Transcriptional activation in Drosophila spermatogenesis involves the mutually dependent function of aly and a novel meiotic arrest gene cookie monster

RpL38 modulates germ cell differentiation by controlling Bam expression in Drosophila testis

Switching from mitotic spermatogonia to meiotic spermatocytes is critical to producing haploid sperms during male germ cell differentiation. However, the underlying mechanisms of this switch remain largely unexplored. In Drosophila melanogaster, the gene RpL38 encodes the ribosomal protein L38, one component of the 60S subunit of ribosomes. We found that its depletion in spermatogonia severely diminished the production of mature sperms and thus led to the infertility of male flies. By examining the germ cell differentiation in testes, we found that RpL38-knockdown blocked the transition from spermatogonia to spermatocytes and accumulated spermatogonia in the testis. To understand the intrinsic reason for this blockage, we conducted proteomic analysis for these spermatogonia populations. Differing from the control spermatogonia, the accumulated spermatogonia in RpL38-knockdown testes already expressed many spermatocyte markers but lacked many meiosis-related proteins, suggesting that spermatogonia need to prepare some important proteins for meiosis to complete their switch into spermatocytes. Mechanistically, we found that the expression of bag of marbles (bam), a crucial determinant in the transition from spermatogonia to spermatocytes, was inhibited at both the mRNA and protein levels upon RpL38 depletion. We also confirmed that the bam loss phenocopied RpL38 RNAi in the testis phenotype and transcriptomic profiling. Strikingly, overexpressing bam was able to fully rescue the testis abnormality and infertility of RpL38-knockdown flies, indicating that bam is the key effector downstream of RpL38 to regulate spermatogonia differentiation. Overall, our data suggested that germ cells start to prepare meiosis-related proteins as early as the spermatogonial stage, and RpL38 in spermatogonia is required to regulate their transition toward spermatocytes in a bam-dependent manner, providing new knowledge for our understanding of the transition process from spermatogonia to spermatocytes in Drosophila spermatogenesis.

Rapid Divergence of Key Spermatogenesis Genes in nasuta-Subgroup of Drosophila

The crosses between closely related Drosophila species usually produce sterile hybrid males with spermatogenesis disrupted at post-meiotic phase, especially in sperm individualization stage than the pre-meiotic stage. This is possibly due to the rapid interspecies divergence of male sex and reproduction-related genes. Here we annotated 11 key spermatogenesis genes in 35 strains of species belonging to nasuta-subgroup of Drosophila, where many interspecies crosses produce sterile males. We characterized the divergence and polymorphism in the protein coding regions by employing gene-wide, codon-wide, and lineage-specific selection analysis to test the mode and strength of selection acting on these genes. Our analysis showed signature of positive selection at bag of marbles (bam) and benign gonial cell neoplasma (bgcn) despite the selection constrains and the absence of endosymbiont infection which could potentially drive rapid divergence due to an arms race while roughex (rux) showed lineage-specific rapid divergence in frontal sheen complex of nasuta-subgroup. cookie monster (comr) showed rapid divergence consistent with the possibility of meiotic arrest observed in sterile hybrids of Drosophila species. Rapid divergence observed at don juan (dj) and Mst98Ca-like was consistent with fused sperm-tail abnormality observed in the hybrids of Drosophila nasuta and Drosophila albomicans. These findings highlight the potential role of rapid nucleotide divergence in bringing about hybrid incompatibility in the form of male sterility; however, additional genetic manipulation studies can widen our understanding of hybrid incompatibilities. Furthermore, our study emphasizes the importance of young species belonging to nasuta-subgroup of Drosophila in studying post-zygotic reproductive isolation mechanisms.

Comparative transcriptomics between Drosophila mojavensis and D. arizonae reveals transgressive gene expression and underexpression of spermatogenesis-related genes in hybrid testes

Interspecific hybridization is a stressful condition that can lead to sterility and/or inviability through improper gene regulation in Drosophila species with a high divergence time. However, the extent of these abnormalities in hybrids of recently diverging species is not well known. Some studies have shown that in Drosophila, the mechanisms of postzygotic isolation may evolve more rapidly in males than in females and that the degree of viability and sterility is associated with the genetic distance between species. Here, we used transcriptomic comparisons between two Drosophila mojavensis subspecies and D. arizonae (repleta group, Drosophila) and identified greater differential gene expression in testes than in ovaries. We tested the hypothesis that the severity of the interspecies hybrid phenotype is associated with the degree of gene misregulation. We showed limited gene misregulation in fertile females and an increase in the amount of misregulation in males with more severe sterile phenotypes (motile vs. amotile sperm). In addition, for these hybrids, we identified candidate genes that were mostly associated with spermatogenesis dysfunction.

Arabidopsis TCX8 functions as a senescence modulator by regulating LOX2 expression

TCX8 localizes to nucleus and has transcriptional repression activity. TCX8 binds to the promoter region of LOX2 encoding lipoxygenase, causing JA biosynthesis suppression, and thereby delays plant senescence. Conserved CXC domain-containing proteins are found in most eukaryotes. Eight TCX proteins, which are homologs of animal CXC-Hinge-CXC (CHC) proteins, were identified in Arabidopsis, and three of them, TSO1, TCX2/SOL2 and TCX3/SOL1, have been reported to affect cell-cycle control. TCX8, one of the TCX family proteins, was believed to be a TF but its precise function has not been reported. Yeast two-hybrid screening revealed TCP20, a TF that binds to the promoter of LOX2 encoding lipoxygenase, as a strong candidate for interaction with TCX8. We confirmed that TCX8 directly interacts with TCP20 using in vitro pull-down assay and in vivo BiFC and observed that TCX8, as a TF, localizes to nucleus. Using EMSA and by analyzing phenotypes of TCX8-overexpression lines, we demonstrated that TCX8 regulates the expression of LOX2 by binding to either cis-element of LOX2 promoter to which TCP20 or TCP4 binds, affecting JA biosynthesis, and thereby delaying plant senescence. Our study provides new information about the role of TCX8 in modulating plant senescence through regulating LOX2 expression.

Mitochondrial-Y chromosome epistasis in Drosophila melanogaster

The coordination between mitochondrial and nuclear genes is crucial to eukaryotic organisms. Predicting the nature of these epistatic interactions can be difficult because of the transmission asymmetry of the genes involved. While autosomes and X-linked genes are transmitted through both sexes, genes on the Y chromosome and in the mitochondrial genome are uniparentally transmitted through males and females, respectively. Here, we generate 36 otherwise isogenic Drosophila melanogaster strains differing only in the geographical origin of their mitochondrial genome and Y chromosome, to experimentally examine the effects of the uniparentally inherited parts of the genome, as well as their interaction, in males. We assay longevity and gene expression through RNA-sequencing. We detect an important role for both mitochondrial and Y-linked genes, as well as extensive mitochondrial-Y chromosome epistasis. In particular, genes involved in male reproduction appear to be especially sensitive to such interactions, and variation on the Y chromosome is associated with differences in longevity. Despite these interactions, we find no evidence that the mitochondrial genome and Y chromosome are co-adapted within a geographical region. Overall, our study demonstrates a key role for the uniparentally inherited parts of the genome for male biology, but also that mito-nuclear interactions are complex and not easily predicted from simple transmission asymmetries.

Sex Differences in Intestinal Carbohydrate Metabolism Promote Food Intake and Sperm Maturation

Physiology and metabolism are often sexually dimorphic, but the underlying mechanisms remain incompletely understood. Here, we use the intestine of Drosophila melanogaster to investigate how gut-derived signals contribute to sex differences in whole-body physiology. We find that carbohydrate handling is male-biased in a specific portion of the intestine. In contrast to known sexual dimorphisms in invertebrates, the sex differences in intestinal carbohydrate metabolism are extrinsically controlled by the adjacent male gonad, which activates JAK-STAT signaling in enterocytes within this intestinal portion. Sex reversal experiments establish roles for this male-biased intestinal metabolic state in controlling food intake and sperm production through gut-derived citrate. Our work uncovers a male gonad-gut axis coupling diet and sperm production, revealing that metabolic communication across organs is physiologically important. The instructive role of citrate in inter-organ communication might be significant in more biological contexts than previously recognized.

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Intestinal carbohydrate metabolism is male-biased and region-specific

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Testes masculinize gut sugar handling by promoting enterocyte JAK-STAT signaling

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The male intestine secretes citrate to the adjacent testes

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Gut-derived citrate promotes food intake and sperm maturation

Integrative analyses of the RNA modification machinery reveal tissue- and cancer-specific signatures

BACKGROUND

RNA modifications play central roles in cellular fate and differentiation. However, the machinery responsible for placing, removing, and recognizing more than 170 RNA modifications remains largely uncharacterized and poorly annotated, and we currently lack integrative studies that identify which RNA modification-related proteins (RMPs) may be dysregulated in each cancer type.

RESULTS

Here, we perform a comprehensive annotation and evolutionary analysis of human RMPs, as well as an integrative analysis of their expression patterns across 32 tissues, 10 species, and 13,358 paired tumor-normal human samples. Our analysis reveals an unanticipated heterogeneity of RMP expression patterns across mammalian tissues, with a vast proportion of duplicated enzymes displaying testis-specific expression, suggesting a key role for RNA modifications in sperm formation and possibly intergenerational inheritance. We uncover many RMPs that are dysregulated in various types of cancer, and whose expression levels are predictive of cancer progression. Surprisingly, we find that several commonly studied RNA modification enzymes such as METTL3 or FTO are not significantly upregulated in most cancer types, whereas several less-characterized RMPs, such as LAGE3 and HENMT1, are dysregulated in many cancers.

CONCLUSIONS

Our analyses reveal an unanticipated heterogeneity in the expression patterns of RMPs across mammalian tissues and uncover a large proportion of dysregulated RMPs in multiple cancer types. We provide novel targets for future cancer research studies targeting the human epitranscriptome, as well as foundations to understand cell type-specific behaviors that are orchestrated by RNA modifications.

Single-cyst transcriptome analysis of Drosophila male germline stem cell lineage

The Drosophila male germline stem cell (GSC) lineage provides a great model to understand stem cell maintenance, proliferation, differentiation and dedifferentiation. Here, we use the Drosophila GSC lineage to systematically analyze the transcriptome of discrete but continuously differentiating germline cysts. We first isolated single cysts at each recognizable stage from wild-type testes, which were subsequently applied for RNA-seq analyses. Our data delineate a high-resolution transcriptome atlas in the entire male GSC lineage: the most dramatic switch occurs from early to late spermatocyte, followed by the change from the mitotic spermatogonia to early meiotic spermatocyte. By contrast, the transit-amplifying spermatogonia cysts display similar transcriptomes, suggesting common molecular features among these stages, which may underlie their similar behavior during both differentiation and dedifferentiation processes. Finally, distinct differentiating germ cell cyst samples do not exhibit obvious dosage compensation of X-chromosomal genes, even considering the paucity of X-chromosomal gene expression during meiosis, which is different from somatic cells. Together, our single cyst-resolution, genome-wide transcriptional profile analyses provide an unprecedented resource to understand many questions in both germ cell biology and stem cell biology fields.

Satellite DNA-containing gigantic introns in a unique gene expression program during Drosophila spermatogenesis

Intron gigantism, where genes contain megabase-sized introns, is observed across species, yet little is known about its purpose or regulation. Here we identify a unique gene expression program utilized for the proper expression of genes with intron gigantism. We find that two Drosophila genes with intron gigantism, kl-3 and kl-5, are transcribed in a spatiotemporal manner over the course of spermatocyte differentiation, which spans ~90 hours. The introns of these genes contain megabases of simple satellite DNA repeats that comprise over 99% of the gene loci, and these satellite-DNA containing introns are transcribed. We identify two RNA-binding proteins that specifically localize to kl-3 and kl-5 transcripts and are needed for the successful transcription or processing of these genes. We propose that genes with intron gigantism require a unique gene expression program, which may serve as a platform to regulate gene expression during cellular differentiation.

Introns are non-coding elements of eukaryotic genes, often containing important regulatory sequences. Curiously, some genes contain introns so large that more than 99% of the gene locus is non-coding. One of the best-studied large genes, Dystrophin, a causative gene for Duchenne Muscular Dystrophy, spans 2.2Mb, only 11kb of which is coding. This phenomenon, ‘intron gigantism’, is observed across species, yet little is known about its purpose or regulation. Here we identify a unique gene expression program utilized for the proper expression of genes with intron gigantism using Drosophila spermatogenic genes as a model system. We show that the gigantic introns of these genes are transcribed in line with the exons, likely as a single transcript. We identify two RNA-binding proteins that specifically localize to the site of transcription and are needed for the successful transcription or processing of these genes. We propose that genes with intron gigantism require a unique gene expression program, which may serve as a platform to regulate gene expression during cellular differentiation.

Protecting and Diversifying the Germline

Gametogenesis represents the most dramatic cellular differentiation pathways in both female and male flies. At the genome level, meiosis ensures that diploid germ cells become haploid gametes. At the epigenome level, extensive changes are required to turn on and shut off gene expression in a precise spatiotemporally controlled manner. Research applying conventional molecular genetics and cell biology, in combination with rapidly advancing genomic tools have helped us to investigate (1) how germ cells maintain lineage specificity throughout their adult reproductive lifetime; (2) what molecular mechanisms ensure proper oogenesis and spermatogenesis, as well as protect genome integrity of the germline; (3) how signaling pathways contribute to germline-soma communication; and (4) if such communication is important. In this chapter, we highlight recent discoveries that have improved our understanding of these questions. On the other hand, restarting a new life cycle upon fertilization is a unique challenge faced by gametes, raising questions that involve intergenerational and transgenerational epigenetic inheritance. Therefore, we also discuss new developments that link changes during gametogenesis to early embryonic development-a rapidly growing field that promises to bring more understanding to some fundamental questions regarding metazoan development.

Genome-wide analysis of gene regulation mechanisms during Drosophila spermatogenesis

BACKGROUND

During Drosophila spermatogenesis, testis-specific meiotic arrest complex (tMAC) and testis-specific TBP-associated factors (tTAF) contribute to activation of hundreds of genes required for meiosis and spermiogenesis. Intriguingly, tMAC is paralogous to the broadly expressed complex Myb-MuvB (MMB)/dREAM and Mip40 protein is shared by both complexes. tMAC acts as a gene activator in spermatocytes, while MMB/dREAM was shown to repress gene activity in many cell types.

RESULTS

Our study addresses the intricate interplay between tMAC, tTAF, and MMB/dREAM during spermatogenesis. We used cell type-specific DamID to build the DNA-binding profiles of Cookie monster (tMAC), Cannonball (tTAF), and Mip40 (MMB/dREAM and tMAC) proteins in male germline cells. Incorporating the whole transcriptome analysis, we characterized the regulatory effects of these proteins and identified their gene targets. This analysis revealed that tTAFs complex is involved in activation of achi, vis, and topi meiosis arrest genes, implying that tTAFs may indirectly contribute to the regulation of Achi, Vis, and Topi targets. To understand the relationship between tMAC and MMB/dREAM, we performed Mip40 DamID in tTAF- and tMAC-deficient mutants demonstrating meiosis arrest phenotype. DamID profiles of Mip40 were highly dynamic across the stages of spermatogenesis and demonstrated a strong dependence on tMAC in spermatocytes. Integrative analysis of our data indicated that MMB/dREAM represses genes that are not expressed in spermatogenesis, whereas tMAC recruits Mip40 for subsequent gene activation in spermatocytes.

CONCLUSIONS

Discovered interdependencies allow to formulate a renewed model for tMAC and tTAFs action in Drosophila spermatogenesis demonstrating how tissue-specific genes are regulated.

Ultrastructural alterations in sperm formation of the beetle, Blaps polycresta (Coleoptera: Tenebrionidae) as a biomonitor of heavy metal soil pollution

Little is known about ultrastructural alterations induced by heavy metals pollution in insects. Therefore, the main objective of the present study is to elucidate ultrastructural changes in sperm formation of the tenebrionid beetle, Blaps polycresta as a biomonitor of heavy metal soil pollution. Metal percentages in testicular tissues of adult insects collected from reference and polluted sites were estimated using energy-dispersive X-ray microanalysis (EDX). Only cadmium, among eight detected metals, showed significantly higher percentages in the polluted testes compared with the reference ones. Ultrastructure investigation revealed severe alterations both in spermatogenic and spermiogenic stages of the polluted insects. Some cells were totally eroded. No spermatozoa were observed in all the examined cysts. Most degenerations were confined to the flagella of spermatids having enlarged vacuolated cytoplasm and malformed mitochondrial derivatives. Groups of multiple axial filaments were appeared in the form of bi-and tetra-flagellate spermatids. Electron dense vesicles were observed in almost all stages of the polluted testes. It is a novel trend in which ultrastructural alterations in sperm formation of insects could be used as a promising biomonitoring and risk assessment tool for heavy metal soil pollution.

Drosophila dany is essential for transcriptional control and nuclear architecture in spermatocytes

The terminal differentiation of adult stem cell progeny depends on transcriptional control. A dramatic change in gene expression programs accompanies the transition from proliferating spermatogonia to postmitotic spermatocytes, which prepare for meiosis and subsequent spermiogenesis. More than a thousand spermatocyte-specific genes are transcriptionally activated in early Drosophila spermatocytes. Here we describe the identification and initial characterization of dany, a gene required in spermatocytes for the large-scale change in gene expression. Similar to tMAC and tTAFs, the known major activators of spermatocyte-specific genes, dany has a recent evolutionary origin, but it functions independently. Like dan and danr, its primordial relatives with functions in somatic tissues, dany encodes a nuclear Psq domain protein. Dany associates preferentially with euchromatic genome regions. In dany mutant spermatocytes, activation of spermatocyte-specific genes and silencing of non-spermatocyte-specific genes are severely compromised and the chromatin no longer associates intimately with the nuclear envelope. Therefore, as suggested recently for Dan/Danr, we propose that Dany is essential for the coordination of change in cell type-specific expression programs and large-scale spatial chromatin reorganization.

Estrogen related receptor is required for the testicular development and for the normal sperm axoneme/mitochondrial derivatives in Drosophila males

Estrogen related receptors (ERRs), categorized as orphan nuclear receptors, are critical for energy homeostasis and somatic development. However, significance of ERRs in the development of reproductive organs/organelles/cells remain poorly understood, albeit their homology to estrogen receptors. In this context, here, we show that knockdown of ERR in the testes leads to improperly developed testes with mis-regulation of genes (aly, mia, bruce, bam, bgcn, fzo and eya) involved in spermatogenesis, resulting in reduced male fertility. The observed testicular deformity is consistent with the down-regulation of SOX-E group of gene (SOX100B) in Drosophila. We also show dispersion/disintegration of fusomes (microtubule based structures associated with endoplasmic reticulum derived vesicle, interconnecting spermatocytes) in ERR knockdown testes. A few ERR knockdown testes go through spermatogenesis but have significantly fewer sperm. Moreover, flagella of these sperm are defective with abnormal axoneme and severely reduced mitochondrial derivatives, suggesting a possible role for ERR in mitochondrial biogenesis, analogous to mammalian ERRα. Interestingly, similar knockdown of remaining seventeen nuclear receptors did not yield a detectable reproductive or developmental defect in Drosophila. These findings add newer dimensions to the functions envisaged for ERR and provide the foundation for deciphering the relevance of orphan nuclear receptors in ciliopathies and testicular dysgenesis.

Analysis of Drosophila p8 and p52 mutants reveals distinct roles for the maintenance of TFIIH stability and male germ cell differentiation

Eukaryotic gene expression is activated by factors that interact within complex machinery to initiate transcription. An important component of this machinery is the DNA repair/transcription factor TFIIH. Mutations in TFIIH result in three human syndromes: xeroderma pigmentosum, Cockayne syndrome and trichothiodystrophy. Transcription and DNA repair defects have been linked to some clinical features of these syndromes. However, how mutations in TFIIH affect specific developmental programmes, allowing organisms to develop with particular phenotypes, is not well understood. Here, we show that mutations in the p52 and p8 subunits of TFIIH have a moderate effect on the gene expression programme in the Drosophila testis, causing germ cell differentiation arrest in meiosis, but no Polycomb enrichment at the promoter of the affected differentiation genes, supporting recent data that disagree with the current Polycomb-mediated repression model for regulating gene expression in the testis. Moreover, we found that TFIIH stability is not compromised in p8 subunit-depleted testes that show transcriptional defects, highlighting the role of p8 in transcription. Therefore, this study reveals how defects in TFIIH affect a specific cell differentiation programme and contributes to understanding the specific syndrome manifestations in TFIIH-afflicted patients.

Bactrocera dorsalis male sterilization by targeted RNA interference of spermatogenesis: empowering sterile insect technique programs

RNA interference (RNAi) is a genetic technique which has novel application for sustainable pest control. The Sterile Insect Technique (SIT) uses releases of mass-produced, sterile male insects to out-compete wild males for mates to reduce pest populations. RNAi sterilization of SIT males would have several advantages over radiation sterilization, but to achieve this appropriate target genes must first be identified and then targeted with interference technology. With this goal, eight spermatogenesis related candidate genes were cloned and tested for potential activity in Bactrocera dorsalis. The knockdown of candidate genes by oral delivery of dsRNAs did not influence the mating of male flies, but significantly affected the daily average number of eggs laid by females, and reduced egg hatching rate by 16-60%. RNAi negatively affected spermatozoa quantitatively and qualitatively. Following the mating of lola-/topi-/rac-/rho-/upd-/magu-silenced males, we recorded a significant decrease in number and length of spermatozoa in female spermatheca compared to gfp-silenced control group. In a greenhouse trial, the number of damaged oranges and B. dorsalis larvae were significantly reduced in a dsrho-treated group compared with the dsgfp group. This study provides strong evidence for the use RNAi in pest management, especially for the improvement of SIT against B. dorsalis and other species.

Differentiation in Stem Cell Lineages and in Life: Explorations in the Male Germ Line Stem Cell Lineage

I have been privileged to work on cellular differentiation during a great surge of discovery that has revealed the molecular mechanisms and genetic regulatory circuitry that control embryonic development and adult tissue maintenance and repair. Studying the regulation of proliferation and differentiation in the male germ line stem cell lineage has allowed us investigate how the developmental program imposes layers of additional controls on fundamental cellular processes like cell cycle progression and gene expression to give rise to the huge variety of specialized cell types in our bodies. We are beginning to understand how local signals from somatic support cells specify self-renewal versus differentiation in the stem cell niche at the apical tip of the testis. We are discovering the molecular events that block cell proliferation and initiate terminal differentiation at the switch from mitosis to meiosis-a signature event of the germ cell program. Our work is beginning to reveal how the developmental program that sets up the dramatic new cell type-specific transcription program that prepares germ cells for meiotic division and spermatid differentiation is turned on when cells become spermatocytes. I have had the privilege of working with incredible students, postdocs, and colleagues who have discovered, brainstormed, challenged, and refined our science and our ideas of how developmental pathways and cellular mechanisms work together to drive differentiation.

Recruitment of Mediator Complex by Cell Type and Stage-Specific Factors Required for Tissue-Specific TAF Dependent Gene Activation in an Adult Stem Cell Lineage

Onset of terminal differentiation in adult stem cell lineages is commonly marked by robust activation of new transcriptional programs required to make the appropriate differentiated cell type(s). In the Drosophila male germ line stem cell lineage, the switch from proliferating spermatogonia to spermatocyte is accompanied by one of the most dramatic transcriptional changes in the fly, as over 1000 new transcripts turn on in preparation for meiosis and spermatid differentiation. Here we show that function of the coactivator complex Mediator is required for activation of hundreds of new transcripts in the spermatocyte program. Mediator appears to act in a sequential hierarchy, with the testis activating Complex (tMAC), a cell type specific form of the Mip/dREAM general repressor, required to recruit Mediator subunits to the chromatin, and Mediator function required to recruit the testis TAFs (tTAFs), spermatocyte specific homologs of subunits of TFIID. Mediator, tMAC and the tTAFs co-regulate expression of a major set of spermatid differentiation genes. The Mediator subunit Med22 binds the tMAC component Topi when the two are coexpressed in S2 cells, suggesting direct recruitment. Loss of Med22 function in spermatocytes causes meiosis I maturation arrest male infertility, similar to loss of function of the tMAC subunits or the tTAFs. Our results illuminate how cell type specific versions of the Mip/dREAM complex and the general transcription machinery cooperate to drive selective gene activation during differentiation in stem cell lineages.

Selective gene expression is crucial to making different cell types over the course of the development of an organism. In stem cell lineages, precursor cells terminally differentiate into defined cell types, with onset of terminal differentiation associated with activation of stage- and cell type-specific transcriptional programs. When spermatogonia initiate differentiation and become spermatocytes in the Drosophila male germ line, they undergo the most dramatic transcriptional changes that occur in the fly, as over 1000 new transcripts turn on in preparation for meiosis and the striking morphological changes that produce sperm. This robust spermatocyte transcription program requires cooperative action of a testis-specific protein complex, tMAC and the testis-specific basal transcription machinery TFIID. Here we show that the transcriptional co-activator complex, Mediator is key in connecting the two classes of players. We found that Mediator is recruited to spermatocyte chromatin through the interaction of its subunit, Med22 and a putative transcription activator in tMAC. Recruitment of Mediator is then required for proper localization and function of the testis-specific TFIID complex to initiate gene transcription for spermatid differentiation, illuminating how transcription factors and cell type-specific versions of the general transcription machinery cooperate to drive gene activation during differentiation in adult stem cell lineages.

The sperm ultrastructure and spermiogenesis of Tribolium castaneum (Coleoptera: Tenebrionidae) with evidence of cyst degeneration

Previous studies on the spermatogenesis of tenebrionid beetles showed the unusual formation of two antiparallel sperm bundles per cyst. In this work we reported this feature also in Tribolium castaneum using light and transmission electron microscopy. The sperm structure of T. castaneum, similar to other tenebrionids, consists of a three-layered acrosome, an elongated nucleus and a flagellum with a 9+9+2 axoneme, two accessory bodies and two asymmetric mitochondrial derivatives. The presence of two antiparallel sperm bundles per cyst also in Meloidae and Rhipiphoridae suggests that it is a strong trait synapomorphic for Tenebrionoidea. The huge degeneration of whole sperm cells in several cysts of testes during spermiogenesis is also described.

Transcriptional regulation during Drosophila spermatogenesis

Drosophila spermatogenesis has become a paradigmatic system for the study of mechanisms that regulate adult stem cell maintenance, proliferation and differentiation. The dramatic cellular differentiation process from germline stem cell (GSC) to mature sperm is accompanied by dynamic changes in gene expression, which are regulated at transcriptional, post-transcriptional (including translational) and post-translational levels. Post-transcriptional regulation has been proposed as a unique feature of germ cells. However, recent studies have provided new insights into transcriptional regulation during Drosophila spermatogenesis. Both signaling pathways and epigenetic mechanisms act to orchestrate the transcriptional regulation of distinct genes at different germ cell differentiation stages. Many of the regulatory pathways that control male gamete differentiation in Drosophila are conserved in mammals. Therefore, studies using Drosophila spermatogenesis will provide insight into the molecular mechanisms that regulate mammalian germ cell differentiation pathways.

BmAly is an important factor in meiotic progression and spermatid differentiation in Bombyx mori (Lepidoptera: Bombycidae)

The Drosophila melanogaster "always early" gene (Dmaly), which is required for G2/M cell-cycle control and spermatid differentiation, is one of the meiotic arrest genes. To study the Bombyx mori aly gene (Bmaly), the cDNA of Bmaly was cloned and sequenced, and the results showed that the open reading frame of Bmaly is 1,713 bp in length, encoding 570 amino acid residues, in which a domain in an Rb-related pathway was found. Phylogenetic analysis based on the amino acid sequence of conserved regions showed that Aly from different insects gathered together, except for DmAly and Culex quinquefasciatus Aly, which were not clustered to a subgroup according to insect order. The Bmaly gene was inserted into expression vector pGS-21a(+) and then the recombinant protein was expressed in Escherichia coli and used to immunize mice to prepare the antibody against BmAly. Immunofluorescence examination showed that BmAly was distributed in both the cytoplasm and nucleus of BmN cell. The Bmaly gene expression could not be detected in the silk gland, malpighian tubule, fat body, or midgut of the silkworm. Expression levels of the Bmaly gene were detected in the gonadal tissues, where the levels in the testes were 10 times higher than that in the ovaries. Moreover, Bmaly expression was detected by quantitative polymerase chain reaction at different stages of B. mori testis development, at which fifth instar was relatively grossly expressed. The result suggested Bmaly was abundantly expressed in primary spermatocytes and prespermatids. To further explore the function of Bmaly, Bmaly siRNA was injected into third and fourth instar silkworm larvae, which markedly inhibited the development of sperm cells. These results together suggest that Bmaly is a meiotic arrest gene that plays an important role in spermatogenesis.

Genome-wide and cell-specific epigenetic analysis challenges the role of polycomb in Drosophila spermatogenesis

The Drosophila spermatogenesis cell differentiation pathway involves the activation of a large set of genes in primary spermatocytes. Most of these genes are activated by testis-specific TATA-binding protein associated factors (tTAFs). In the current model for the activation mechanism, Polycomb plays a key role silencing these genes in the germline precursors, and tTAF-dependent activation in primary spermatocytes involves the displacement of Polycomb from gene promoters. We investigated the genome-wide binding of Polycomb in wild type and tTAF mutant testes. According to the model we expected to see a clear enhancement in Polycomb binding at tTAF-dependent spermatogenesis genes in tTAF mutant testes. However, we find little evidence for such an enhancement in tTAF mutant testes compared to wild type. To avoid problems arising from cellular heterogeneity in whole testis analysis, we further tested the model by analysing Polycomb binding in purified germline precursors, representing cells before tTAF-dependent gene activation. Although we find Polycomb associated with its canonical targets, we find little or no evidence of Polycomb at spermatogenesis genes. The lack of Polycomb at tTAF-dependent spermatogenesis genes in precursor cells argues against a model where Polycomb displacement is the mechanism of spermatogenesis gene activation.

The RNA export factor, Nxt1, is required for tissue specific transcriptional regulation

The highly conserved, Nxf/Nxt (TAP/p15) RNA nuclear export pathway is important for export of most mRNAs from the nucleus, by interacting with mRNAs and promoting their passage through nuclear pores. Nxt1 is essential for viability; using a partial loss of function allele, we reveal a role for this gene in tissue specific transcription. We show that many Drosophila melanogaster testis-specific mRNAs require Nxt1 for their accumulation. The transcripts that require Nxt1 also depend on a testis-specific transcription complex, tMAC. We show that loss of Nxt1 leads to reduced transcription of tMAC targets. A reporter transcript from a tMAC-dependent promoter is under-expressed in Nxt1 mutants, however the same transcript accumulates in mutants if driven by a tMAC-independent promoter. Thus, in Drosophila primary spermatocytes, the transcription factor used to activate expression of a transcript, rather than the RNA sequence itself or the core transcription machinery, determines whether this expression requires Nxt1. We additionally find that transcripts from intron-less genes are more sensitive to loss of Nxt1 function than those from intron-containing genes and propose a mechanism in which transcript processing feeds back to increase activity of a tissue specific transcription complex.

Rapid male-specific regulatory divergence and down regulation of spermatogenesis genes in Drosophila species hybrids

In most crosses between closely related species of Drosophila, the male hybrids are sterile and show postmeiotic abnormalities. A series of gene expression studies using genomic approaches have found significant down regulation of postmeiotic spermatogenesis genes in sterile male hybrids. These results have led some to suggest a direct relationship between down regulation in gene expression and hybrid sterility. An alternative explanation to a cause-and-effect relationship between misregulation of gene expression and male sterility is rapid divergence of male sex regulatory elements leading to incompatible interactions in an interspecies hybrid genome. To test the effect of regulatory divergence in spermatogenesis gene expression, we isolated 35 fertile D. simulans strains with D. mauritiana introgressions in either the X, second or third chromosome. We analyzed gene expression in these fertile hybrid strains for a subset of spermatogenesis genes previously reported as significantly under expressed in sterile hybrids relative to D. simulans. We found that fertile autosomal introgressions can cause levels of gene down regulation similar to that of sterile hybrids. We also found that X chromosome heterospecific introgressions cause significantly less gene down regulation than autosomal introgressions. Our results provide evidence that rapid male sex gene regulatory divergence can explain misexpression of spermatogenesis genes in hybrids.

Transcriptional and post-transcriptional regulation of Drosophila germline stem cells and their differentiating progeny

In this chapter we will concentrate on the transcriptional and translational regulations that govern the development and differentiation of male germline cells. Our focus will be on the processes that occur during differentiation, that distinguish the differentiating population of cells from their stem cell parents. We discuss how these defining features are established as cells transit from a stem cell character to that of a fully committed differentiating cell. The focus will be on how GSCs differentiate, via spermatogonia, to spermatocytes. We will achieve this by first describing the transcriptional activity in the differentiating spermatocytes, cataloguing the known transcriptional regulators in these cells and then investigating how the transcription programme is set up by processes in the progentior cells. This process is particularly interesting to study from a stem cell perspective as the male GSCs are unipotent, so lineage decisions in differentiating progeny of stem cells, which occurs in many other stem cell systems, do not impinge on the behaviour of these cells.

BmTGIF, a Bombyx mori homolog of Drosophila DmTGIF, regulates progression of spermatogenesis

TG-interacting factor (TGIF) in Drosophila consists of two tandemly-repeated genes, achintya (Dmachi) and vismay (Dmvis), which act as transcriptional activators in Drosophila spermatogenesis. In contrast, TGIF in humans is a transcriptional repressor that binds directly to DNA or interacts with corepressors to repress the transcription of target genes. In this study, we investigated the characteristics and functions of BmTGIF, a Bombyx mori homolog of DmTGIF. Like DmTGIF, BmTGIF is predominantly expressed in the testes and ovaries. Four alternatively spliced isoforms could be isolated from testes, and two isoforms from ovaries. Quantitative polymerase chain reaction indicated BmTGIF was abundantly expressed in the testis of 3rd instar larvae, when the testis is almost full of primary spermatocytes. The results of luciferase assays indicated that BmTGIF contains two adjacent acidic domains that activate the transcription of reporter genes. Immunofluorescence assay in BmN cells showed that the BmTGIF protein was located mainly in the nucleus, and paraffin sections of testis showed BmTGIF was grossly expressed in primary spermatocytes and mature sperms. Consistent with the role of DmVis in Drosophila development, BmTGIF significantly affected spermatid differentiation, as indicated by hematoxylin-eosin staining of paraffin sections of testis from BmTGIF-small interfering RNA (siRNA)-injected male silkworms. Co-immunoprecipitation experiments suggested that BmTGIF interacted with BmAly, and that they may recruit other factors to form a complex to regulate the genes required for meiotic divisions and spermatid differentiation. The results of this analysis of BmTGIF will improve our understanding of the mechanism of spermatid differentiation in B. mori, with potential applications for pest control.

The bromodomain-containing protein tBRD-1 is specifically expressed in spermatocytes and is essential for male fertility

By a conserved cellular differentiation process, spermatogenesis leads to formation of haploid sperm for successful reproduction. In Drosophila and in mammals, post-meiotic spermatid differentiation depends on several translationally repressed and stored mRNAs that are often expressed exclusively in the testis through a cell type specific transcriptional program. In Drosophila, the mRNAs of proteins required for post-meiotic chromatin reorganisation, like ProtB and Mst77F, are transcribed in meiotic spermatocytes and subjected to translational repression for days. Transcription of many of these translationally repressed mRNAs depends on testis-specific homologs of TATA box binding protein-associated factors (tTAFs). Here, we identified the testis-specific bromodomain protein, tBRD-1, that is only expressed in primary spermatocytes. Bromodomain proteins are able to recognise and bind acetylated histones and non-histone proteins. We generated tbrd-1 mutant flies and observed that function of tBRD-1 is required for male fertility. tBRD-1 partially colocalised with tTAFs, TAF1 and Polycomb to a Fibrillarin-deficient region within the spermatocyte nucleolus. The nucleolar localisation of tBRD-1 depended on tTAF function but not the other way round. Further, we could show that ectopically expressed tBRD-1-eGFP is able to bind to the interbands of polytene chromosomes. By inhibitor treatment of cultured testis we observed that sub-cellular localisation of tBRD-1 may depend on the acetylation status of primary spermatocytes.

The THO complex is required for nucleolar integrity in Drosophila spermatocytes

The THO complex is a conserved multisubunit protein complex that functions in the formation of export-competent messenger ribonucleoprotein (mRNP). Although the complex has been studied extensively at the single-cell level, its exact role at the multicellular organism level has been poorly understood. Here, we isolated a novel Drosophila male sterile mutant, garmcho (garm). Positional cloning indicated that garm encodes a subunit of the Drosophila THO complex, THOC5. Flies lacking THOC5 showed a meiotic arrest phenotype with severe nucleolar disruption in primary spermatocytes. A functional GFP-tagged fusion protein, THOC5-GFP, revealed a unique pattern of THOC5 localization near the nucleolus. The nucleolar distribution of a testis-specific TATA binding protein (TBP)-associated factor (tTAF), SA, which is required for the expression of genes responsible for sperm differentiation, was severely disrupted in mutant testes lacking THOC5. But THOC5 appeared to be largely dispensable for the expression and nuclear export of either tTAF target mRNAs or tTAF-independent mRNAs. Taken together, our study suggests that the Drosophila THO complex is necessary for proper spermatogenesis by contribution to the establishment or maintenance of nucleolar integrity rather than by nuclear mRNA export in spermatocytes.

Sequential changes at differentiation gene promoters as they become active in a stem cell lineage

Transcriptional silencing of terminal differentiation genes by the Polycomb group (PcG) machinery is emerging as a key feature of precursor cells in stem cell lineages. How, then, is this epigenetic silencing reversed for proper cellular differentiation? Here, we investigate how the developmental program reverses local PcG action to allow expression of terminal differentiation genes in the Drosophila male germline stem cell (GSC) lineage. We find that the silenced state, set up in precursor cells, is relieved through developmentally regulated sequential events at promoters once cells commit to spermatocyte differentiation. The programmed events include global downregulation of Polycomb repressive complex 2 (PRC2) components, recruitment of hypophosphorylated RNA polymerase II (Pol II) to promoters, as well as the expression and action of testis-specific homologs of TATA-binding protein-associated factors (tTAFs). In addition, action of the testis-specific meiotic arrest complex (tMAC), a tissue-specific version of the MIP/dREAM complex, is required both for recruitment of tTAFs to target differentiation genes and for proper cell type-specific localization of PRC1 components and tTAFs within the spermatocyte nucleolus. Together, the action of the tMAC and tTAF cell type-specific chromatin and transcription machinery leads to loss of Polycomb and release of stalled Pol II from the terminal differentiation gene promoters, allowing robust transcription.

Unique aspects of transcription regulation in male germ cells

Spermatogenesis is a complex and ordered differentiation process in which the spermatogonial stem cell population gives rise to primary spermatocytes that undergo two successive meiotic divisions followed by a major biochemical and structural reorganization of the haploid cells to generate mature elongate spermatids. The transcriptional regulatory programs that orchestrate this process have been intensively studied in model organisms such as Drosophila melanogaster and mouse. Genetic and biochemical approaches have identified the factors involved and revealed mechanisms of action that are unique to male germ cells. In a well-studied example, cofactors and pathways distinct from those used in somatic tissues mediate the action of CREM in male germ cells. But perhaps the most striking feature concerns the paralogs of somatically expressed transcription factors and of components of the general transcription machinery that act in distinct regulatory mechanisms in both Drosophila and murine spermatogenesis.

Wake-up-call, a lin-52 paralogue, and Always early, a lin-9 homologue physically interact, but have opposing functions in regulating testis-specific gene expression

A conserved multi-subunit complex (MybMuvB, MMB), regulates transcriptional activity of many different target genes in Drosophila somatic cells. A paralogous complex, tMAC, controls expression of at least 1500 genes in the male germline, and is essential for sperm production. The roles of specific subunits of tMAC, MMB or orthologous complexes in regulating target gene expression are not understood. MMB and orthologous complexes have Lin-52 as a subunit, but Lin-52 did not co-purify with tMAC. We identified wake-up-call (wuc), a lin-52 paralogue, via a physical interaction with the tMAC lin-9-related subunit Aly, and find that Wuc co-localises with known tMAC subunits. We show that wuc, like aly, is required for spermatogenesis. However, despite phenotypic similarities, the role of wuc is very different from that of previously characterised tMAC mutants. Unlike aly, loss of wuc results in only relatively mild defects in testis-specific gene expression. Strikingly, wuc loss of function partially rescues expression of target genes in aly mutant testes. We propose that wuc represses testis-specific gene expression, that this repression is counteracted by aly, and that aly and a testis-specific TF_IID complex work together to promote high transcriptional activity of spermiogenic genes specifically in primary spermatocytes.

Male sex interspecies divergence and down regulation of expression of spermatogenesis genes in Drosophila sterile hybrids

Male sex genes have shown a pattern of rapid interspecies divergence at both the coding and gene expression level. A common outcome from crosses between closely-related species is hybrid male sterility. Phenotypic and genetic studies in Drosophila sterile hybrid males have shown that spermatogenesis arrest is postmeiotic with few exceptions, and that most misregulated genes are involved in late stages of spermatogenesis. Comparative studies of gene regulation in sterile hybrids and parental species have mainly used microarrays providing a whole genome representation of regulatory problems in sterile hybrids. Real-time PCR studies can reject or reveal differences not observed in microarray assays. Moreover, differences in gene expression between samples can be dependant on the source of RNA (e.g., whole body vs. tissue). Here we survey expression in D. simulans, D. mauritiana and both intra and interspecies hybrids using a real-time PCR approach for eight genes expressed at the four main stages of sperm development. We find that all genes show a trend toward under expression in the testes of sterile hybrids relative to parental species with only the two proliferation genes (bam and bgcn) and the two meiotic class genes (can and sa) showing significant down regulation. The observed pattern of down regulation for the genes tested can not fully explain hybrid male sterility. We discuss the down regulation of spermatogenesis genes in hybrids between closely-related species within the contest of rapid divergence experienced by the male genome, hybrid sterility and possible allometric changes due to subtle testes-specific developmental abnormalities.

Molecular mechanisms of gene regulation during Drosophila spermatogenesis

The differentiation of sperm from morphologically unremarkable cells into highly specialised free-living, motile cells requires the co-ordinated action of a very large number of gene products. The expression of these products must be regulated in a developmental context to ensure normal cellular differentiation. Many genes essential for spermatogenesis are not used elsewhere in the animal, or are expressed elsewhere, but using a different transcription regulation module. Spermatogenesis is thus a good system for elucidating the principles of tissue-specific gene expression, as well as being interesting in its own right. Here, I discuss the regulation of gene expression during spermatogenesis in Drosophila, focussing on the processes underlying the expression of testis-specific genes in the male germline.

FlyTED: the Drosophila Testis Gene Expression Database

FlyTED, the Drosophila Testis Gene Expression Database, is a biological research database for gene expression images from the testis of the fruit fly Drosophila melanogaster. It currently contains 2762 mRNA in situ hybridization images and ancillary metadata revealing the patterns of gene expression of 817 Drosophila genes in testes of wild type flies and of seven meiotic arrest mutant strains in which spermatogenesis is defective. This database has been built by adapting a widely used digital library repository software system, EPrints (http://eprints.org/software/), and provides both web-based search and browse interfaces, and programmatic access via an SQL dump, OAI-PMH and SPARQL. FlyTED is available at http://www.fly-ted.org/.

Gene expression disruptions of organism versus organ in Drosophila species hybrids

Hybrid dysfunctions, such as sterility, may result in part from disruptions in the regulation of gene expression. Studies of hybrids within the Drosophila simulans clade have reported genes expressed above or below the expression observed in their parent species, and such misexpression is associated with male sterility in multigenerational backcross hybrids. However, these studies often examined whole bodies rather than testes or had limited replication using less-sensitive but global techniques. Here, we use a new RNA isolation technique to re-examine hybrid gene expression disruptions in both testes and whole bodies from single Drosophila males by real-time quantitative RT-PCR. We find two early-spermatogenesis transcripts are underexpressed in hybrid whole-bodies but not in assays of testes alone, while two late-spermatogenesis transcripts seem to be underexpressed in both whole-bodies and testes alone. Although the number of transcripts surveyed is limited, these results provide some support for a previous hypothesis that the spermatogenesis pathway in these sterile hybrids may be disrupted sometime after the expression of the early meiotic arrest genes.

Drosophila Uri, a PP1alpha binding protein, is essential for viability, maintenance of DNA integrity and normal transcriptional activity

BACKGROUND

Protein phosphatase 1 (PP1) is involved in diverse cellular processes, and is targeted to substrates via interaction with many different protein binding partners. PP1 catalytic subunits (PP1c) fall into PP1alpha and PP1beta subfamilies based on sequence analysis, however very few PP1c binding proteins have been demonstrated to discriminate between PP1alpha and PP1beta.

RESULTS

URI (unconventional prefoldin RPB5 interactor) is a conserved molecular chaperone implicated in a variety of cellular processes, including the transcriptional response to nutrient signalling and maintenance of DNA integrity. We show that Drosophila Uri binds PP1alpha with much higher affinity than PP1beta, and that this ability to discriminate between PP1c forms is conserved to humans. Most Uri is cytoplasmic, however we found some protein associated with active RNAPII on chromatin. We generated a uri loss of function allele, and show that uri is essential for viability in Drosophila. uri mutants have transcriptional defects, reduced cell viability and differentiation in the germline, and accumulate DNA damage in their nuclei.

CONCLUSION

Uri is the first PP1alpha specific binding protein to be described in Drosophila. Uri protein plays a role in transcriptional regulation. Activity of uri is required to maintain DNA integrity and cell survival in normal development.

Organization and regulation of sex-specific thioredoxin encoding genes in the genus Drosophila

Thioredoxins are small thiol proteins that have a conserved active site sequence, WCGPC, and reduce disulfide bonds in various proteins using the two active site cysteines, a reaction that oxidizes thioredoxin and renders it inactive. Thioredoxin reductase returns thioredoxin to its reduced, active form in a reaction that converts NADPH to NADP(+). The biological functions of thioredoxins vary widely; they have roles in oxidative stress protection, act as electron donors for ribonucleotide reductase, and form structural components of enzymes. To date, three thioredoxin genes have been characterized in Drosophila melanogaster: the generally expressed Thioredoxin-2 (Trx-2) and the two sex-specific genes ThioredoxinT (TrxT) and deadhead (dhd). The male-specific TrxT and the female-specific dhd are located as a gene pair, transcribed in opposite directions, with only 470 bp between their transcription start points. We show in this study that all three D. melanogaster thioredoxins are conserved in 11 other Drosophilid species, which are believed to have diverged up to 40 Ma ago and that Trx-2 is conserved all the way to Tribolium castaneum. We have found that the intriguing gene organization and regulation of TrxT and dhd is remarkably well conserved and identified potential conserved regulatory sequences. In addition, we show that the 50-70 C terminal amino acids of TrxT constitute a hyper-variable domain, which could play a role in sexual conflict and male-female co-evolution.

A novel eIF4G homolog, Off-schedule, couples translational control to meiosis and differentiation in Drosophila spermatocytes

During spermatogenesis, cells coordinate differentiation with the meiotic cell cycle to generate functional gametes. We identified a novel gene, which we named off-schedule (ofs), as being essential for this coordinated control. During the meiotic G(2) phase, Drosophila ofs mutant germ cells do not reach their proper size and fail to execute meiosis or significant differentiation. The accumulation of four cell cycle regulators--Cyclin A, Boule, Twine and Roughex--is altered in these mutants, indicating that ofs reveals a novel branch of the pathway controlling meiosis and differentiation. Ofs is homologous to eukaryotic translation initiation factor eIF4G. The level of ofs expression in spermatocytes is much higher than for the known eIF4G ortholog (known as eIF-4G or eIF4G), suggesting that Ofs substitutes for this protein. Consistent with this, assays for association with mRNA cap complexes, as well as RNA-interference and phenotypic-rescue experiments, demonstrate that Ofs has eIF4G activity. Based on these studies, we speculate that spermatocytes monitor G(2) growth as one means to coordinate the initiation of meiotic division and differentiation.

Discovery of tMAC: a Drosophila testis-specific meiotic arrest complex paralogous to Myb-Muv B

The Drosophila Myb-Muv B (MMB)/dREAM complex regulates gene expression and DNA replication site-specifically, but its activities in vivo have not been thoroughly explored. In ovarian amplification-stage follicle cell nuclei, the largest subunit, Mip130, is a negative regulator of replication, whereas another subunit, Myb, is a positive regulator. Here, we identified a mutation in mip40 and generated a mutation in mip120, two additional MMB subunits. Both mutants were viable, but mip120 mutants had many complex phenotypes including shortened longevity and severe eye defects. mip40 mutant females had severely reduced fertility, whereas mip120 mutant females were sterile, substantiating ovarian regulatory role(s) for MMB. Myb accumulation and binding to polytene chromosomes was dependent on the core factors of the MMB complex. In contrast to the documented mip130 mutant phenotypes, both mip40 and mip120 mutant males were sterile. We purified Mip40-containing complexes from testis nuclear extracts and identified tMAC, a new testis-specific meiotic arrest complex that contained Mip40, Caf1/p55, the Mip130 family member, Always early (Aly), and a Mip120 family member, Tombola (Tomb). Together, these data demonstrate that MMB serves diverse roles in different developmental pathways, and members of MMB can be found in alternative, noninteracting complexes in different cell types.

Tombola, a tesmin/TSO1-family protein, regulates transcriptional activation in the Drosophila male germline and physically interacts with always early

During male gametogenesis, a developmentally regulated and cell type-specific transcriptional programme is activated in primary spermatocytes to prepare for differentiation of sperm. The Drosophila aly-class meiotic-arrest loci (aly, comr, achi/vis and topi) are essential for activation of transcription of many differentiation-specific genes, and several genes important for meiotic cell cycle progression, thus linking meiotic divisions to cellular differentiation during spermatogenesis. Protein interaction studies suggest that the aly-class gene products form a chromatin-associated complex in primary spermatocytes. We identify, clone and characterise a new aly-class meiotic-arrest gene, tombola (tomb), which encodes a testis-specific CXC-domain protein that interacts with Aly. The tomb mutant phenotype is more like that of aly and comr mutants than that of achi/vis or topi mutants in terms of target gene profile and chromosome morphology. tomb encodes a chromatin-associated protein required for localisation of Aly and Comr, but not Topi, to chromatin Reciprocally, aly and comr, but not topi or achi/vis, are required to maintain the normal localisation of Tomb. tomb and aly might be components of a complex paralogous to the Drosophila dREAM/Myb-MuvB and C. elegans DRM transcriptional regulatory complexes.

Rapid evolution and gene-specific patterns of selection for three genes of spermatogenesis in Drosophila

Hybrid males resulting from crosses between closely related species of Drosophila are sterile. The F1 hybrid sterility phenotype is mainly due to defects occurring during late stages of development that relate to sperm individualization, and so genes controlling sperm development may have been subjected to selective diversification between species. It is also possible that genes of spermatogenesis experience selective constraints given their role in a developmental pathway. We analyzed the molecular evolution of three genes playing a role during the sperm developmental pathway in Drosophila at an early (bam), a mid (aly), and a late (dj) stage. The complete coding region of these genes was sequenced in different strains of Drosophila melanogaster and Drosophila simulans. All three genes showed rapid divergence between species, with larger numbers of nonsynonymous to synonymous differences between species than polymorphisms. Although this could be interpreted as evidence for positive selection at all three genes, formal tests of selection do not support such a conclusion. Departures from neutrality were detected only for dj and bam but not aly. The role played by selection is unique and determined by gene-specific characteristics rather than site of expression. In dj, the departure was due to a high proportion of neutral synonymous polymorphisms in D. simulans, and there was evidence of purifying selection maintaining a high lysine amino acid protein content that is characteristic of other DNA-binding proteins. The earliest spermatogenesis gene surveyed, which plays a role in both male and female gametogenesis, was bam, and its significant departure from neutrality was due to an excess of nonsynonymous substitutions between species. Bam is degraded at the end of mitosis, and rapid evolutionary changes among species might be a characteristic shared with other degradable transient proteins. However, the large number of nonsynonymous changes between D. melanogaster and D. simulans and a phylogenetic comparative analysis among species confirms evidence of positive selection driving the evolution of Bam and suggests an yet unknown germ cell line developmental adaptive change between these two species.

Inhibition of oncogenic transformation by mammalian Lin-9, a pRB-associated protein

Genetic studies in Caenorhabditis elegans identified lin-9 to function together with the retinoblastoma homologue lin-35 in vulva differentiation. We have now identified a human homologue of Lin-9 (hLin-9) and provide evidence about its function in the mammalian pRB pathway. hLin-9 binds to pRB and cooperates with pRB in flat cell formation in Saos-2 cells. In addition, hLin-9 synergized with pRB and Cbfal to transactivate an osteoblast-specific reporter gene. In contrast, hLin-9 was not involved in pRB-mediated inhibition of cell cycle progression or repression of E2F-dependent transactivation. Consistent with these data, hLin-9 was able to associate with partially penetrant pRB mutants that do not bind to E2F, but retain the ability to activate transcription and to promote differentiation. hLin-9 can also inhibit oncogenic transformation, dependent on the presence of a functional pRB protein. RNAi-mediated knockdown of Lin-9 can substitute for the loss of pRB in transformation of human primary fibroblasts. These data suggest that hLin-9 has tumor-suppressing activities and that the ability of hLin-9 to inhibit transformation is mediated through its association with pRB.

Testis-specific TAF homologs collaborate to control a tissue-specific transcription program

Alternate forms of the PolII transcription initiation machinery have been proposed to play a role in selective activation of cell-type-specific gene expression programs during cellular differentiation. The cannonball(can) gene of Drosophila encodes a homolog of a TBP-associated factor (dTAF5) protein expressed only in spermatocytes, where it is required for normal transcription of genes required for spermatid differentiation. We show that Drosophila primary spermatocytes also express four additional tissue-specific TAFs: nht (homolog of dTAF4), mia (homolog of dTAF6), sa (homolog of dTAF8) and rye (homolog of dTAF12). Mutations in nht, mia and sa have similar effects in primary spermatocytes on transcription of several target genes involved in spermatid differentiation, and cause the same phenotypes as mutations in can, blocking both meiotic cell cycle progression and spermatid differentiation. The nht, mia, sa and rye proteins contain histone fold domain dimerization motifs. The nht and rye proteins interact structurally when co-expressed in bacteria, similarly to their generally expressed homologs TAF4 and TAF12,which heterodimerize. Strikingly, the structural interaction is tissue specific: nht did not interact with dTAF12 and dTAF4 did not interact with rye in a bacterial co-expression assay. We propose that the products of the five Drosophila genes encoding testis TAF homologs collaborate in an alternative TAF-containing protein complex to regulate a testis-specific gene expression program in primary spermatocytes required for terminal differentiation of male germ cells.

Gene structure and molecular analysis of Arabidopsis thaliana ALWAYS EARLY homologs

Drosophila always early (aly) is essential for spermatogenesis, and is related to the LIN-9 protein of Caenorhabditis elegans; lin-9 is a class B Synthetic Multivulva gene (synMuvB) required for gonadal sheath development. Aly/LIN-9 have two conserved regions, called domains 1 and 2, which have been identified in homologous proteins from several multicellular eukaryotes, including the model plant Arabidopsis thaliana. We cloned and sequenced cDNAs of three different A. thaliana ALWAYS EARLY homologs (AtALY1, AtALY2 and AtALY3), analysed the expression pattern of these three genes and show that AtALY1, like Aly, is nuclear localised. We also demonstrate that the plant homologs of aly/lin-9 contain an additional N-terminal myb domain not present in the animal Aly/LIN-9 proteins, and that part of the ALY/LIN-9 conserved domain 1 in the predicted plant proteins is related to the TUDOR domain.

Regulation of transcription of meiotic cell cycle and terminal differentiation genes by the testis-specific Zn-finger protein matotopetli

A robust developmentally regulated and cell type specific transcriptional programme is activated in primary spermatocytes in preparation for differentiation of the male gametes during spermatogenesis. Work in Drosophila is beginning to reveal the genetic networks that regulate this gene expression. The Drosophila aly-class meiotic arrest loci are essential for activation of transcription of many differentiation-specific genes, as well as several genes important for meiotic cell cycle progression, thus linking meiotic cell cycle progression to cellular differentiation during spermatogenesis. The three previously described aly-class proteins (aly, comr and achi/vis) form a complex and are associated with chromatin in primary spermatocytes. We identify, clone and characterize a new aly-class meiotic arrest gene, matotopetli (topi), which encodes a testis-specific Zn-finger protein that physically interacts with Comr. The topi mutant phenotype is most like achi/vis in that topi function is not required for the nuclear localization of Aly or Comr, but is required for their accumulation on chromatin. Most target genes in the transcriptional programme depend on both topi and achi/vis; however, a small subset of target genes are differentially sensitive to loss of topi or achi/vis, suggesting that these aly-class predicted DNA binding proteins can act independently in some contexts.

Drosophila TGIF is essential for developmentally regulated transcription in spermatogenesis

We have investigated the role of TGIF, a TALE-class homeodomain transcription factor, in Drosophila development. In vertebrates, TGIF has been implicated, by in vitro analysis, in several pathways, most notably as a repressor modulating the response to TGFbeta signalling. Human TGIF has been associated with the developmental disorder holoprosencephaly. Drosophila TGIF is represented by the products of two tandemly repeated highly similar genes, achintya and vismay. We have generated mutations that delete both genes. Homozygous mutant flies are viable and appear morphologically normal, but the males are completely sterile. The defect lies at the primary spermatocyte stage and differentiation is blocked prior to the onset of the meiotic divisions. We show that mutants lacking TGIF function fail to activate transcription of many genes required for sperm manufacture and of some genes required for entry into the meiotic divisions. This groups TGIF together with two other genes producing similar phenotypes, always early and cookie monster, as components of the machinery required for the activation of the spermatogenic programme of transcription. TGIF is the first sequence-specific transcription factor identified in this pathway. By immunolabelling in mouse testes we show that TGIF is expressed in the early stages of spermatogenesis consistent with a conserved role in the activation of the spermatogenesis transcription programme.

Requirement for two nearly identical TGIF-related homeobox genes in Drosophila spermatogenesis

The genetic analysis of spermatogenesis in Drosophila melanogaster has led to the identification of several genes that control the onset of meiosis, spermatid differentiation, or both. We described two tightly linked and nearly identical homeobox genes of the TGIF (TG-interacting factor) subclass called vismay and achintya that are essential for spermatogenesis in Drosophila. In flies deficient for both genes, spermatogenesis is blocked prior to any spermatid differentiation and before the first meiotic division. This suggests that vismay and achintya function at the same step as two previously characterized meiotic arrest genes, always early and cookie monster. Consistent with this idea, both always early and cookie monster are still expressed in flies deficient in vismay and achintya. Conversely, Vismay and Achintya proteins are present in always early mutant testes. Co-immunoprecipitation experiments further suggest that Vismay and Achintya proteins exist in a complex with Always early and Cookie monster proteins. Because Vismay and Achintya are likely to be sequence-specific DNA binding factors, these results suggest that they help to specify the spermatogenesis program by recruiting or stabilizing Always early and Cookie monster to specific target genes that need to be transcriptionally regulated during testes development.