Prtl99C Acts Together with Protamines and Safeguards Male Fertility in Drosophila

An orphan gene is essential for efficient sperm entry into eggs in Drosophila melanogaster

While spermatogenesis has been extensively characterized in the Drosophila melanogaster model system, very little is known about the genes required for fly sperm entry into eggs. We identified a lineage-specific gene, which we named katherine johnson (kj), that is required for efficient fertilization. Males that do not express kj produce and transfer sperm that are stored normally in females, but sperm from these males enter eggs with severely reduced efficiency. Using a tagged transgenic rescue construct, we observed that the KJ protein localizes around the edge of the nucleus at various stages of spermatogenesis but is undetectable in mature sperm. These data suggest that kj exerts an effect on sperm development, the loss of which results in reduced fertilization ability. Interestingly, KJ protein lacks detectable sequence similarity to any other known protein, suggesting that kj could be a lineage-specific orphan gene. While previous bioinformatic analyses indicated that kj was restricted to the melanogaster group of Drosophila, we identified putative orthologs with conserved synteny, male-biased expression, and predicted protein features across the genus, as well as likely instances of gene loss in some lineages. Thus, kj was likely present in the Drosophila common ancestor. It is unclear whether its role in fertility had already evolved at that time or developed later in the lineage leading to D. melanogaster. Our results demonstrate a new aspect of male reproduction that has been shaped by a lineage-specific gene and provide a molecular foothold for further investigating the mechanism of sperm entry into eggs in Drosophila.

Cytoplasmic incompatibility factor proteins from Wolbachia prophage are costly to sperm development in Drosophila melanogaster

The symbiosis between arthropods and Wolbachia bacteria is globally widespread, largely due to selfish-drive systems that favour the fitness of symbiont-transmitting females. The most common drive, cytoplasmic incompatibility (CI), is central to arboviral control efforts. In Drosophila melanogaster carrying wMel Wolbachia deployed in mosquito control, two prophage genes in Wolbachia, cifA and cifB, cause CI that results in a paternal-effect lethality of embryos in crosses between Wolbachia-bearing males and aposymbiotic females. While the CI mechanism by which Cif proteins alter sperm development has recently been elucidated in D. melanogaster and Aedes aegypti mosquitoes, the Cifs' extended impact on male reproductive fitness such as sperm morphology and quantity remains unclear. Here, using cytochemical, microscopic and transgenic assays in D. melanogaster, we demonstrate that both CifA and CifB cause a significant portion of defects in elongating spermatids, culminating in malformed mature sperm nuclei. Males expressing Cifs have reduced spermatid bundles and sperm counts, and transgenic expression of Cifs can occasionally result in no mature sperm formation. We reflect on Cifs' varied functional impacts on the Host Modification model of CI as well as host evolution, behaviour and vector control strategies.

An orphan gene is essential for efficient sperm entry into eggs in Drosophila melanogaster

While spermatogenesis has been extensively characterized in the Drosophila melanogaster model system, very little is known about the genes required for fly sperm entry into eggs. We identified a lineage-specific gene, which we named katherine johnson (kj), that is required for efficient fertilization. Males that do not express kj produce and transfer sperm that are stored normally in females, but sperm from these males enter eggs with severely reduced efficiency. Using a tagged transgenic rescue construct, we observed that the KJ protein localizes around the edge of the nucleus at various stages of spermatogenesis but is undetectable in mature sperm. These data suggest that kj exerts an effect on sperm development, the loss of which results in reduced fertilization ability. Interestingly, KJ protein lacks detectable sequence similarity to any other known protein, suggesting that kj could be a lineage-specific orphan gene. While previous bioinformatic analyses indicated that kj was restricted to the melanogaster group of Drosophila, we identified putative orthologs with conserved synteny, male-biased expression, and predicted protein features across the genus, as well as likely instances of gene loss in some lineages. Thus, kj was likely present in the Drosophila common ancestor and subsequently evolved an essential role in fertility in D. melanogaster. Our results demonstrate a new aspect of male reproduction that has been shaped by a lineage-specific gene and provide a molecular foothold for further investigating the mechanism of sperm entry into eggs in Drosophila.

Histone removal in sperm protects paternal chromosomes from premature division at fertilization

The global replacement of histones with protamines in sperm chromatin is widespread in animals, including insects, but its actual function remains enigmatic. We show that in the Drosophila paternal effect mutant paternal loss (pal), sperm chromatin retains germline histones H3 and H4 genome wide without impairing sperm viability. However, after fertilization, pal sperm chromosomes are targeted by the egg chromosomal passenger complex and engage into a catastrophic premature division in synchrony with female meiosis II. We show that pal encodes a rapidly evolving transition protein specifically required for the eviction of (H3-H4)2 tetramers from spermatid DNA after the removal of H2A-H2B dimers. Our study thus reveals an unsuspected role of histone eviction from insect sperm chromatin: safeguarding the integrity of the male pronucleus during female meiosis.

APOLLO, a testis-specific Drosophila ortholog of importin-4, mediates the loading of protamine-like protein Mst77F into sperm chromatin

DNA in sperm is packed with small, charged proteins termed SNBPs (sperm nuclear basic proteins), including mammalian and Drosophila protamines. During spermiogenesis, somatic-type chromatin is taken apart and replaced with sperm chromatin in a multistep process leading to an extraordinary condensation of the genome. During fertilization, the ova face a similarly challenging task of SNBP eviction and reassembly of nucleosome-based chromatin. Despite its importance for the animal life cycle, sperm chromatin metabolism, including the biochemical machinery mediating the mutual replacement of histones and SNBPs, remains poorly studied. In Drosophila, Mst77F is one of the first SNBPs loaded into the spermatid nuclei. It persists in mature spermatozoa and is essential for sperm compaction and male fertility. Here, by using in vitro biochemical assays, we identify chaperones that can mediate the eviction and loading of Mst77F on DNA, thus facilitating the interconversions of chromatin forms in the male gamete. Unlike NAP1 and TAP/p32 chaperones that disassemble Mst77F-DNA complexes, ARTEMIS and APOLLO, orthologs of mammalian importin-4 (IPO4), mediate the deposition of Mst77F on DNA or oligonucleosome templates, accompanied by the dissociation of histone-DNA complexes. In vivo, a mutation of testis-specific Apollo brings about a defect of Mst77F loading, abnormal sperm morphology, and male infertility. We identify IPO4 ortholog APOLLO as a critical component of sperm chromatin assembly apparatus in Drosophila. We discover that in addition to recognized roles in protein traffic, a nuclear transport receptor (IPO4) can function directly in chromatin remodeling as a dual, histone- and SNBP-specific, chaperone.

A rapidly evolving single copy histone H1 variant is associated with male fertility in a parasitoid wasp

The linker histone H1 binds to the nucleosome core particle at the site where DNA enters and exits, and facilitates folding of the nucleosomes into a higher-order chromatin structure in eukaryotes. Additionally, some variant H1s promote specialized chromatin functions in cellular processes. Germline-specific H1 variants have been reported in some model species with diverse roles in chromatin structure changes during gametogenesis. In insects, the current understanding of germline-specific H1 variants comes mainly from the studies in Drosophila melanogaster, and the information on this set of genes in other non-model insects remains largely unknown. Here, we identify two H1 variants (PpH1V1 and PpH1V2) that are specifically predominantly expressed in the testis of the parasitoid wasp Pteromalus puparum. Evolutionary analyses suggest that these H1 variant genes evolve rapidly, and are generally maintained as a single copy in Hymenoptera. Disruption of PpH1V1 function in the late larval stage male by RNA interference experiments has no phenotype on spermatogenesis in the pupal testis, but results in abnormal chromatin structure and low sperm fertility in the adult seminal vesicle. In addition, PpH1V2 knockdown has no detectable effect on spermatogenesis or male fertility. Collectively, our discovery indicates distinct functions of male germline-enriched H1 variants between parasitoid wasp Pteromalus and Drosophila, providing new insights into the role of insect H1 variants in gametogenesis. This study also highlights the functional complexity of germline-specific H1s in animals.

Derepression of Y-linked multicopy protamine-like genes interferes with sperm nuclear compaction in D. melanogaster

Across species, sperm maturation involves the dramatic reconfiguration of chromatin into highly compact nuclei that enhance hydrodynamic ability and ensure paternal genomic integrity. This process is mediated by the replacement of histones by sperm nuclear basic proteins, also referred to as protamines. In humans, a carefully balanced dosage between two known protamine genes is required for optimal fertility. However, it remains unknown how their proper balance is regulated and how defects in balance may lead to compromised fertility. Here, we show that a nucleolar protein, modulo, a homolog of nucleolin, mediates the histone-to-protamine transition during Drosophila spermatogenesis. We find that modulo mutants display nuclear compaction defects during late spermatogenesis due to decreased expression of autosomal protamine genes (including Mst77F) and derepression of Y-linked multicopy Mst77F homologs (Mst77Y), leading to the mutant's known sterility. Overexpression of Mst77Y in a wild-type background is sufficient to cause nuclear compaction defects, similar to modulo mutant, indicating that Mst77Y is a dominant-negative variant interfering with the process of histone-to-protamine transition. Interestingly, ectopic overexpression of Mst77Y caused decompaction of X-bearing spermatids nuclei more frequently than Y-bearing spermatid nuclei, although this did not greatly affect the sex ratio of offspring. We further show that modulo regulates these protamine genes at the step of transcript polyadenylation. We conclude that the regulation of protamines mediated by modulo, ensuring the expression of functional ones while repressing dominant-negative ones, is critical for male fertility.

Expansion and loss of sperm nuclear basic protein genes in Drosophila correspond with genetic conflicts between sex chromosomes

Many animal species employ sperm nuclear basic proteins (SNBPs) or protamines to package sperm genomes tightly. SNBPs vary across animal lineages and evolve rapidly in mammals. We used a phylogenomic approach to investigate SNBP diversification in Drosophila species. We found that most SNBP genes in Drosophila melanogaster evolve under positive selection except for genes essential for male fertility. Unexpectedly, evolutionarily young SNBP genes are more likely to be critical for fertility than ancient, conserved SNBP genes. For example, CG30056 is dispensable for male fertility despite being one of three SNBP genes universally retained in Drosophila species. We found 19 independent SNBP gene amplification events that occurred preferentially on sex chromosomes. Conversely, the montium group of Drosophila species lost otherwise-conserved SNBP genes, coincident with an X-Y chromosomal fusion. Furthermore, SNBP genes that became linked to sex chromosomes via chromosomal fusions were more likely to degenerate or relocate back to autosomes. We hypothesize that autosomal SNBP genes suppress meiotic drive, whereas sex-chromosomal SNBP expansions lead to meiotic drive. X-Y fusions in the montium group render autosomal SNBPs dispensable by making X-versus-Y meiotic drive obsolete or costly. Thus, genetic conflicts between sex chromosomes may drive SNBP rapid evolution during spermatogenesis in Drosophila species.

The Cif proteins from Wolbachia prophage WO modify sperm genome integrity to establish cytoplasmic incompatibility

Inherited microorganisms can selfishly manipulate host reproduction to drive through populations. In Drosophila melanogaster, germline expression of the native Wolbachia prophage WO proteins CifA and CifB cause cytoplasmic incompatibility (CI) in which embryos from infected males and uninfected females suffer catastrophic mitotic defects and lethality; however, in infected females, CifA expression rescues the embryonic lethality and thus imparts a fitness advantage to the maternally transmitted Wolbachia. Despite widespread relevance to sex determination, evolution, and vector control, the mechanisms underlying when and how CI impairs male reproduction remain unknown and a topic of debate. Here, we use cytochemical, microscopic, and transgenic assays in D. melanogaster to demonstrate that CifA and CifB proteins of wMel localize to nuclear DNA throughout the process of spermatogenesis. Cif proteins cause abnormal histone retention in elongating spermatids and protamine deficiency in mature sperms that travel to the female reproductive tract with Cif proteins. Notably, protamine gene knockouts enhance wild-type CI. In ovaries, CifA localizes to germ cell nuclei and cytoplasm of early-stage egg chambers; however, Cifs are absent in late-stage oocytes and subsequently in fertilized embryos. Finally, CI and rescue are contingent upon a newly annotated CifA bipartite nuclear localization sequence. Together, our results strongly support the Host modification model of CI in which Cifs initially modify the paternal and maternal gametes to bestow CI-defining embryonic lethality and rescue.

A putative de novo evolved gene required for spermatid chromatin condensation in Drosophila melanogaster

Comparative genomics has enabled the identification of genes that potentially evolved de novo from non-coding sequences. Many such genes are expressed in male reproductive tissues, but their functions remain poorly understood. To address this, we conducted a functional genetic screen of over 40 putative de novo genes with testis-enriched expression in Drosophila melanogaster and identified one gene, atlas, required for male fertility. Detailed genetic and cytological analyses showed that atlas is required for proper chromatin condensation during the final stages of spermatogenesis. Atlas protein is expressed in spermatid nuclei and facilitates the transition from histone- to protamine-based chromatin packaging. Complementary evolutionary analyses revealed the complex evolutionary history of atlas. The protein-coding portion of the gene likely arose at the base of the Drosophila genus on the X chromosome but was unlikely to be essential, as it was then lost in several independent lineages. Within the last ~15 million years, however, the gene moved to an autosome, where it fused with a conserved non-coding RNA and evolved a non-redundant role in male fertility. Altogether, this study provides insight into the integration of novel genes into biological processes, the links between genomic innovation and functional evolution, and the genetic control of a fundamental developmental process, gametogenesis.

Distinct spermiogenic phenotypes underlie sperm elimination in the Segregation Distorter meiotic drive system

Segregation Distorter (SD) is a male meiotic drive system in Drosophila melanogaster. Males heterozygous for a selfish SD chromosome rarely transmit the homologous SD+ chromosome. It is well established that distortion results from an interaction between Sd, the primary distorting locus on the SD chromosome and its target, a satellite DNA called Rsp, on the SD+ chromosome. However, the molecular and cellular mechanisms leading to post-meiotic SD+ sperm elimination remain unclear. Here we show that SD/SD+ males of different genotypes but with similarly strong degrees of distortion have distinct spermiogenic phenotypes. In some genotypes, SD+ spermatids fail to fully incorporate protamines after the removal of histones, and degenerate during the individualization stage of spermiogenesis. In contrast, in other SD/SD+ genotypes, protamine incorporation appears less disturbed, yet spermatid nuclei are abnormally compacted, and mature sperm nuclei are eventually released in the seminal vesicle. Our analyses of different SD+ chromosomes suggest that the severity of the spermiogenic defects associates with the copy number of the Rsp satellite. We propose that when Rsp copy number is very high (> 2000), spermatid nuclear compaction defects reach a threshold that triggers a checkpoint controlling sperm chromatin quality to eliminate abnormal spermatids during individualization.

RNA from a simple-tandem repeat is required for sperm maturation and male fertility in Drosophila melanogaster

Tandemly-repeated DNAs, or satellites, are enriched in heterochromatic regions of eukaryotic genomes and contribute to nuclear structure and function. Some satellites are transcribed, but we lack direct evidence that specific satellite RNAs are required for normal organismal functions. Here, we show satellite RNAs derived from AAGAG tandem repeats are transcribed in many cells throughout Drosophila melanogaster development, enriched in neurons and testes, often localized within heterochromatic regions, and important for viability. Strikingly, we find AAGAG transcripts are necessary for male fertility, and that AAGAG RNA depletion results in defective histone-protamine exchange, sperm maturation and chromatin organization. Since these events happen late in spermatogenesis when the transcripts are not detected, we speculate that AAGAG RNA in primary spermatocytes ‘primes’ post-meiosis steps for sperm maturation. In addition to demonstrating essential functions for AAGAG RNAs, comparisons between closely related Drosophila species suggest that satellites and their transcription evolve quickly to generate new functions.

Nejire/dCBP-mediated histone H3 acetylation during spermatogenesis is essential for male fertility in Drosophila melanogaster

Spermatogenesis in many species including Drosophila melanogaster is accompanied by major reorganisation of chromatin in post-meiotic stages, involving a nearly genome-wide displacement of histones by protamines, Mst77F and Protamine-like 99C. A proposed prerequisite for the histone-to-protamine transition is massive histone H4 hyper-acetylation prior to the switch. Here, we investigated the pattern of histone H3 lysine acetylation and general lysine crotonylation in D. melanogaster spermiogenesis to elucidate a possible role of these marks in chromatin reorganisation. Lysine crotonylation was strongest prior to remodelling and the deposition of this mark depended on the acetylation status of the spermatid chromatin. In contrast to H4 acetylation, individual H3 acetylation marks displayed surprisingly distinct patterns during the histone-to-protamine transition. We observed that Nejire, a histone acetyl transferase, is expressed during the time of histone-to-protamine transition. Nejire knock down led to strongly reduced fertility, which correlated with misshaped spermatid nuclei and a lack of mature sperm. protA and prtl99C transcript levels were reduced after knocking down Nejire. ProtB-eGFP, Mst77F-eGFP and Prtl99C-eGFP were synthesized at the late canoe stage, while histones were often not detectable. However, in some cysts histones persist in parallel to protamines. Therefore, we hypothesize that complete histone removal requires multiple histone modifications besides H3K18ac and H3K27ac. In summary, H3K18 and H3K27 acetylation during Drosophila spermatogenesis is dependent on Nejire or a yet uncharacterized acetyl transferase. We show that Nejire is required for male fertility since Nejire contributes to efficient transcription of protA and prtl99C, but not Mst77F, in spermatocytes, and to maturation of sperm.

Gordon Dixon, protamines, and the atypical patterns of gene expression in spermatogenic cells

Gordon Dixon's pioneering work on the replacement of histones by protamines during spermatogenesis inspired research as recombinant DNA became widely used to analyze gene expression in mammalian spermatogenic cells. The impact of recombinant DNA began immediately with the identification of mouse protamine 1 as a haploid-expressed mRNA, resolving a decades-long controversy whether gene expression in haploid spermatogenic cells distorts transmission of alleles to progeny. Numerous insights into the biology of spermatogenesis followed as the sequences of many mRNAs revealed that the patterns of gene expression in spermatogenic cells are astonishingly different from those in other cells in the mammalian body. Studies of these phenomena have generated fundamental insights across reproductive, molecular and evolutionary biology. Abbreviations: PRM1: protamine 1; PRM2: protamine 2; TCE: translation control element.

The Drosophila chromosomal protein Mst77F is processed to generate an essential component of mature sperm chromatin

In most animals, the bulk of sperm DNA is packaged with sperm nuclear basic proteins (SNBPs), a diverse group of highly basic chromosomal proteins notably comprising mammalian protamines. The replacement of histones with SNBPs during spermiogenesis allows sperm DNA to reach an extreme level of compaction, but little is known about how SNBPs actually function in vivo. Mst77F is a Drosophila SNBP with unique DNA condensation properties in vitro, but its role during spermiogenesis remains unclear. Here, we show that Mst77F is required for the compaction of sperm DNA and the production of mature sperm, through its cooperation with protamine-like proteins Mst35Ba/b. We demonstrate that Mst77F is incorporated in spermatid chromatin as a precursor protein, which is subsequently processed through the proteolysis of its N-terminus. The cleavage of Mst77F is very similar to the processing of protamine P2 during human spermiogenesis and notably leaves the cysteine residues in the mature protein intact, suggesting that they participate in the formation of disulfide cross-links. Despite the rapid evolution of SNBPs, sperm chromatin condensation thus involves remarkably convergent mechanisms in distantly related animals.

Analysis of Chromatin Dynamics During Drosophila Spermatogenesis

In the course of spermatogenesis, germ cells undergo dramatic morphological changes that affect almost all cellular components. Therefore, it is impossible to study the process of spermatogenesis in its entirety without detailed morphological analyses. Here, we describe a method to visualize chromatin dynamics in differentiating Drosophila male germ cells using immunofluorescence staining. In addition, we demonstrate how to treat Drosophila sperm before immunofluorescence staining to help reveal epitopes in the highly condensed sperm chromatin that otherwise may be inaccessible to antibodies.

Unlocking sperm chromatin at fertilization requires a dedicated egg thioredoxin in Drosophila

In most animals, the extreme compaction of sperm DNA is achieved after the massive replacement of histones with sperm nuclear basic proteins (SNBPs), such as protamines. In some species, the ultracompact sperm chromatin is stabilized by a network of disulfide bonds connecting cysteine residues present in SNBPs. Studies in mammals have established that the reduction of these disulfide crosslinks at fertilization is required for sperm nuclear decondensation and the formation of the male pronucleus. Here, we show that the Drosophila maternal thioredoxin Deadhead (DHD) is specifically required to unlock sperm chromatin at fertilization. In dhd mutant eggs, the sperm nucleus fails to decondense and the replacement of SNBPs with maternally-provided histones is severely delayed, thus preventing the participation of paternal chromosomes in embryo development. We demonstrate that DHD localizes to the sperm nucleus to reduce its disulfide targets and is then rapidly degraded after fertilization.