory formation and alternative homolog conjunction, we have analyzed spermatocytes with chromosomal aberrations. We examined territory formation after acute chromosome cleavage by Cas9, targeted to the dodeca satellite adjacent to the centromere of chromosome 3 specifically in spermatocytes. Moreover, we studied territory organization, as well as the eventual orientation of chromosomes during meiosis I, in spermatocytes with stable structural aberrations, including heterozygous reciprocal autosomal translocations. Our observations indicate that alternative homolog conjunction is applied in a spatially confined manner. Comparable to crossovers, only a single conjunction spot per chromosome arm appears to be applied usually. These conjunction spots resist separation by the dispersing forces that drive apart homologous pericentromeric heterochromatin and embedded centromeres within territories, as well as the distinct chromosomal entities into peripheral, maximally separated territories within the spermatocyte nucleus.

Already the primordial eukaryote appears to have used meiosis for sexual reproduction, because this sophisticated process follows a canonical program in lineages ranging from unicellular organisms to plants and animals. The maternal and paternal copies of a particular chromosome, i.e., the homologs, are first physically linked into a bivalent before the first meiotic division. Linkage is essential for error-free chromosome segregation. In canonical meiosis, linkage is achieved by crossovers. These are regulated so that each chromosome pair is linked, but only by very few crossovers. Surprisingly, crossovers are absent during meiosis in males of the fruit fly Drosophila melanogaster. Instead, an alternative homolog conjunction system is used. It is not yet clear how this functions. Here, we demonstrate that the alternative chromosome glue appears to be applied in a locally restricted manner rather than all along the paired homologs. Just two spots of glue appear to conjoin the two homologous chromosomes usually, with one spot linking the left and another the right chromosome arm. Thus, number and location of linkages appear to be similar as crossovers, raising the possibility of mechanistic similarities in the establishment of the two distinct types of homolog linkage.

Phase separation in controlling meiotic chromosome dynamics

Sexually reproducing organisms produce haploid gametes through meiotic cell division, during which a single round of DNA replication is followed by two consecutive chromosome segregation. A series of meiosis-specific events take place during the meiotic prophase to ensure successful chromosome segregation. These events include programmed DNA double-strand break formation, chromosome movement driven by cytoplasmic forces, homologous pairing, synaptonemal complex installation, and inter-homolog crossover formation. Phase separation has emerged as a key principle controlling cellular biomolecular material organization and biological processes. Recent studies have revealed the involvements of phase separation in assembling meiotic chromosome-associated structures. Here we review and discuss how phase separation may participate in meiotic chromosome dynamics and propose that it may provide opportunities to understand the mysteries in meiotic regulations.

Bivalent individualization during chromosome territory formation in Drosophila spermatocytes by controlled condensin II protein activity and additional force generators

Reduction of genome ploidy from diploid to haploid necessitates stable pairing of homologous chromosomes into bivalents before the start of the first meiotic division. Importantly, this chromosome pairing must avoid interlocking of non-homologous chromosomes. In spermatocytes of Drosophila melanogaster, where homolog pairing does not involve synaptonemal complex formation and crossovers, associations between non-homologous chromosomes are broken up by chromosome territory formation in early spermatocytes. Extensive non-homologous associations arise from the coalescence of the large blocks of pericentromeric heterochromatin into a chromocenter and from centromere clustering. Nevertheless, during territory formation, bivalents are moved apart into spatially separate subnuclear regions. The condensin II subunits, Cap-D3 and Cap-H2, have been implicated, but the remarkable separation of bivalents during interphase might require more than just condensin II. For further characterization of this process, we have applied time-lapse imaging using fluorescent markers of centromeres, telomeres and DNA satellites in pericentromeric heterochromatin. We describe the dynamics of the disruption of centromere clusters and the chromocenter in normal spermatocytes. Mutations in Cap-D3 and Cap-H2 abolish chromocenter disruption, resulting in excessive chromosome missegregation during M I. Chromocenter persistence in the mutants is not mediated by the special system, which conjoins homologs in compensation for the absence of crossovers in Drosophila spermatocytes. However, overexpression of Cap-H2 precluded conjunction between autosomal homologs, resulting in random segregation of univalents. Interestingly, Cap-D3 and Cap-H2 mutant spermatocytes displayed conspicuous stretching of the chromocenter, as well as occasional chromocenter disruption, suggesting that territory formation might involve forces unrelated to condensin II. While the molecular basis of these forces remains to be clarified, they are not destroyed by inhibitors of F actin and microtubules. Our results indicate that condensin II activity promotes chromosome territory formation in co-operation with additional force generators and that careful co-ordination with alternative homolog conjunction is crucial.

Dynamic localization of DNA topoisomerase I and its functional relevance during Drosophila development

DNA topoisomerase I (Top1) maintains chromatin conformation during transcription. While Top1 is not essential in simple eukaryotic organisms such as yeast, it is required for the development of multicellular organisms. In fact, tissue and cell-type-specific functions of Top1 have been suggested in the fruit fly Drosophila. A better understanding of Top1’s function in the context of development is important as Top1 inhibitors are among the most widely used anticancer drugs. As a step toward such a better understanding, we studied its localization in live cells of Drosophila. Consistent with prior results, Top1 is highly enriched at the nucleolus in transcriptionally active polyploid cells, and this enrichment responds to perturbation of transcription. In diploid cells, we uncovered evidence for Top1 foci formation at genomic regions not limited to the active rDNA locus, suggestive of novel regulation of Top1 recruitment. In the male germline, Top1 is highly enriched at the paired rDNA loci on sex chromosomes suggesting that it might participate in regulating their segregation during meiosis. Results from RNAi-mediated Top1 knockdown lend support to this hypothesis. Our study has provided one of the most comprehensive descriptions of Top1 localization during animal development.

Divergent Gene Expression Following Duplication of Meiotic Genes in the Stick Insect Clitarchus hookeri

Some animal groups, such as stick insects (Phasmatodea), have repeatedly evolved alternative reproductive strategies, including parthenogenesis. Genomic studies have found modification of the genes underlying meiosis exists in some of these animals. Here we examine the evolution of copy number, evolutionary rate, and gene expression in candidate meiotic genes of the New Zealand geographic parthenogenetic stick insect Clitarchus hookeri. We characterized 101 genes from a de novo transcriptome assembly from female and male gonads that have homology with meiotic genes from other arthropods. For each gene we determined copy number, the pattern of gene duplication relative to other arthropod orthologs, and the potential for meiosis-specific expression. There are five genes duplicated in C. hookeri, including one also duplicated in the stick insect Timema cristinae, that are not or are uncommonly duplicated in other arthropods. These included two sister chromatid cohesion associated genes (SA2 and SCC2), a recombination gene (HOP1), an RNA-silencing gene (AGO2) and a cell-cycle regulation gene (WEE1). Interestingly, WEE1 and SA2 are also duplicated in the cyclical parthenogenetic aphid Acyrthosiphon pisum and Daphnia duplex, respectively, indicating possible roles in the evolution of reproductive mode. Three of these genes (SA2, SCC2, and WEE1) have one copy displaying gonad-specific expression. All genes, with the exception of WEE1, have significantly different nonsynonymous/synonymous ratios between the gene duplicates, indicative of a shift in evolutionary constraints following duplication. These results suggest that stick insects may have evolved genes with novel functions in gamete production by gene duplication.

Adenosine Receptor and Its Downstream Targets, Mod(mdg4) and Hsp70, Work as a Signaling Pathway Modulating Cytotoxic Damage in Drosophila

Adenosine (Ado) is an important signaling molecule involved in stress responses. Studies in mammalian models have shown that Ado regulates signaling mechanisms involved in “danger-sensing” and tissue-protection. Yet, little is known about the role of Ado signaling in Drosophila. In the present study, we observed lower extracellular Ado concentration and suppressed expression of Ado transporters in flies expressing mutant huntingtin protein (mHTT). We altered Ado signaling using genetic tools and found that the overexpression of Ado metabolic enzymes, as well as the suppression of Ado receptor (AdoR) and transporters (ENTs), were able to minimize mHTT-induced mortality. We also identified the downstream targets of the AdoR pathway, the modifier of mdg4 (Mod(mdg4)) and heat-shock protein 70 (Hsp70), which modulated the formation of mHTT aggregates. Finally, we showed that a decrease in Ado signaling affects other Drosophila stress reactions, including paraquat and heat-shock treatments. Our study provides important insights into how Ado regulates stress responses in Drosophila.

The Interchromosomal Effect: Different Meanings for Different Organisms

The term interchromosomal effect was originally used to describe a change in the distribution of exchange in the presence of an inversion. First characterized in the 1920s by early Drosophila researchers, it has been observed in multiple organisms. Nearly half a century later, the term began to appear in the human genetics literature to describe the hypothesis that parental chromosome differences, such as translocations or inversions, may increase the frequency of meiotic chromosome nondisjunction. Although it remains unclear if chromosome aberrations truly affect the segregation of structurally normal chromosomes in humans, the use of the term interchromosomal effect in this context persists. This article explores the history of the use of the term interchromosomal effect and discusses how chromosomes with structural aberrations are segregated during meiosis.

Chromosome separation during Drosophila male meiosis I requires separase-mediated cleavage of the homolog conjunction protein UNO

Regular chromosome segregation during the first meiotic division requires prior pairing of homologous chromosomes into bivalents. During canonical meiosis, linkage between homologous chromosomes is maintained until late metaphase I by chiasmata resulting from meiotic recombination in combination with distal sister chromatid cohesion. Separase-mediated elimination of cohesin from chromosome arms at the end of metaphase I permits terminalization of chiasmata and homolog segregation to opposite spindle poles during anaphase I. Interestingly, separase is also required for bivalent splitting during meiosis I in Drosophila males, where homologs are conjoined by an alternative mechanism independent of meiotic recombination and cohesin. Here we report the identification of a novel alternative homolog conjunction protein encoded by the previously uncharacterized gene univalents only (uno). The univalents that are present in uno null mutants at the start of meiosis I, instead of normal bivalents, are segregated randomly. In wild type, UNO protein is detected in dots associated with bivalent chromosomes and most abundantly at the localized pairing site of the sex chromosomes. UNO is cleaved by separase. Expression of a mutant UNO version with a non-functional separase cleavage site restores homolog conjunction in a uno null background. However, separation of bivalents during meiosis I is completely abrogated by this non-cleavable UNO version. Therefore, we propose that homolog separation during Drosophila male meiosis I is triggered by separase-mediated cleavage of UNO.

Sex Chromosome Pairing Mediated by Euchromatic Homology in Drosophila Male Meiosis

Diploid germline cells must undergo two consecutive meiotic divisions before differentiating as haploid sex cells. During meiosis I, homologs pair and remain conjoined until segregation at anaphase. Drosophila melanogaster spermatocytes are unique in that the canonical events of meiosis I including synaptonemal complex formation, double-strand DNA breaks, and chiasmata are absent. Sex chromosomes pair at intergenic spacer sequences within the ribosomal DNA (rDNA). Autosomes pair at numerous euchromatic homologies, but not at heterochromatin, suggesting that pairing may be limited to specific sequences. However, previous work generated from genetic segregation assays or observations of late prophase I/prometaphase I chromosome associations fail to differentiate pairing from maintenance of pairing (conjunction). Here, we separately examined the capability of X euchromatin to pair and conjoin using an rDNA-deficient X and a series of Dp(1;Y) chromosomes. Genetic assays showed that duplicated X euchromatin can substitute for endogenous rDNA pairing sites. Segregation was not proportional to homology length, and pairing could be mapped to nonoverlapping sequences within a single Dp(1;Y) Using fluorescence in situ hybridization to early prophase I spermatocytes, we showed that pairing occurred with high fidelity at all homologies tested. Pairing was unaffected by the presence of X rDNA, nor could it be explained by rDNA magnification. By comparing genetic and cytological data, we determined that centromere proximal pairings were best at segregation. Segregation was dependent on the conjunction protein Stromalin in Meiosis, while the autosomal-specific Teflon was dispensable. Overall, our results suggest that pairing may occur at all homologies, but there may be sequence or positional requirements for conjunction.

Drosophila Doublefault protein coordinates multiple events during male meiosis by controlling mRNA translation

During the extended prophase of Drosophila gametogenesis, spermatocytes undergo robust gene transcription and store many transcripts in the cytoplasm in a repressed state, until translational activation of select mRNAs in later steps of spermatogenesis. Here, we characterize the Drosophila Doublefault (Dbf) protein as a C2H2 zinc-finger protein, primarily expressed in testes, that is required for normal meiotic division and spermiogenesis. Loss of Dbf causes premature centriole disengagement and affects spindle structure, chromosome segregation and cytokinesis. We show that Dbf interacts with the RNA-binding protein Syncrip/hnRNPQ, a key regulator of localized translation in Drosophila We propose that the pleiotropic effects of dbf loss-of-function mutants are associated with the requirement of dbf function for translation of specific transcripts in spermatocytes. In agreement with this hypothesis, Dbf protein binds cyclin B mRNA and is essential for translation of cyclin B in mature spermatocytes.

Studying Interactions between the Mod(mdg4)-67.2 Protein and Other Mod(mdg4) Isoforms in the Embryonic Cells of Drosophila melanogaster

It is found that, in embryonic D. melanogaster cells, Mod(mdg4) protein isoforms can interact with each other through BTB domains. However, this nonspecific interaction is destroyed as a result of recruitment of protein complexes to the chromatin sites.

MNM and SNM maintain but do not establish achiasmate homolog conjunction during Drosophila male meiosis

The first meiotic division reduces genome ploidy. This requires pairing of homologous chromosomes into bivalents that can be bi-oriented within the spindle during prometaphase I. Thereafter, pairing is abolished during late metaphase I, and univalents are segregated apart onto opposite spindle poles during anaphase I. In contrast to canonical meiosis, homologous chromosome pairing does not include the formation of a synaptonemal complex and of cross-overs in spermatocytes of Drosophila melanogaster. The alternative pairing mode in these cells depends on mnm and snm. These genes are required exclusively in spermatocytes specifically for successful conjunction of chromosomes into bivalents. Available evidence suggests that MNM and SNM might be part of a physical linkage that directly conjoins chromosomes. Here this notion was analyzed further. Temporal variation in delivery of mnm and snm function was realized by combining various transgenes with null mutant backgrounds. The observed phenotypic consequences provide strong evidence that MNM and SNM contribute directly to chromosome linkage. Premature elimination of these proteins results in precocious bivalent splitting. Delayed provision results in partial conjunction defects that are more pronounced in autosomal bivalents compared to the sex chromosome bivalent. Overall, our findings suggest that MNM and SNM cannot re-establish pairing of chromosomes into bivalents if provided after a chromosome-specific time point of no return. When delivered before this time point, they fortify preformed linkages in order to preclude premature bivalent splitting by the disruptive forces that drive chromosome territory formation during spermatocyte maturation and chromosome condensation during entry into meiosis I.

Multiple interactions are involved in a highly specific association of the Mod(mdg4)-67.2 isoform with the Su(Hw) sites in Drosophila

The best-studied Drosophila insulator complex consists of two BTB-containing proteins, the Mod(mdg4)-67.2 isoform and CP190, which are recruited to the chromatin through interactions with the DNA-binding Su(Hw) protein. It was shown previously that Mod(mdg4)-67.2 is critical for the enhancer-blocking activity of the Su(Hw) insulators and it differs from more than 30 other Mod(mdg4) isoforms by the C-terminal domain required for a specific interaction with Su(Hw) only. The mechanism of the highly specific association between Mod(mdg4)-67.2 and Su(Hw) is not well understood. Therefore, we have performed a detailed analysis of domains involved in the interaction of Mod(mdg4)-67.2 with Su(Hw) and CP190. We found that the N-terminal region of Su(Hw) interacts with the glutamine-rich domain common to all the Mod(mdg4) isoforms. The unique C-terminal part of Mod(mdg4)-67.2 contains the Su(Hw)-interacting domain and the FLYWCH domain that facilitates a specific association between Mod(mdg4)-67.2 and the CP190/Su(Hw) complex. Finally, interaction between the BTB domain of Mod(mdg4)-67.2 and the M domain of CP190 has been demonstrated. By using transgenic lines expressing different protein variants, we have shown that all the newly identified interactions are to a greater or lesser extent redundant, which increases the reliability in the formation of the protein complexes.

Spermiogenesis and Male Fertility Require the Function of Suppressor of Hairy-Wing in Somatic Cyst Cells of Drosophila

Drosophila Suppressor of Hairy-wing [Su(Hw)] protein is an example of a multivalent transcription factor. Although best known for its role in establishing the chromatin insulator of the gypsy retrotransposon, Su(Hw) functions as an activator and repressor at non-gypsy genomic sites. It remains unclear how the different regulatory activities of Su(Hw) are utilized during development. Motivated from observations of spatially restricted expression of Su(Hw) in the testis, we investigated the role of Su(Hw) in spermatogenesis to advance an understanding of its developmental contributions as an insulator, repressor, and activator protein. We discovered that Su(Hw) is required for sustained male fertility. Although dynamics of Su(Hw) expression coincide with changes in nuclear architecture and activation of coregulated testis-specific gene clusters, we show that loss of Su(Hw) does not disrupt meiotic chromosome pairing or transcription of testis-specific genes, suggesting that Su(Hw) has minor architectural or insulator functions in the testis. Instead, Su(Hw) has a prominent role as a repressor of neuronal genes, consistent with suggestions that Su(Hw) is a functional homolog of mammalian REST, a repressor of neuronal genes in non-neuronal tissues. We show that Su(Hw) regulates transcription in both germline and somatic cells. Surprisingly, the essential spermatogenesis function of Su(Hw) resides in somatic cyst cells, implying context-specific consequences due to loss of this transcription factor. Together, our studies highlight that Su(Hw) has a major developmental function as a transcriptional repressor, with the effect of its loss dependent upon the cell-specific factors.

Dynamics and control of sister kinetochore behavior during the meiotic divisions in Drosophila spermatocytes

Sister kinetochores are connected to the same spindle pole during meiosis I and to opposite poles during meiosis II. The molecular mechanisms controlling the distinct behavior of sister kinetochores during the two meiotic divisions are poorly understood. To study kinetochore behavior during meiosis, we have optimized time lapse imaging with Drosophila spermatocytes, enabling kinetochore tracking with high temporal and spatial resolution through both meiotic divisions. The correct bipolar orientation of chromosomes within the spindle proceeds rapidly during both divisions. Stable bi-orientation of the last chromosome is achieved within ten minutes after the onset of kinetochore-microtubule interactions. Our analyses of mnm and tef mutants, where univalents instead of bivalents are present during meiosis I, indicate that the high efficiency of normal bi-orientation depends on pronounced stabilization of kinetochore attachments to spindle microtubules by the mechanical tension generated by spindle forces upon bi-orientation. Except for occasional brief separation episodes, sister kinetochores are so closely associated that they cannot be resolved individually by light microscopy during meiosis I, interkinesis and at the start of meiosis II. Permanent evident separation of sister kinetochores during M II depends on spindle forces resulting from bi-orientation. In mnm and tef mutants, sister kinetochore separation can be observed already during meiosis I in bi-oriented univalents. Interestingly, however, this sister kinetochore separation is delayed until the metaphase to anaphase transition and depends on the Fzy/Cdc20 activator of the anaphase-promoting complex/cyclosome. We propose that univalent bi-orientation in mnm and tef mutants exposes a release of sister kinetochore conjunction that occurs also during normal meiosis I in preparation for bi-orientation of dyads during meiosis II.

For production of oocytes and sperm, cells have to complete meiosis which includes two successive divisions. These divisions convert diploid cells with a maternal and a paternal copy of each chromosome into haploid cells with only one copy of each chromosome. Chromosome copy reduction requires regulation of sister kinetochore behavior during the meiotic divisions. Kinetochores are protein networks assembled at the start of divisions within the centromeric region of chromosomes. They provide attachment sites for spindle microtubules which in turn exert poleward pulling forces. During pre-meiotic S phase, each chromosome is duplicated into two closely associated sister chromatids. At the start of the first meiotic division, both sister chromatids together assemble only one functional kinetochore, permitting subsequent separation of paired homologous chromosomes to opposite spindle poles. In contrast, at the onset of the second meiotic division, each sister chromatid organizes its own kinetochore followed by separation of sister chromatids to opposite spindle poles. To analyze when and how sister kinetochores are individualized, we have improved time lapse imaging with Drosophila spermatocytes. Our analyses in normal and genetically altered spermatocytes suggest that the release of sister kinetochore conjunction occurs during the first meiotic division after activation of the anaphase promoting complex/cyclosome.

Retention of Core Meiotic Genes Across Diverse Hymenoptera

The cellular mechanisms of meiosis are critical for proper gamete formation in sexual organisms. Functional studies in model organisms have identified genes essential for meiosis, yet the extent to which this core meiotic machinery is conserved across non-model systems is not fully understood. Moreover, it is unclear whether deviation from canonical modes of sexual reproduction is accompanied by modifications in the genetic components involved in meiosis. We used a robust approach to identify and catalogue meiosis genes in Hymenoptera, an insect order typically characterized by haplodiploid reproduction. Using newly available genome data, we searched for 43 genes involved in meiosis in 18 diverse hymenopterans. Seven of eight genes with roles specific to meiosis were found across a majority of surveyed species, suggesting the preservation of core meiotic machinery in haplodiploid hymenopterans. Phylogenomic analyses of the inventory of meiosis genes and the identification of shared gene duplications and losses provided support for the grouping of species within Proctotrupomorpha, Ichneumonomorpha, and Aculeata clades, along with a paraphyletic Symphyta. The conservation of meiosis genes across Hymenoptera provides a framework for studying transitions between reproductive modes in this insect group.

Recombination rate plasticity: revealing mechanisms by design

For over a century, scientists have known that meiotic recombination rates can vary considerably among individuals, and that environmental conditions can modify recombination rates relative to the background. A variety of external and intrinsic factors such as temperature, age, sex and starvation can elicit 'plastic' responses in recombination rate. The influence of recombination rate plasticity on genetic diversity of the next generation has interesting and important implications for how populations evolve. Further, many questions remain regarding the mechanisms and molecular processes that contribute to recombination rate plasticity. Here, we review 100 years of experimental work on recombination rate plasticity conducted in Drosophila melanogaster We categorize this work into four major classes of experimental designs, which we describe via classic studies in D. melanogaster Based on these studies, we highlight molecular mechanisms that are supported by experimental results and relate these findings to studies in other systems. We synthesize lessons learned from this model system into experimental guidelines for using recent advances in genotyping technologies, to study recombination rate plasticity in non-model organisms. Specifically, we recommend (1) using fine-scale genome-wide markers, (2) collecting time-course data, (3) including crossover distribution measurements, and (4) using mixed effects models to analyse results. To illustrate this approach, we present an application adhering to these guidelines from empirical work we conducted in Drosophila pseudoobscuraThis article is part of the themed issue 'Evolutionary causes and consequences of recombination rate variation in sexual organisms'.

Interactions between BTB domain of CP190 and two adjacent regions in Su(Hw) are required for the insulator complex formation

The best-studied Drosophila insulator complex consists of two BTB-containing proteins, the Mod(mdg4)-67.2 isoform and CP190, which are recruited cooperatively to chromatin through interactions with the DNA-binding architectural protein Su(Hw). While Mod(mdg4)-67.2 interacts only with Su(Hw), CP190 interacts with many other architectural proteins. In spite of the fact that CP190 is critical for the activity of Su(Hw) insulators, interaction between these proteins has not been studied yet. Therefore, we have performed a detailed analysis of domains involved in the interaction between the Su(Hw) and CP190. The results show that the BTB domain of CP190 interacts with two adjacent regions at the N-terminus of Su(Hw). Deletion of either region in Su(Hw) only weakly affected recruiting of CP190 to the Su(Hw) sites in the presence of Mod(mdg4)-67.2. Deletion of both regions in Su(Hw) prevents its interaction with CP190. Using mutations in vivo, we found that interactions with Su(Hw) and Mod(mdg4)-67.2 are essential for recruiting of CP190 to the Su(Hw) genomic sites.

Conservation and Variability of Meiosis Across the Eukaryotes

Comparisons among a variety of eukaryotes have revealed considerable variability in the structures and processes involved in their meiosis. Nevertheless, conventional forms of meiosis occur in all major groups of eukaryotes, including early-branching protists. This finding confirms that meiosis originated in the common ancestor of all eukaryotes and suggests that primordial meiosis may have had many characteristics in common with conventional extant meiosis. However, it is possible that the synaptonemal complex and the delicate crossover control related to its presence were later acquisitions. Later still, modifications to meiotic processes occurred within different groups of eukaryotes. Better knowledge on the spectrum of derived and uncommon forms of meiosis will improve our understanding of many still mysterious aspects of the meiotic process and help to explain the evolutionary basis of functional adaptations to the meiotic program.

Achiasmy: Male Fruit Flies Are Not Ready to Mix

Maintenance of the chromosomal copy number over generations and recombination between homologous chromosomes are hallmarks of meiotic cell division. This genetic exchange that take place during gamete formation leads to genetic diversity, the main driving force behind natural selection. Formation of chiasmata, the physical link between homologous chromosomes during meiosis, is a requisite for recombination. In addition, chiasmata also aid in proper segregation of homologous chromosomes and has a major impact on reproductive fitness. Given these facts it is intriguing that many insect species have forgone the need for genetic exchange between homologous chromosomes during meiosis. Geneticists for several decades knew that meiotic crossover and recombination is absent in Drosophila males and some female lepidopterans, a condition termed achiasmy. However, a good understanding of the mechanisms that cause achiasmy and the evolutionary benefits of achiasmy is currently lacking. In this article we will discuss possible genetic and molecular basis of achiasmy in male Drosophila.

A genomic approach to identify hybrid incompatibility genes

Uncovering the genetic and molecular basis of barriers to gene flow between populations is key to understanding how new species are born. Intrinsic postzygotic reproductive barriers such as hybrid sterility and hybrid inviability are caused by deleterious genetic interactions known as hybrid incompatibilities. The difficulty in identifying these hybrid incompatibility genes remains a rate-limiting step in our understanding of the molecular basis of speciation. We recently described how whole genome sequencing can be applied to identify hybrid incompatibility genes, even from genetically terminal hybrids. Using this approach, we discovered a new hybrid incompatibility gene, gfzf, between Drosophila melanogaster and Drosophila simulans, and found that it plays an essential role in cell cycle regulation. Here, we discuss the history of the hunt for incompatibility genes between these species, discuss the molecular roles of gfzf in cell cycle regulation, and explore how intragenomic conflict drives the evolution of fundamental cellular mechanisms that lead to the developmental arrest of hybrids.

The insulator protein BEAF-32 is required for Hippo pathway activity in the terminal differentiation of neuronal subtypes

The Hippo pathway is crucial for not only normal growth and apoptosis but also cell fate specification during development. What controls Hippo pathway activity during cell fate specification is incompletely understood. In this article, we identify the insulator protein BEAF-32 as a regulator of Hippo pathway activity in Drosophila photoreceptor differentiation. Though morphologically uniform, the fly eye is composed of two subtypes of R8 photoreceptor neurons defined by expression of light-detecting Rhodopsin proteins. In one R8 subtype, active Hippo signaling induces Rhodopsin 6 (Rh6) and represses Rhodopsin 5 (Rh5), whereas in the other subtype, inactive Hippo signaling induces Rh5 and represses Rh6. The activity state of the Hippo pathway in R8 cells is determined by the expression of warts, a core pathway kinase, which interacts with the growth regulator melted in a double-negative feedback loop. We show that BEAF-32 is required for expression of warts and repression of melted Furthermore, BEAF-32 plays a second role downstream of Warts to induce Rh6 and prevent Rh5 fate. BEAF-32 is dispensable for Warts feedback, indicating that BEAF-32 differentially regulates warts and Rhodopsins. Loss of BEAF-32 does not noticeably impair the functions of the Hippo pathway in eye growth regulation. Our study identifies a context-specific regulator of Hippo pathway activity in post-mitotic neuronal fate, and reveals a developmentally specific role for a broadly expressed insulator protein.

Separase Is Required for Homolog and Sister Disjunction during Drosophila melanogaster Male Meiosis, but Not for Biorientation of Sister Centromeres

Spatially controlled release of sister chromatid cohesion during progression through the meiotic divisions is of paramount importance for error-free chromosome segregation during meiosis. Cohesion is mediated by the cohesin protein complex and cleavage of one of its subunits by the endoprotease separase removes cohesin first from chromosome arms during exit from meiosis I and later from the pericentromeric region during exit from meiosis II. At the onset of the meiotic divisions, cohesin has also been proposed to be present within the centromeric region for the unification of sister centromeres into a single functional entity, allowing bipolar orientation of paired homologs within the meiosis I spindle. Separase-mediated removal of centromeric cohesin during exit from meiosis I might explain sister centromere individualization which is essential for subsequent biorientation of sister centromeres during meiosis II. To characterize a potential involvement of separase in sister centromere individualization before meiosis II, we have studied meiosis in Drosophila melanogaster males where homologs are not paired in the canonical manner. Meiosis does not include meiotic recombination and synaptonemal complex formation in these males. Instead, an alternative homolog conjunction system keeps homologous chromosomes in pairs. Using independent strategies for spermatocyte-specific depletion of separase complex subunits in combination with time-lapse imaging, we demonstrate that separase is required for the inactivation of this alternative conjunction at anaphase I onset. Mutations that abolish alternative homolog conjunction therefore result in random segregation of univalents during meiosis I also after separase depletion. Interestingly, these univalents become bioriented during meiosis II, suggesting that sister centromere individualization before meiosis II does not require separase.

A few of our favorite things: Pairing, the bouquet, crossover interference and evolution of meiosis

Meiosis presents many important mysteries that await elucidation. Here we discuss two such aspects. First, we consider how the current meiotic program might have evolved. We emphasize the central feature of this program: how homologous chromosomes find one another ("pair") so as to create the connections required for their regular segregation at Meiosis I. Points of emphasis include the facts that: (i) the classical "bouquet stage" is not required for initial homolog contacts in the current evolved meiotic program; and (ii) diverse observations point to commonality between molecules that mediate meiotic inter-homolog interactions and molecules that are integral to centromeres and/or to microtubule organizing centers (a.k.a. spindle pole bodies or centrosomes). Second, we provide an overview of the classical phenomenon of crossover (CO) interference in an effort to bridge the gap between description on the one hand versus logic and mechanism on the other.

Centromere pairing--tethering partner chromosomes in meiosis I

In meiosis, homologous chromosomes face the obstacle of finding, holding onto and segregating away from their partner chromosome. There is increasing evidence, in a diverse range of organisms, that centromere–centromere interactions that occur in late prophase are an important mechanism in ensuring segregation fidelity. Centromere pairing appears to initiate when homologous chromosomes synapse in meiotic prophase. Structural proteins of the synaptonemal complex have been shown to help mediate centromere pairing, but how the structure that maintains centromere pairing differs from the structure of the synaptonemal complex along the chromosomal arms remains unknown. When the synaptonemal complex proteins disassemble from the chromosome arms in late prophase, some of these synaptonemal complex components persist at the centromeres. In yeast and Drosophila these centromere-pairing behaviors promote the proper segregation of chromosome partners that have failed to become linked by chiasmata. Recent studies of mouse spermatocytes have described centromere pairing behaviors that are similar in several respects to what has been described in the fly and yeast systems. In humans, chromosomes that fail to experience crossovers in meiosis are error-prone and are a major source of aneuploidy. The finding that centromere pairing is a conserved phenomenon raises the possibility that it may play a role in promoting the segregation fidelity of non-exchange chromosome pairs in humans.

From equator to pole: splitting chromosomes in mitosis and meiosis

During eukaryotic cell division, chromosomes must be precisely partitioned to daughter cells. This relies on a mechanism to move chromosomes in defined directions within the parental cell. While sister chromatids are segregated from one another in mitosis and meiosis II, specific adaptations enable the segregation of homologous chromosomes during meiosis I to reduce ploidy for gamete production. Many of the factors that drive these directed chromosome movements are known, and their molecular mechanism has started to be uncovered. Here we review the mechanisms of eukaryotic chromosome segregation, with a particular emphasis on the modifications that ensure the segregation of homologous chromosomes during meiosis I.

Drosophila Yemanuclein associates with the cohesin and synaptonemal complexes

Meiosis is characterized by two chromosome segregation rounds (meiosis I and II), which follow a single round of DNA replication, resulting in haploid genome formation. Chromosome reduction occurs at meiosis I. It relies on key structures, such as chiasmata, which are formed by repair of double-strand breaks (DSBs) between the homologous chromatids. In turn, to allow for segregation of homologs, chiasmata rely on the maintenance of sister chromatid cohesion. In most species, chiasma formation requires the prior synapsis of homologous chromosome axes, which is mediated by the synaptonemal complex, a tripartite proteinaceous structure specific to prophase I of meiosis. Yemanuclein (Yem) is a maternal factor that is crucial for sexual reproduction. It is required in the zygote for chromatin assembly of the male pronucleus, where it acts as a histone H3.3 chaperone in complex with Hira. We report here that Yem associates with the synaptonemal complex and the cohesin complex. A genetic interaction between yem(1) (V478E) and the Spo11 homolog mei-W68, modified a yem(1) dominant effect on crossover distribution, suggesting that Yem has an early role in meiotic recombination. This is further supported by the impact of yem mutations on DSB kinetics. A Hira mutation gave a similar effect, presumably through disruption of Hira-Yem complex.

Sisters unbound is required for meiotic centromeric cohesion in Drosophila melanogaster

Regular meiotic chromosome segregation requires sister centromeres to mono-orient (orient to the same pole) during the first meiotic division (meiosis I) when homologous chromosomes segregate, and to bi-orient (orient to opposite poles) during the second meiotic division (meiosis II) when sister chromatids segregate. Both orientation patterns require cohesion between sister centromeres, which is established during meiotic DNA replication and persists until anaphase of meiosis II. Meiotic cohesion is mediated by a conserved four-protein complex called cohesin that includes two structural maintenance of chromosomes (SMC) subunits (SMC1 and SMC3) and two non-SMC subunits. In Drosophila melanogaster, however, the meiotic cohesion apparatus has not been fully characterized and the non-SMC subunits have not been identified. We have identified a novel Drosophila gene called sisters unbound (sunn), which is required for stable sister chromatid cohesion throughout meiosis. sunn mutations disrupt centromere cohesion during prophase I and cause high frequencies of non-disjunction (NDJ) at both meiotic divisions in both sexes. SUNN co-localizes at centromeres with the cohesion proteins SMC1 and SOLO in both sexes and is necessary for the recruitment of both proteins to centromeres. Although SUNN lacks sequence homology to cohesins, bioinformatic analysis indicates that SUNN may be a structural homolog of the non-SMC cohesin subunit stromalin (SA), suggesting that SUNN may serve as a meiosis-specific cohesin subunit. In conclusion, our data show that SUNN is an essential meiosis-specific Drosophila cohesion protein.

Meiosis in male Drosophila

Meiosis entails sorting and separating both homologous and sister chromatids. The mechanisms for connecting sister chromatids and homologs during meiosis are highly conserved and include specialized forms of the cohesin complex and a tightly regulated homolog synapsis/recombination pathway designed to yield regular crossovers between homologous chromatids. Drosophila male meiosis is of special interest because it dispenses with large segments of the standard meiotic script, particularly recombination, synapsis and the associated structures. Instead, Drosophila relies on a unique protein complex composed of at least two novel proteins, SNM and MNM, to provide stable connections between homologs during meiosis I. Sister chromatid cohesion in Drosophila is mediated by cohesins, ring-shaped complexes that entrap sister chromatids. However, unlike other eukaryotes Drosophila does not rely on the highly conserved Rec8 cohesin in meiosis, but instead utilizes two novel cohesion proteins, ORD and SOLO, which interact with the SMC1/3 cohesin components in providing meiotic cohesion.

The analysis of mutant alleles of different strength reveals multiple functions of topoisomerase 2 in regulation of Drosophila chromosome structure

Topoisomerase II is a major component of mitotic chromosomes but its role in the assembly and structural maintenance of chromosomes is rather controversial, as different chromosomal phenotypes have been observed in various organisms and in different studies on the same organism. In contrast to vertebrates that harbor two partially redundant Topo II isoforms, Drosophila and yeasts have a single Topo II enzyme. In addition, fly chromosomes, unlike those of yeast, are morphologically comparable to vertebrate chromosomes. Thus, Drosophila is a highly suitable system to address the role of Topo II in the assembly and structural maintenance of chromosomes. Here we show that modulation of Top2 function in living flies by means of mutant alleles of different strength and in vivo RNAi results in multiple cytological phenotypes. In weak Top2 mutants, meiotic chromosomes of males exhibit strong morphological abnormalities and dramatic segregation defects, while mitotic chromosomes of larval brain cells are not affected. In mutants of moderate strength, mitotic chromosome organization is normal, but anaphases display frequent chromatin bridges that result in chromosome breaks and rearrangements involving specific regions of the Y chromosome and 3L heterochromatin. Severe Top2 depletion resulted in many aneuploid and polyploid mitotic metaphases with poorly condensed heterochromatin and broken chromosomes. Finally, in the almost complete absence of Top2, mitosis in larval brains was virtually suppressed and in the rare mitotic figures observed chromosome morphology was disrupted. These results indicate that different residual levels of Top2 in mutant cells can result in different chromosomal phenotypes, and that the effect of a strong Top2 depletion can mask the effects of milder Top2 reductions. Thus, our results suggest that the previously observed discrepancies in the chromosomal phenotypes elicited by Topo II downregulation in vertebrates might depend on slight differences in Topo II concentration and/or activity.

Which one is the real matchmaker for the pair?

A fundamental question for meiosis is how homologous chromosomes (homologs) find each other and pair together to ensure homologous recombination and segregation. Intuitively, the answer to the question is related to the interaction between homologous sequences. However, that is not the whole story according to some studies on the role of cohesins in homolog pairing. The most recent one by Ishiguro et al.[1] of the Watanabe group indicates that chromosome architecture defined by a meiosis-specific cohesin protein RAD21L is the key to homology searching. Moreover, they report that homologous pairing is dependent on neither SPO11, an evolutionarily conserved type 2 isomerase responsible for generating DNA double-strand breaks (DSBs), nor SUN1, which tethers the ends of chromosomes to the nuclear envelop (NE) and facilitates chromosome movement and bouquet formation. These discoveries are quite some surprises!

Meiosis-specific cohesin component, Stag3 is essential for maintaining centromere chromatid cohesion, and required for DNA repair and synapsis between homologous chromosomes

Cohesins are important for chromosome structure and chromosome segregation during mitosis and meiosis. Cohesins are composed of two structural maintenance of chromosomes (SMC1-SMC3) proteins that form a V-shaped heterodimer structure, which is bridged by a α-kleisin protein and a stromal antigen (STAG) protein. Previous studies in mouse have shown that there is one SMC1 protein (SMC1β), two α-kleisins (RAD21L and REC8) and one STAG protein (STAG3) that are meiosis-specific. During meiosis, homologous chromosomes must recombine with one another in the context of a tripartite structure known as the synaptonemal complex (SC). From interaction studies, it has been shown that there are at least four meiosis-specific forms of cohesin, which together with the mitotic cohesin complex, are lateral components of the SC. STAG3 is the only meiosis-specific subunit that is represented within all four meiosis-specific cohesin complexes. In Stag3 mutant germ cells, the protein level of other meiosis-specific cohesin subunits (SMC1β, RAD21L and REC8) is reduced, and their localization to chromosome axes is disrupted. In contrast, the mitotic cohesin complex remains intact and localizes robustly to the meiotic chromosome axes. The instability of meiosis-specific cohesins observed in Stag3 mutants results in aberrant DNA repair processes, and disruption of synapsis between homologous chromosomes. Furthermore, mutation of Stag3 results in perturbation of pericentromeric heterochromatin clustering, and disruption of centromere cohesion between sister chromatids during meiotic prophase. These defects result in early prophase I arrest and apoptosis in both male and female germ cells. The meiotic defects observed in Stag3 mutants are more severe when compared to single mutants for Smc1β, Rec8 and Rad21l, however they are not as severe as the Rec8, Rad21l double mutants. Taken together, our study demonstrates that STAG3 is required for the stability of all meiosis-specific cohesin complexes. Furthermore, our data suggests that STAG3 is required for structural changes of chromosomes that mediate chromosome pairing and synapsis, DNA repair and progression of meiosis.

Dynamic trans interactions in yeast chromosomes

Three-dimensional organization of the genome is important for regulation of gene expression and maintenance of genomic stability. It also defines, and is defined by, contacts between different chromosomal loci. Interactions between loci positioned on different chromosomes, i.e. "trans" interactions are one type of such contacts. Here, we describe a case of inducible trans interaction in chromosomes of the budding yeast S. cerevisiae. Special DNA sequences, inserted in two ectopic chromosomal loci positioned in trans, pair with one another in an inducible manner. The spatial proximity diagnostic of pairing is observable by both chromosome capture analysis (3C) and epifluorescence microscopy in whole cells. Protein synthesis de novo appears to be required for this process. The three-dimensional organization of the yeast nucleus imposes a constraint on such pairing, presumably by dictating the probability with which the two sequences collide with one another.

Collaborative homologous pairing during C. elegans meiosis

In preparation for meiotic chromosome segregation, homologous chromosomes need to pair, synapse (i.e., assemble the synaptonemal complex, SC), and then recombine to generate a physical linkage (i.e., chiasma) between them. In many organisms meiotic pairing capacity distributed along the entire chromosome length supports presynaptic alignment. In contrast, the prevailing model for C. elegans proposes that presynaptic homologous pairing is performed solely by a master pairing-site, the pairing center (PC). In this model, the remaining chromosomal regions (the non-PC regions) are not actively involved in presynaptic pairing, and the SC assembling from the PC aligns the homologous chromosomes along non-PC regions and holds them together. Our recent work, however, demonstrates that C. elegans chromosomes establish presynaptic alignment along the entire chromosome length, suggesting that the non-PC regions are also actively involved in the presynaptic pairing process. Furthermore, we have also discovered that the chromodomain protein MRG-1 facilitates this presynaptic non-PC pairing. The phenotype of the mrg-1 mutant indicates that the PC and the non-PC collaborate in successful pairing and synapsis. Therefore, homologous pairing mechanisms in C. elegans possibly share more similarity with those in other organisms than previously thought. Here, we elaborate on these observations and discuss a hypothetical model for presynaptic pairing in C. elegans based on our novel findings.

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. In this study, we show 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. Our 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). We propose 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.

Sexual reproduction entails an intricate 2-step division called meiosis in which homologous chromosomes and sister chromatids are sequentially segregated to yield gametes (eggs and sperm) with exactly one copy of each chromosome. The Drosophila meiosis protein SOLO is essential for cohesion between sister chromatids. SOLO localizes to centromeres throughout meiosis where it collaborates with the conserved cohesin complex to enable sister centromeres to orient properly – to the same pole during the first division and to opposite poles during the second division. In solo mutants, sister chromatids become disconnected early in meiosis and segregate randomly through both meiotic divisions generating gametes with random (and mostly wrong) numbers of chromosomes. In this study we show that SOLO also localizes to chromosome arms where it is required to construct stable synaptonemal complexes that connect homologs while they recombine. In addition, SOLO is required to prevent crossovers between sister chromatids, as only homolog crossovers are useful for forming the interhomolog connections (chiasmata) needed for homolog segregation. SOLO collaborates with cohesin for these tasks as well. We propose that SOLO is a subunit of a specialized meiotic cohesin complex and a multi-purpose cohesion protein that regulates several meiotic processes needed for proper chromosome segregation.

Meiotic development in Caenorhabditis elegans

Caenorhabditis elegans has become a powerful experimental organism with which to study meiotic processes that promote the accurate segregation of chromosomes during the generation of haploid gametes. Haploid reproductive cells are produced through one round of chromosome replication followed by two -successive cell divisions. Characteristic meiotic chromosome structure and dynamics are largely conserved in C. elegans. Chromosomes adopt a meiosis-specific structure by loading cohesin proteins, assembling axial elements, and acquiring chromatin marks. Homologous chromosomes pair and form physical connections though synapsis and recombination. Synaptonemal complex and crossover formation allow for the homologs to stably associate prior to remodeling that facilitates their segregation. This chapter will cover conserved meiotic processes as well as highlight aspects of meiosis that are unique to C. elegans.

Transgenerational propagation and quantitative maintenance of paternal centromeres depends on Cid/Cenp-A presence in Drosophila sperm

In Drosophila melanogaster, as in many animal and plant species, centromere identity is specified epigenetically. In proliferating cells, a centromere-specific histone H3 variant (CenH3), named Cid in Drosophila and Cenp-A in humans, is a crucial component of the epigenetic centromere mark. Hence, maintenance of the amount and chromosomal location of CenH3 during mitotic proliferation is important. Interestingly, CenH3 may have different roles during meiosis and the onset of embryogenesis. In gametes of Caenorhabditis elegans, and possibly in plants, centromere marking is independent of CenH3. Moreover, male gamete differentiation in animals often includes global nucleosome for protamine exchange that potentially could remove CenH3 nucleosomes. Here we demonstrate that the control of Cid loading during male meiosis is distinct from the regulation observed during the mitotic cycles of early embryogenesis. But Cid is present in mature sperm. After strong Cid depletion in sperm, paternal centromeres fail to integrate into the gonomeric spindle of the first mitosis, resulting in gynogenetic haploid embryos. Furthermore, after moderate depletion, paternal centromeres are unable to re-acquire normal Cid levels in the next generation. We conclude that Cid in sperm is an essential component of the epigenetic centromere mark on paternal chromosomes and it exerts quantitative control over centromeric Cid levels throughout development. Hence, the amount of Cid that is loaded during each cell cycle appears to be determined primarily by the preexisting centromeric Cid, with little flexibility for compensation of accidental losses.

Homologous pairing and the role of pairing centers in meiosis

Homologous pairing establishes the foundation for accurate reductional segregation during meiosis I in sexual organisms. This Commentary summarizes recent progress in our understanding of homologous pairing in meiosis, and will focus on the characteristics and mechanisms of specialized chromosome sites, called pairing centers (PCs), in Caenorhabditis elegans and Drosophila melanogaster. In C. elegans, each chromosome contains a single PC that stabilizes chromosome pairing and initiates synapsis of homologous chromosomes. Specific zinc-finger proteins recruited to PCs link chromosomes to nuclear envelope proteins--and through them to the microtubule cytoskeleton--thereby stimulating chromosome movements in early prophase, which are thought to be important for homolog sorting. This mechanism appears to be a variant of the 'telomere bouquet' process, in which telomeres cluster on the nuclear envelope, connect chromosomes through nuclear envelope proteins to the cytoskeleton and lead chromosome movements that promote homologous synapsis. In Drosophila males, which undergo meiosis without recombination, pairing of the largely non-homologous X and Y chromosomes occurs at specific repetitive sequences in the ribosomal DNA. Although no other clear examples of PC-based pairing mechanisms have been described, there is evidence for special roles of telomeres and centromeres in aspects of chromosome pairing, synapsis and segregation; these roles are in some cases similar to those of PCs.

Cohesin: a catenase with separate entry and exit gates?

Cohesin confers both intrachromatid and interchromatid cohesion through formation of a tripartite ring within which DNA is thought to be entrapped. Here, I discuss what is known about the four stages of the cohesin ring cycle using the ring model as an intellectual framework. I postulate that cohesin loading onto chromosomes, catalysed by a separate complex called kollerin, is mediated by the entry of DNA into cohesin rings, whereas dissociation, catalysed by Wapl and several other cohesin subunits (an activity that will be called releasin here), is mediated by the subsequent exit of DNA. I suggest that the ring's entry and exit gates may be separate, with the former and latter taking place at Smc1-Smc3 and Smc3-kleisin interfaces, respectively. Establishment of cohesion during S phase involves neutralization of releasin through acetylation of Smc3 at a site close to the putative exit gate of DNA, which locks rings shut until opened irreversibly by kleisin cleavage through the action of separase, an event that triggers the metaphase to anaphase transition.

Nuclear structure and chromosome segregation in Drosophila male meiosis depend on the ubiquitin ligase dTopors

In many organisms, homolog pairing and synapsis at meiotic prophase depend on interactions between chromosomes and the nuclear membrane. Male Drosophila lack synapsis, but nonetheless, their chromosomes closely associate with the nuclear periphery at prophase I. To explore the functional significance of this association, we characterize mutations in nuclear blebber (nbl), a gene required for both spermatocyte nuclear shape and meiotic chromosome transmission. We demonstrate that nbl corresponds to dtopors, the Drosophila homolog of the mammalian dual ubiquitin/small ubiquitin-related modifier (SUMO) ligase Topors. We show that mutations in dtopors cause abnormalities in lamin localizations, centriole separation, and prophase I chromatin condensation and also cause anaphase I bridges that likely result from unresolved homolog connections. Bridge formation does not require mod(mdg4) in meiosis, suggesting that bridges do not result from misregulation of the male homolog conjunction complex. At the ultrastructural level, we observe disruption of nuclear shape, an uneven perinuclear space, and excess membranous structures. We show that dTopors localizes to the nuclear lamina at prophase, and also transiently to intranuclear foci. As a role of dtopors at gypsy insulator has been reported, we also asked whether these new alleles affected expression of the gypsy-induced mutation ct(6) and found that it was unaltered in dtopors homozygotes. Our results indicate that dTopors is required for germline nuclear structure and meiotic chromosome segregation, but in contrast, is not necessary for gypsy insulator function. We suggest that dtopors plays a structural role in spermatocyte lamina that is critical for multiple aspects of meiotic chromosome transmission.

Homolog pairing and sister chromatid cohesion in heterochromatin in Drosophila male meiosis I

Drosophila males undergo meiosis without recombination or chiasmata but homologous chromosomes pair and disjoin regularly. The X-Y pair utilizes a specific repeated sequence within the heterochromatic ribosomal DNA blocks as a pairing site. No pairing sites have yet been identified for the autosomes. To search for such sites, we utilized probes targeting specific heterochromatic regions to assay heterochromatin pairing sequences and behavior in meiosis by fluorescence in situ hybridization (FISH). We found that the small fourth chromosome pairs at heterochromatic region 61 and associates with the X chromosome throughout prophase I. Homolog pairing of the fourth chromosome is disrupted when the homolog conjunction complex is perturbed by mutations in SNM or MNM. On the other hand, six tested heterochromatic regions of the major autosomes proved to be largely unpaired after early prophase I, suggesting that stable homolog pairing sites do not exist in heterochromatin of the major autosomes. Furthermore, FISH analysis revealed two distinct patterns of sister chromatid cohesion in heterochromatin: regions with stable cohesion and regions lacking cohesion. This suggests that meiotic sister chromatid cohesion is incomplete within heterochromatin and may occur at specific preferential sites.

A single mutation results in diploid gamete formation and parthenogenesis in a Drosophila yemanuclein-alpha meiosis I defective mutant

BACKGROUND

Sexual reproduction relies on two key events: formation of cells with a haploid genome (the gametes) and restoration of diploidy after fertilization. Therefore the underlying mechanisms must have been evolutionary linked and there is a need for evidence that could support such a model.

RESULTS

We describe the identification and the characterization of yem1, the first yem-alpha mutant allele (V478E), which to some extent affects diploidy reduction and its restoration. Yem-alpha is a member of the Ubinuclein/HPC2 family of proteins that have recently been implicated in playing roles in chromatin remodeling in concert with HIRA histone chaperone. The yem1 mutant females exhibited disrupted chromosome behavior in the first meiotic division and produced very low numbers of viable progeny. Unexpectedly these progeny did not display paternal chromosome markers, suggesting that they developed from diploid gametes that underwent gynogenesis, a form of parthenogenesis that requires fertilization.

CONCLUSIONS

We focus here on the analysis of the meiotic defects exhibited by yem1 oocytes that could account for the formation of diploid gametes. Our results suggest that yem1 affects chromosome segregation presumably by affecting kinetochores function in the first meiotic division. This work paves the way to further investigations on the evolution of the mechanisms that support sexual reproduction.