The maternal effect mutation sésame affects the formation of the male pronucleus in Drosophila melanogaster

HIRA and dPCIF1 coordinately establish totipotent chromatin and control orderly ZGA in Drosophila embryos

Early embryos undergo profound changes in their genomic architecture to establish the totipotent state, enabling pioneer factors to access chromatin and drive zygotic genome activation (ZGA). However, the mechanisms by which the totipotent state is established and properly interpreted by pioneer factors to allow orderly ZGA remain unknown. Here, we identify the H3.3-specific chaperone HIRA as a factor involving establishing totipotent-state chromatin in Drosophila early embryos. Through cophase separation with HIRA, the pioneer factor GAGA factor (GAF) efficiently binds to H3.3-marked nucleosomes to activate major-wave zygotic genes. Importantly, dPCIF1, a chromatin-associated protein, antagonized the GAF-HIRA interaction by competitively binding to HIRA, thereby restricting GAF on earlier chromatin and avoiding premature ZGA. Hence, the coordinated action of HIRA and dPCIF1 ensures sequential ZGA from the minor to major wave in early embryos. This study provides insights into understanding how a totipotent state is established and properly controlled during ZGA.

Local nuclear to cytoplasmic ratio regulates chaperone-dependent H3 variant incorporation during zygotic genome activation

Early embryos often have relatively unstructured chromatin that lacks active and inactive domains typical of differentiated cells. In many species, these regulatory domains are established during zygotic genome activation (ZGA). In Drosophila, ZGA occurs after 13 fast, reductive, syncytial nuclear divisions during which the nuclear to cytoplasmic (N/C) ratio grows exponentially. These divisions incorporate maternally-loaded, cytoplasmic pools of histones into chromatin. Previous work found that chromatin incorporation of replication-coupled histone H3 decreases while its variant H3.3 increases in the cell cycles leading up to ZGA. In other cell types, H3.3 is associated with sites of active transcription as well as heterochromatin, suggesting a link between H3.3 incorporation and ZGA. Here, we examine the factors that contribute to H3.3 incorporation at ZGA. We identify a more rapid decrease in the nuclear availability of H3 than H3.3 over the final pre-ZGA cycles. We also observe an N/C ratio-dependent increase in H3.3 incorporation in mutant embryos with non-uniform local N/C ratios. We find that chaperone binding, not gene expression, controls incorporation patterns using H3/H3.3 chimeric proteins at the endogenous H3.3A locus. We test the specificity of the H3.3 chaperone pathways for H3.3 incorporation using Hira (H3.3 chaperone) mutant embryos. Overall, we propose a model in which local N/C ratios and specific chaperone binding regulate differential incorporation of H3.3 during ZGA.

CircHIRA sponges miR-196b-5p to promote porcine early embryonic development

Circular RNA (circRNA) is abundantly expressed in preimplantation embryos and embryonic stem cells in mice and humans. However, its function and mechanism in early development of mammalian embryos remain unclear. Here, we showed that circHIRA mediated miR-196b-5p to regulate porcine early embryonic development. We verified the circular feature of circHIRA by sanger sequencing, and proved the authenticity of circHIRA by enzyme digestion test. HIRA and circHIRA were expressed in porcine early embryos, and its expression levels significantly increased from 8-cell stage onwards and reached the maximum at the blastocyst stage. Functional studies revealed that circHIRA knockdown not only significantly reduced the developmental efficiency of embryos from 8-cell stage to blastocyst stage, but also impaired the blastocyst quality. Mechanistically, integrated analysis of miRNA prediction and gene expression showed that circHIRA knockdown significantly increased the expression of miR-196b-5p in porcine early embryos. Furthermore, miR-196b-5p inhibitor injection could rescue the early development of circHIRA knockdown embryos. Taken together, the findings reveal that circHIRA regulates porcine early embryonic development via inhibiting the expression of miR-196b-5p.

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.

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.

Restriction of subapical proteins during cellularization depends on the onset of zygotic transcription and the formin Dia

Cortical domains are characterized by spatially restricted polarity proteins. The pattern of cortical domains is dynamic and changes during cell differentiation and development. Although there is a good understanding for how the cortical pattern is maintained, e. g. by mutual antagonism, less is known about how the initial pattern is established, and its dynamics coordinated with developmental progression. Here we investigate the initial restriction of subapical marker proteins during the syncytial-cellular transition in Drosophila embryos. The subapical markers Canoe/Afadin, the complex ELMO-Sponge, Baz and Arm become initially restricted between apical and lateral domains during cellularization. We define the role of zygotic genome activation as a timer for subapical domain formation. Subapical markers remained widely spread in embryos treated with α-amanitin and became precociously restricted in mutant embryos with premature zygotic transcription. In contrast, remodeling of the nuclear division cycle without cytokinesis to a full cell cycle is not a prerequisite for subapical domain formation, since we observed timely subapical restriction in embryos undergoing an extra nuclear cycle. We provide evidence that earliest subapical markers ELMO-Sponge and Canoe are required for subapical accumulation of Baz. Supporting an important role of cortical F-actin in subapical restriction, we found that the formin Dia was required for Baz restriction, and its distribution depended on the onset of zygotic gene expression. In summary, we define zygotic transcription as a timer, to which subapical markers respond in a dia-dependent mechanism.

CHD1 controls H3.3 incorporation in adult brain chromatin to maintain metabolic homeostasis and normal lifespan

The ATP-dependent chromatin remodeling factor CHD1 is essential for the assembly of variant histone H3.3 into paternal chromatin during sperm chromatin remodeling in fertilized eggs. It remains unclear, however, if CHD1 has a similar role in normal diploid cells. Using a specifically tailored quantitative mass spectrometry approach, we show that Chd1 disruption results in reduced H3.3 levels in heads of Chd1 mutant flies. Chd1 deletion perturbs brain chromatin structure in a similar way as H3.3 deletion and leads to global de-repression of transcription. The physiological consequences are reduced food intake, metabolic alterations, and shortened lifespan. Notably, brain-specific CHD1 expression rescues these phenotypes. We further demonstrate a strong genetic interaction between Chd1 and H3.3 chaperone Hira. Thus, our findings establish CHD1 as a factor required for the assembly of H3.3-containing chromatin in adult cells and suggest a crucial role for CHD1 in the brain as a regulator of organismal health and longevity.

Gametic specialization of centromeric histone paralogs in Drosophila virilis

In most eukaryotes, centromeric histone (CenH3) proteins mediate mitosis and meiosis and ensure epigenetic inheritance of centromere identity. We hypothesized that disparate chromatin environments in soma versus germline might impose divergent functional requirements on single CenH3 genes, which could be ameliorated by gene duplications and subsequent specialization. Here, we analyzed the cytological localization of two recently identified CenH3 paralogs, Cid1 and Cid5, in Drosophila virilis using specific antibodies and epitope-tagged transgenic strains. We find that only ancestral Cid1 is present in somatic cells, whereas both Cid1 and Cid5 are expressed in testes and ovaries. However, Cid1 is lost in male meiosis but retained throughout oogenesis, whereas Cid5 is lost during female meiosis but retained in mature sperm. Following fertilization, only Cid1 is detectable in the early embryo, suggesting that maternally deposited Cid1 is rapidly loaded onto paternal centromeres during the protamine-to-histone transition. Our studies reveal mutually exclusive gametic specialization of divergent CenH3 paralogs. Duplication and divergence might allow essential centromeric genes to resolve an intralocus conflict between maternal and paternal centromeric requirements in many animal species.

The nuclear to cytoplasmic ratio directly regulates zygotic transcription in Drosophila through multiple modalities

Early embryos must rapidly generate large numbers of cells to form an organism. Many species accomplish this through a series of rapid, reductive, and transcriptionally silent cleavage divisions. Previous work has demonstrated that the number of divisions before both cell cycle elongation and zygotic genome activation (ZGA) is regulated by the ratio of nuclear content to cytoplasm (N/C). To understand how the N/C ratio affects the timing of ZGA, we directly assayed the behavior of several previously identified N/C ratio-dependent genes using the MS2-MCP reporter system in living Drosophila embryos with altered ploidy and cell cycle durations. For every gene that we examined, we found that nascent RNA output per cycle is delayed in haploid embryos. Moreover, we found that the N/C ratio influences transcription through three overlapping modes of action. For some genes (knirps, fushi tarazu, and snail), the effect of ploidy can be primarily attributed to changes in cell cycle duration. However, additional N/C ratio-mediated mechanisms contribute significantly to transcription delays for other genes. For giant and bottleneck, the kinetics of transcription activation are significantly disrupted in haploids, while for frühstart and Krüppel, the N/C ratio controls the probability of transcription initiation. Our data demonstrate that the regulatory elements of N/C ratio-dependent genes respond directly to the N/C ratio through multiple modes of regulation.

The Enigma of Centriole Loss in the 1182-4 Cell Line

The Drosophila melanogaster cell line 1182-4, which constitutively lacks centrioles, was established many years ago from haploid embryos laid by females homozygous for the maternal haploid (mh) mutation. This was the first clear example of animal cells regularly dividing in the absence of this organelle. However, the cause of the acentriolar nature of the 1182-4 cell line remained unclear and could not be clearly assigned to a particular genetic event. Here, we detail historically the longstanding mystery of the lack of centrioles in this Drosophila cell line. Recent advances, such as the characterization of the mh gene and the genomic analysis of 1182-4 cells, allow now a better understanding of the physiology of these cells. By combining these new data, we propose three reasonable hypotheses of the genesis of this remarkable phenotype.

From mother to embryo: A molecular perspective on zygotic genome activation

In animals, the early embryo is mostly transcriptionally silent and development is fueled by maternally supplied mRNAs and proteins. These maternal products are important not only for survival, but also to gear up the zygote's genome for activation. Over the last three decades, research with different model organisms and experimental approaches has identified molecular factors and proposed mechanisms for how the embryo transitions from being transcriptionally silent to transcriptionally competent. In this chapter, we discuss the molecular players that shape the molecular landscape of ZGA and provide insights into their mode of action in activating the transcription program in the developing embryo.

Genetic Variation and Potential for Resistance Development to the tTA Overexpression Lethal System in Insects

Release of insect pests carrying the dominant lethal tetracycline transactivator (tTA) overexpression system has been proposed as a means for population suppression. High levels of the tTA transcription factor are thought to be toxic due to either transcriptional squelching or interference with protein ubiquitination. Here we utilized the Drosophila melanogaster Genetic Reference Panel (DGRP) to examine the influence of genetic variation on the efficacy of a female-specific tTA overexpression system. The level of female lethality between DGRP lines varied from 11 to 97% with a broad sense heritability of 0.89. A genome-wide association analysis identified 192 allelic variants associated with high or low lethality (P < 10^-5), although none were significant when corrected for multiple testing. 151 of the variants fell within 108 genes that were associated with several biological processes including transcription and protein ubiquitination. In four lines with high female lethality, tTA RNA levels were similar or higher than in the parental tTA overexpression strain. In two lines with low lethality, tTA levels were about two fold lower than in the parental strain. However, in two other lines with low lethality, tTA levels were similar or approximately 30% lower. RNAseq analysis identified genes that were up or downregulated in the four low female lethal lines compared to the four high lethal lines. For example, genes associated with RNA processing and rRNA maturation were significantly upregulated in low lethal lines. Our data suggest that standing genetic variation in an insect population could provide multiple mechanisms for resistance to the tTA overexpression system.

A Link between Deoxyribonucleotide Metabolites and Embryonic Cell-Cycle Control

The egg contains maternal RNAs and proteins, which have instrumental functions in patterning and morphogenesis. Besides these, the egg also contains metabolites, whose developmental functions have been little investigated. For example, the rapid increase of DNA content during the fast embryonic cell cycles poses high demands on the supply of deoxyribonucleotides (dNTPs), which may be synthesized in the embryo or maternally provided [1, 2]. Here, we analyze the role of dNTP in early Drosophila embryos. We found that dNTP levels initially decreased about 2-fold before reaching stable levels at the transition from syncytial to cellular blastoderm. Employing a mutant of the metabolic enzyme serine hydroxymethyl transferase (SHMT), which is impaired in the embryonic synthesis of deoxythymidine triphosphate (dTTP), we found that the maternal supply of dTTP was specifically depleted by interphase 13. SHMT mutants showed persistent S phase, replication stress, and a checkpoint-dependent cell-cycle arrest in NC13, depending on the loss of dTTP. The cell-cycle arrest in SHMT mutants was suppressed by reduced zygotic transcription. Consistent with the requirement of dTTP for cell-cycle progression, increased dNTP levels accelerated the cell cycle in embryos lacking zygotic transcription. We propose a model that both a limiting dNTP supply and interference of zygotic transcription with DNA replication [3] elicit DNA replication stress and checkpoint activation. Our study reveals a specific mechanism of how dNTP metabolites contribute to the embryonic cell-cycle control.

ASF1 is required to load histones on the HIRA complex in preparation of paternal chromatin assembly at fertilization

Background

Anti-Silencing Factor 1 (ASF1) is a conserved H3–H4 histone chaperone involved in both Replication-Coupled and Replication-Independent (RI) nucleosome assembly pathways. At DNA replication forks, ASF1 plays an important role in regulating the supply of H3.1/2 and H4 to the CAF-1 chromatin assembly complex. ASF1 also provides H3.3–H4 dimers to HIRA and DAXX chaperones for RI nucleosome assembly. The early Drosophila embryo is an attractive system to study chromatin assembly in a developmental context. The formation of a diploid zygote begins with the unique, genome-wide RI assembly of paternal chromatin following sperm protamine eviction. Then, within the same cytoplasm, syncytial embryonic nuclei undergo a series of rapid, synchronous S and M phases to form the blastoderm embryo. Here, we have investigated the implication of ASF1 in these two distinct assembly processes.

Results

We show that depletion of the maternal pool of ASF1 with a specific shRNA induces a fully penetrant, maternal effect embryo lethal phenotype. Unexpectedly, despite the depletion of ASF1 protein to undetectable levels, we show that asf1 knocked-down (KD) embryos can develop to various stages, thus demonstrating that ASF1 is not absolutely required for the amplification of cleavage nuclei. Remarkably, we found that ASF1 is required for the formation of the male pronucleus, although ASF1 protein does not reside in the decondensing sperm nucleus. In asf1 KD embryos, HIRA localizes to the male nucleus but is only capable of limited and insufficient chromatin assembly. Finally, we show that the conserved HIRA B domain, which is involved in ASF1-HIRA interaction, is dispensable for female fertility.

Conclusions

We conclude that ASF1 is critically required to load H3.3–H4 dimers on the HIRA complex prior to histone deposition on paternal DNA. This separation of tasks could optimize the rapid assembly of paternal chromatin within the gigantic volume of the egg cell. In contrast, ASF1 is surprisingly dispensable for the amplification of cleavage nuclei, although chromatin integrity is likely compromised in KD embryos.

Electronic supplementary material

The online version of this article (10.1186/s13072-018-0189-x) contains supplementary material, which is available to authorized users.

Sperm morphology and the evolution of intracellular sperm-egg interactions

Sperm morphology is incredibly diverse, even among closely related species, yet the coevolution between males and females of fertilization recognition systems is necessary for successful karyogamy (male and female pronuclear fusion). In most species, the entire sperm enters the egg during fertilization so sperm morphological diversity may impact the intracellular sperm-egg interactions necessary for karyogamy. We quantified morphological variation of sperm inside eggs prior to and following karyogamy in several species of Drosophila to understand whether evolution of sperm morphology could influence intracellular sperm-egg interactions (ISEIs). We measured seven parameters that describe ISEIs among species to determine whether these parameters varied both within a species across development and across species at the same developmental stage. We used heterospecific crosses to test the relative role of male origin, female origin, and interaction between the male and female in determining ISEIs. We found that sperm shape changed within a species as development proceeded and, at particular development stages, species varied in some ISEIs. Parental origin had an effect on some ISEIs, with a general trend for a stronger female effect. Overall, our findings identify conserved and variable ISEIs among species and demonstrate the potential to contribute understanding to gamete evolution and development.

Establishment and maintenance of heritable chromatin structure during early Drosophila embryogenesis

During embryogenesis, the initial chromatin state is established during a period of rapid proliferative activity. We have measured with 3-min time resolution how heritable patterns of chromatin structure are initially established and maintained during the midblastula transition (MBT). We find that regions of accessibility are established sequentially, where enhancers are opened in advance of promoters and insulators. These open states are stably maintained in highly condensed mitotic chromatin to ensure faithful inheritance of prior accessibility status across cell divisions. The temporal progression of establishment is controlled by the biological timers that control the onset of the MBT. In general, acquisition of promoter accessibility is controlled by the biological timer that measures the nucleo-cytoplasmic (N:C) ratio, whereas timing of enhancer accessibility is regulated independently of the N:C ratio. These different timing classes each associate with binding sites for two transcription factors, GAGA-factor and Zelda, previously implicated in controlling chromatin accessibility at ZGA.

DOI:http://dx.doi.org/10.7554/eLife.20148.001

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.

The intimate genetics of Drosophila fertilization

The union of haploid gametes at fertilization initiates the formation of the diploid zygote in sexually reproducing animals. This founding event of embryogenesis includes several fascinating cellular and nuclear processes, such as sperm-egg cellular interactions, sperm chromatin remodelling, centrosome formation or pronuclear migration. In comparison with other aspects of development, the exploration of animal fertilization at the functional level has remained so far relatively limited, even in classical model organisms. Here, we have reviewed our current knowledge of fertilization in Drosophila melanogaster, with a special emphasis on the genes involved in the complex transformation of the fertilizing sperm nucleus into a replicated set of paternal chromosomes.

Mitotic fidelity requires transgenerational action of a testis-restricted HP1

Sperm-packaged DNA must undergo extensive reorganization to ensure its timely participation in embryonic mitosis. Whereas maternal control over this remodeling is well described, paternal contributions are virtually unknown. In this study, we show that Drosophila melanogaster males lacking Heterochromatin Protein 1E (HP1E) sire inviable embryos that undergo catastrophic mitosis. In these embryos, the paternal genome fails to condense and resolve into sister chromatids in synchrony with the maternal genome. This delay leads to a failure of paternal chromosomes, particularly the heterochromatin-rich sex chromosomes, to separate on the first mitotic spindle. Remarkably, HP1E is not inherited on mature sperm chromatin. Instead, HP1E primes paternal chromosomes during spermatogenesis to ensure faithful segregation post-fertilization. This transgenerational effect suggests that maternal control is necessary but not sufficient for transforming sperm DNA into a mitotically competent pronucleus. Instead, paternal action during spermiogenesis exerts post-fertilization control to ensure faithful chromosome segregation in the embryo.

DOI:http://dx.doi.org/10.7554/eLife.07378.001

The genetic information of cells is packaged into structures called chromosomes, which are made up of long strands of DNA that are wrapped around proteins to form a structure called chromatin. The cells of most animals contain two copies of every chromosome, but the egg and sperm cells contain only one copy. This means that when an egg fuses with a sperm cell during fertilization, the resulting ‘zygote’ will contain two copies of each chromosome—one inherited from the mother, and one from the father. These chromosomes duplicate and divide many times within the developing embryo in a process known as mitosis.

The first division of the zygote is particularly complicated, as the egg and sperm chromosomes must go through extensive—and yet different—chromatin reorganization processes. For instance, paternal DNA is inherited via sperm, where specialized sperm proteins package the DNA more tightly than in the maternal DNA, which is packaged by histone proteins used throughout development. For paternal DNA to participate in mitosis in the embryo, it must first undergo a transition to a histone-packaged state. Despite these differences, both maternal and paternal chromosomes must undergo mitosis at the same time if the zygote is to successfully divide. Although it is known that the egg cell contributes essential proteins that are incorporated into the sperm chromatin to help it reorganize, the importance of paternal proteins in coordinating this process remains poorly understood.

Many members of a family of proteins called Heterochromatin Protein 1 (or HP1 for short) have previously been shown to control chromatin organization in plants and animals. In 2012, researchers found that several HP1 proteins are found only in the testes of the fruit fly species Drosophila melanogaster. They predicted that these proteins might help control the reorganization of the paternal chromosomes following fertilization.

Levine et al.—including researchers involved in the 2012 study—have now used genetic and cell-based techniques to show that one member of the HP1 family (called HP1E) ensures that the paternal chromosomes are ready for cell division at the same time as the maternal chromosomes. Male flies that are unable to produce this protein do not have any offspring because, while these flies' sperm can fertilize eggs, the resulting zygotes cannot divide as normal.

Further experiments revealed that HP1E is not inherited through the chromatin of mature sperm, but instead influences the structure of the chromosomes during the final stages of the development of the sperm cells in the fly testes.

This study shows that both maternal and paternal proteins are needed to control how the paternal chromosomes reorganize in fruit fly embryos. Although difficult to discover and decipher, this work re-emphasizes the importance of paternal epigenetic contributions—changes that alter how DNA is read, without changing the DNA sequence itself—for ensuring the viability of resulting offspring. Future work will reveal both the molecular mechanism of this epigenetic transfer of information, as well as why certain Drosophila species are able to naturally overcome the loss of the essential HP1E protein.

DOI:http://dx.doi.org/10.7554/eLife.07378.002

Histone storage and deposition in the early Drosophila embryo

Drosophila development initiates with the formation of a diploid zygote followed by the rapid division of embryonic nuclei. This syncytial phase of development occurs almost entirely under maternal control and ends when the blastoderm embryo cellularizes and activates its zygotic genome. The biosynthesis and storage of histones in quantity sufficient for chromatin assembly of several thousands of genome copies represent a unique challenge for the developing embryo. In this article, we have reviewed our current understanding of the mechanisms involved in the production, storage, and deposition of histones in the fertilized egg and during the exponential amplification of cleavage nuclei.

Modeling of the dorsal gradient across species reveals interaction between embryo morphology and Toll signaling pathway during evolution

Morphogenetic gradients are essential to allocate cell fates in embryos of varying sizes within and across closely related species. We previously showed that the maternal NF-κB/Dorsal (Dl) gradient has acquired different shapes in Drosophila species, which result in unequally scaled germ layers along the dorso-ventral axis and the repositioning of the neuroectodermal borders. Here we combined experimentation and mathematical modeling to investigate which factors might have contributed to the fast evolutionary changes of this gradient. To this end, we modified a previously developed model that employs differential equations of the main biochemical interactions of the Toll (Tl) signaling pathway, which regulates Dl nuclear transport. The original model simulations fit well the D. melanogaster wild type, but not mutant conditions. To broaden the applicability of this model and probe evolutionary changes in gradient distributions, we adjusted a set of 19 independent parameters to reproduce three quantified experimental conditions (i.e. Dl levels lowered, nuclear size and density increased or decreased). We next searched for the most relevant parameters that reproduce the species-specific Dl gradients. We show that adjusting parameters relative to morphological traits (i.e. embryo diameter, nuclear size and density) alone is not sufficient to reproduce the species Dl gradients. Since components of the Tl pathway simulated by the model are fast-evolving, we next asked which parameters related to Tl would most effectively reproduce these gradients and identified a particular subset. A sensitivity analysis reveals the existence of nonlinear interactions between the two fast-evolving traits tested above, namely the embryonic morphological changes and Tl pathway components. Our modeling further suggests that distinct Dl gradient shapes observed in closely related melanogaster sub-group lineages may be caused by similar sequence modifications in Tl pathway components, which are in agreement with their phylogenetic relationships.

Embryo size can vary greatly among closely related species. How tissue specification either scales or is modified in the developing embryo in different species is an ongoing investigation in developmental biology. Here we asked how embryo morphology and specific molecular pathways influence tissue specification by altering the distribution of morphogens. Morphogens are molecules that form gradients that regulate gene expression patterns in a dosage-dependent fashion that result in tissue specification, and therefore are a prime target for evolution in order to adjust or maintain tissue proportions in relation to overall embryo size. We used a mathematical model to identify factors that influence the distribution of the Dorsal morphogen gradient that is responsible for patterning the dorsal-ventral axis of the Drosophila fruit fly embryo. We obtained experimental data from mutant conditions and different species of Drosophila to calibrate our model and found an interaction between embryo morphology and regulation of the Toll pathway, which regulates the Dorsal gradient. Furthermore, the model predicts that closely related species share similar modifications in Toll pathway components resulting in their species-specific gradient shapes, which are supported by interspecies amino acid comparison of the components Dorsal and Cactus.

Wolbachia-induced paternal defect in Drosophila is likely by interaction with the juvenile hormone pathway

Wolbachia are endosymbionts that infect many insect species. They can manipulate the host's reproduction to increase their own maternal transmission. Cytoplasmic incompatibility (CI) is one such manipulation, which is expressed as embryonic lethality when Wolbachia-infected males mate with uninfected females. However, matings between males and females carrying the same Wolbachia strain result in viable progeny. The molecular mechanisms of CI are currently not clear. We have previously reported that the gene Juvenile hormone-inducible protein 26 (JhI-26) exhibited the highest upregulation in the 3rd instar larval testes of Drosophila melanogaster when infected by Wolbachia. This is reminiscent of an interaction between Wolbachia and juvenile hormone (JH) pathway in flies. Considering that Jhamt gene encodes JH acid methyltransferase, a key regulatory enzyme of JH biosynthesis, and that methoprene-tolerant (Met) has been regarded as the best JH receptor candidate, we first compared the expression of Jhamt and Met between Wolbachia-infected and uninfected fly testes to investigate whether Wolbachia infection influence the JH signaling pathway. We found that the expressions of Jhamt and Met were significantly increased in the presence of Wolbachia, suggesting an interaction of Wolbachia with the JH signaling pathway. Then, we found that overexpression of JhI-26 in Wolbachia-free transgenic male flies caused paternal-effect lethality that mimics the defects associated with CI. JhI-26 overexpressing males resulted in significantly decrease in hatch rate. Surprisingly, Wolbachia-infected females could rescue the egg hatch. In addition, we showed that overexpression of JhI-26 caused upregulation of the male accessory gland protein (Acp) gene CG10433, but not vice versa. This result suggests that JhI-26 may function at the upstream of CG10433. Likewise, overexpression of CG10433 also resulted in paternal-effect lethality. Both JhI-26 and CG10433 overexpressing males resulted in nuclear division defects in the early embryos. Finally, we found that Wolbachia-infected males decreased the propensity of the mated females to remating, a phenotype also caused by both JhI-26 and CG10433 overexpressing males. Taken together, our results provide a working hypothesis whereby Wolbachia induce paternal defects in Drosophila probably by interaction with the JH pathway via JH response genes JhI-26 and CG10433.

H3.3 turnover: a mechanism to poise chromatin for transcription, or a response to open chromatin?

Histone H3.3 turnover displays distinct dynamics at various genomic elements such as promoters, enhancers, gene bodies, and heterochromatic regions, suggesting that it is differentially regulated according to chromatin context. Incorporation of variant histones into chromatin provides a mechanism to modulate chromatin states in addition to histone modifications. The replication-independent deposition and replacement of histone variant H3.3, i.e. H3.3 turnover, is mainly associated with transcriptional activity. H3.3 or H3.3-like histone turnover has been studied in various organisms from yeast to mammals. Here, we review the recent progress on this topic. The diversified turnover profiles of H3.3, and their corresponding underlying mechanisms, may reflect distinct requirements for chromatin accessibility in different biological events.

HIRA is essential for the development of gibel carp

HIRA is one of the chaperones of histone H3.3. Mutation of Hira results in embryonic lethality in mice, suggesting a critical role in embryogenesis. However, Hira-mutated Drosophila may survive to adults, indicating that it is dispensable in Drosophila development. The role of Hira in fish development is unknown. In this study we first investigated the expression of Hira during embryogenesis of gibel carp (Carassius auratus gibelio) by whole-mount in situ hybridization. We found that Hira signal appeared ubiquitously in the early embryos. After gastrulation, it appeared mainly along the anterior-posterior axis, including the tail bud. In hatching period, the signal was detected in head, heart, and the endoderm region on the back of yolk. Then by microinjection with morpholino-HIRA at the beginning of development, we observed delayed gastrulation and abnormal somitogenesis in gibel carp embryos. The HIRA morphants exhibited short trunk, limited yolk extension, and twisted tail. Most of the mutants died during embryogenesis or shortly after hatching. The rest of the HIRA morphants could survive to larvae but with severe defects in organogenesis. These data suggest that HIRA may be essential for the development of gibel carp, and this function is conserved in vertebrates.

Variation in the dorsal gradient distribution is a source for modified scaling of germ layers in Drosophila

Specification of germ layers along the dorsoventral axis by morphogenetic gradients is an ideal model to study scaling properties of gradients and cell fate changes during evolution. Classical anatomical studies in divergent insects (e.g., flies and grasshoppers) revealed that the neuroectodermal size is conserved and originates similar numbers of neuroblasts of homologous identity. In contrast, mesodermal domains vary significantly in closely related Drosophila species. To further investigate the underlying mechanisms of scaling of germ layers across Drosophila species, we quantified the Dorsal (Dl)/NF-κB gradient, the main morphogenetic gradient that initiates separation of the mesoderm, neuroectoderm, and ectoderm. We discovered a variable range of Toll activation across species and found that Dl activates mesodermal genes at the same threshold levels in melanogaster sibling species. We also show that the Dl gradient distribution can be modulated by nuclear size and packing densities. We propose that variation in mesodermal size occurs at a fast evolutionary rate and is an important mechanism to define the ventral boundary of the neuroectoderm.

Drosophila Yemanuclein and HIRA cooperate for de novo assembly of H3.3-containing nucleosomes in the male pronucleus

The differentiation of post-meiotic spermatids in animals is characterized by a unique reorganization of their nuclear architecture and chromatin composition. In many species, the formation of sperm nuclei involves the massive replacement of nucleosomes with protamines, followed by a phase of extreme nuclear compaction. At fertilization, the reconstitution of a nucleosome-based paternal chromatin after the removal of protamines requires the deposition of maternally provided histones before the first round of DNA replication. This process exclusively uses the histone H3 variant H3.3 and constitutes a unique case of genome-wide replication-independent (RI) de novo chromatin assembly. We had previously shown that the histone H3.3 chaperone HIRA plays a central role for paternal chromatin assembly in Drosophila. Although several conserved HIRA-interacting proteins have been identified from yeast to human, their conservation in Drosophila, as well as their actual implication in this highly peculiar RI nucleosome assembly process, is an open question. Here, we show that Yemanuclein (YEM), the Drosophila member of the Hpc2/Ubinuclein family, is essential for histone deposition in the male pronucleus. yem loss of function alleles affect male pronucleus formation in a way remarkably similar to Hira mutants and abolish RI paternal chromatin assembly. In addition, we demonstrate that HIRA and YEM proteins interact and are mutually dependent for their targeting to the decondensing male pronucleus. Finally, we show that the alternative ATRX/XNP-dependent H3.3 deposition pathway is not involved in paternal chromatin assembly, thus underlining the specific implication of the HIRA/YEM complex for this essential step of zygote formation.

Chromosome organization relies on a basic functional unit called the nucleosome, in which DNA is wrapped around a core of histone proteins. However, during male gamete formation, the majority of histones are replaced by sperm-specific proteins that are adapted to sexual reproduction but incompatible with the formation of the first zygotic nucleus. These proteins must therefore be replaced by histones upon fertilization, in a replication-independent chromatin assembly process that requires the histone deposition factor HIRA. In this study, we identified the protein Yemanuclein (YEM) as a new partner of HIRA at fertilization. We show that, in eggs laid by yem mutant females, the male pronucleus fails to assemble its nucleosomes, resulting in the loss of paternal chromosomes at the first zygotic division. In addition, we found that YEM and HIRA are mutually dependent to perform chromatin assembly at fertilization, demonstrating that they tightly cooperate in vivo. Finally, we demonstrate that the replication-independent chromatin assembly factor ATRX/XNP is not involved in the assembly of paternal nucleosomes. In conclusion, our results shed new light into critical mechanisms controlling paternal chromosome formation at fertilization.

Mechanism and regulation of Cdc25/Twine protein destruction in embryonic cell-cycle remodeling

BACKGROUND

In Drosophila embryos, the midblastula transition (MBT) dramatically remodels the cell cycle during the 14(th) interphase. Before the MBT, each cycle is composed of only a short S phase and mitosis. At the MBT, S phase is dramatically lengthened by the onset of late replication, and a G2 phase is introduced. Both changes set the stage for gastrulation and require downregulation of Cdc25 phosphatase, which was previously attributed to the elimination of its transcripts at the MBT.

RESULTS

Premature removal of cdc25 transcripts by RNAi did not affect progression to the MBT. Instead, an antibody against the Cdc25 isoform Twine showed that Twine protein was abundant and stable until the MBT, when it was destabilized and rapidly eliminated. Persistence of pre-MBT levels of Twine was sufficient to prevent cell-cycle slowing. Twine protein destruction was timed by the nucleocytoplasmic ratio and depended on the activation of zygotic transcription at the MBT, including expression of the gene tribbles, whose activity was sufficient to trigger Twine destruction and was required for prompt Twine disappearance.

CONCLUSIONS

We propose that the developmentally regulated destruction of Twine protein is a critical switch that contributes to the cell-cycle change at the MBT, including the addition of a G2 phase and onset of late replication. Moreover, we show that this destruction is triggered by the nucleocytoplasmic ratio-dependent onset of zygotic transcription of tribbles and other unknown genes.

Posttranslational control of Cdc25 degradation terminates Drosophila's early cell-cycle program

In most metazoans, early embryonic development is characterized by rapid mitotic divisions that are controlled by maternal mRNAs and proteins that accumulate during oogenesis. These rapid divisions pause at the midblastula transition (MBT), coinciding with a dramatic increase in gene transcription and the degradation of a subset of maternal mRNAs. In Drosophila, the cell-cycle pause is controlled by inhibitory phosphorylation of Cdk1, which in turn is driven by downregulation of the activating Cdc25 phosphatases. Here, we show that the two Drosophila Cdc25 homologs, String and Twine, differ in their dynamics and that, contrary to current models, their downregulations are not controlled by mRNA degradation but through different posttranslational mechanisms. The degradation rate of String protein gradually increases during the late syncytial cycles in a manner dependent on the nuclear-to-cytoplasmic ratio and on the DNA replication checkpoints. Twine, on the other hand, is targeted for degradation at the onset of the MBT through a switch-like mechanism controlled, like String, by the nuclear-to-cytoplasmic ratio, but not requiring the DNA replication checkpoints. We demonstrate that posttranslational control of Twine degradation ensures that the proper number of mitoses precede the MBT.

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

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

The role of HIRA and maternal histones in sperm nucleus decondensation in the gibel carp and color crucian carp

The histone H3.3 chaperone HIRA is essential for chromatin assembly during male pronucleus formation in Drosophila. However, the role of HIRA during fertilization in vertebrates remains unclear. The gibel carp (Carassius auratus gibelio) is a unique gynogenetic crucian carp (gyno-carp). Heterologous sperm nuclei cannot decondense when incorporated in the egg, thus the eggs produce a clonal lineage of all females by typical gynogenesis. In contrast, after entering the egg, homologous sperm can undergo decondensation and sexual reproduction is activated, which may produce both female and male offspring. Therefore, this fish is a useful model for studying the mechanisms of fertilization. Herein, we first compared HIRA expression during embryogenesis between gyno-carp and the gonochoristic color crucian carp (Carassius auratus; gono-carp). In gono-carp, a dramatic reduction of HIRA protein occurs shortly after fertilization, whereas HIRA protein is consistently expressed during embryogenesis of gyno-carp. Next, we used immunodepletion and an in vitro sperm decondensation system, and found that complete removal of HIRA inhibited sperm decondensation in both of the fish. Immunofluorescence localization showed that in the condensed sperm nuclei of gono-carp incubated in gyno-carp egg extracts, HIRA was detected, but neither the histone H2A variant H2af1o nor acetylated histone H4 was observed. These results suggest that HIRA may be a critical factor required for sperm nucleus decondensation, while the defect in deposition of some maternal histones in the sperm nucleus could be one reason why heterologous sperm cannot decondense in the gibel carp egg.

The double face of the histone variant H3.3

Histone proteins wrap DNA to form nucleosome particles that compact eukaryotic genomes while still allowing access for cellular processes such as transcription, replication and DNA repair. Histones exist as different variants that have evolved crucial roles in specialized functions in addition to their fundamental role in packaging DNA. H3.3--a conserved histone variant that is structurally very close to the canonical histone H3--has been associated with active transcription. Furthermore, its role in histone replacement at active genes and promoters is highly conserved and has been proposed to participate in the epigenetic transmission of active chromatin states. Unexpectedly, recent data have revealed accumulation of this specific variant at silent loci in pericentric heterochromatin and telomeres, raising questions concerning the actual function of H3.3. In this review, we describe the known properties of H3.3 and the current view concerning its incorporation modes involving particular histone chaperones. Finally, we discuss the functional significance of the use of this H3 variant, in particular during germline formation and early development in different species.

[The function of histone chaperones during development]

Histone chaperones may participate the decondensation and assembly of chromatins, thus regulate gene expression. They play important roles in almost all developmental processes, such as gametogenesis, fertilization, embryogenesis, growth and senescence. In this review, we used well studied examples to illustrate various functions of histone chaperones during developmental processes. Focus is given to nucleoplasmin, CAF-1, HIRA, ASF1/CIA, and NAP1.

Identical sequences but different expression patterns of Hira gene in gynogenetic and gonochoristic crucian carps

Hir/Hira (histone regulation) genes were first identified in yeast as negative regulators of histone gene expression. It has been confirmed that HIRA is a conserved family of proteins present in various animals and plants. In this paper, the cDNAs of the Hira homolog named CagHira and CaHira were isolated from gynogenetic gibel carp (gyno-carp) and gonochoristic color crucian carp (gono-carp) respectively. The full-length CagHira is 3,860 bp in length with an open reading frame (ORF) of 3,033 bp that encodes 1,011 amino acids, while the full-length CaHira is 3,748 bp in length and also has an ORF of 3,033 bp. The deduced amino acid sequences of both Hira homologs contain seven WD domains and show high identity with other HIRA family members. RT-PCR analyses revealed strong expression of Hira in the ovaries, whereas no expression was detected in the testes of either of the fishes. Hira transcription was not detected in the liver of gyno-carp, but a high level of Hira mRNA was observed in gono-carp. The temporal expression pattern showed that the Hira mRNA is consistently expressed during all embryonic development stages in gyno-carp. However, the abundance of CaHira mRNA significantly decreased (P < 0.05) shortly after fertilization and then increased again and remained stable from gastrula till hatching. The varying spatiotemporal expression patterns of Hira genes in gyno-carp and gono-carp may be associated with the differing reproductive modes used by these two closely related fishes. Our results suggest that Hira may play a role not only in the decondensation of sperm nucleus and the formation of pronucleus during fertilization, but also in gastrulation and the subsequent development of embryos.

Clothing up DNA for all seasons: Histone chaperones and nucleosome assembly pathways

In eukaryotes, the packaging of DNA into chromatin is essential for cell viability. Several important DNA metabolic events require the transient disruption of chromatin structure, but cells have evolved a number of elaborate pathways that operate throughout the cell cycle to prevent the deleterious effects of chromatin erosion. In this review, we describe a number of distinct nucleosome assembly pathways that function during DNA replication, transcription, cellular senescence and early embryogenesis. In addition, we illustrate some of the physiological consequences associated with defects in nucleosome assembly pathways.

Indirect effects of ploidy suggest X chromosome dose, not the X:A ratio, signals sex in Drosophila

In the textbook view, the ratio of X chromosomes to autosome sets, X:A, is the primary signal specifying sexual fate in Drosophila. An alternative idea is that X chromosome number signals sex through the direct actions of several X-encoded signal element (XSE) proteins. In this alternative, the influence of autosome dose on X chromosome counting is largely indirect. Haploids (1X;1A), which possess the male number of X chromosomes but the female X:A of 1.0, and triploid intersexes (XX;AAA), which possess a female dose of two X chromosomes and the ambiguous X:A ratio of 0.67, represent critical tests of these hypotheses. To directly address the effects of ploidy in primary sex determination, we compared the responses of the signal target, the female-specific SxlPe promoter of the switch gene Sex-lethal, in haploid, diploid, and triploid embryos. We found that haploids activate SxlPe because an extra precellular nuclear division elevates total X chromosome numbers and XSE levels beyond those in diploid males. Conversely, triploid embryos cellularize one cycle earlier than diploids, causing premature cessation of SxlPe expression. This prevents XX;AAA embryos from fully engaging the autoregulatory mechanism that maintains subsequent Sxl expression, causing them to develop as sexual mosaics. We conclude that the X:A ratio predicts sexual fate, but does not actively specify it. Instead, the instructive X chromosome signal is more appropriately seen as collective XSE dose in the early embryo. Our findings reiterate that correlations between X:A ratios and cell fates in other organisms need not implicate the value of the ratio as an active signal.

The essential role of Drosophila HIRA for de novo assembly of paternal chromatin at fertilization

In many animal species, the sperm DNA is packaged with male germ line–specific chromosomal proteins, including protamines. At fertilization, these non-histone proteins are removed from the decondensing sperm nucleus and replaced with maternally provided histones to form the DNA replication competent male pronucleus. By studying a point mutant allele of the Drosophila Hira gene, we previously showed that HIRA, a conserved replication-independent chromatin assembly factor, was essential for the assembly of paternal chromatin at fertilization. HIRA permits the specific assembly of nucleosomes containing the histone H3.3 variant on the decondensing male pronucleus. We report here the analysis of a new mutant allele of Drosophila Hira that was generated by homologous recombination. Surprisingly, phenotypic analysis of this loss of function allele revealed that the only essential function of HIRA is the assembly of paternal chromatin during male pronucleus formation. This HIRA-dependent assembly of H3.3 nucleosomes on paternal DNA does not require the histone chaperone ASF1. Moreover, analysis of this mutant established that protamines are correctly removed at fertilization in the absence of HIRA, thus demonstrating that protamine removal and histone deposition are two functionally distinct processes. Finally, we showed that H3.3 deposition is apparently not affected in Hira mutant embryos and adults, suggesting that different chromatin assembly machineries could deposit this histone variant.

Chromatin is composed of basic units called nucleosomes, in which DNA wraps around a core of histone proteins. HIRA is a histone chaperone that is specifically involved in the assembly of nucleosomes containing H3.3, a universally conserved type of histone 3. To understand the function of HIRA in vivo, the authors generated mutant fruit flies with a non-functional Hira gene. Surprisingly, mutant flies were viable, but females were completely sterile. By analysing the female fruit flies' eggs, the authors found that in the absence of HIRA protein, the sperm nucleus was unable to participate in the formation of the zygote. In Drosophila, as in many animals, the condensed sperm chromatin contains protamines instead of histones. The authors found that the only crucial role of HIRA in flies was to assemble nucleosomes containing H3.3 in the male pronucleus, after the removal of protamines. This fundamental process, which is presumably also controlled by HIRA in vertebrates, allows the paternal DNA to reconstitute its chromatin and participate in the development of the embryo.

2. Chromatin assembly with H3 histones: full throttle down multiple pathways

The typical eukaryotic genome packages roughly 6 feet of DNA into a nucleus about 5 mum in diameter, yet this compaction blocks access to the DNA. At the first level of compaction, DNA is wrapped around octamers of core histone proteins to form arrays of nucleosomes. Nucleosomes are sufficient to block access to DNA, and cells must therefore manipulate nucleosomes in the course of activating the genome. Dramatic progress has been made in understanding the mechanisms by which nucleosomes are manipulated. In addition to the major core histones, most eukaryotic genomes also encode additional variant histones, which have some structural similarity. These are targeted to specific loci by coupling specialized nucleosome assembly pathways to DNA replication, transcription, or to developmental processes. We review evidence that nucleosome assembly pathways are interlinked with histone-modification systems, and may thereby perpetuate epigenetic chromatin states.

The histone H3.3 chaperone HIRA is essential for chromatin assembly in the male pronucleus

In sexually reproducing animals, a crucial step in zygote formation is the decondensation of the fertilizing sperm nucleus into a DNA replication-competent male pronucleus. Genome-wide nucleosome assembly on paternal DNA implies the replacement of sperm chromosomal proteins, such as protamines, by maternally provided histones. This fundamental process is specifically impaired in sésame (ssm), a unique Drosophila maternal effect mutant that prevents male pronucleus formation. Here we show that ssm is a point mutation in the Hira gene, thus demonstrating that the histone chaperone protein HIRA is required for nucleosome assembly during sperm nucleus decondensation. In vertebrates, HIRA has recently been shown to be critical for a nucleosome assembly pathway independent of DNA synthesis that specifically involves the H3.3 histone variant. We also show that nucleosomes containing H3.3, and not H3, are specifically assembled in paternal Drosophila chromatin before the first round of DNA replication. The exclusive marking of paternal chromosomes with H3.3 represents a primary epigenetic distinction between parental genomes in the zygote, and underlines an important consequence of the critical and highly specialized function of HIRA at fertilization.

Replacement by Drosophila melanogaster protamines and Mst77F of histones during chromatin condensation in late spermatids and role of sesame in the removal of these proteins from the male pronucleus

Chromatin condensation is a typical feature of sperm cells. During mammalian spermiogenesis, histones are first replaced by transition proteins and then by protamines, while little is known for Drosophila melanogaster. Here we characterize three genes in the fly genome, Mst35Ba, Mst35Bb, and Mst77F. The results indicate that Mst35Ba and Mst35Bb encode dProtA and dProtB, respectively. These are considerably larger than mammalian protamines, but, as in mammals, both protamines contain typical cysteine/arginine clusters. Mst77F encodes a linker histone-like protein showing significant similarity to mammalian HILS1 protein. ProtamineA-enhanced green fluorescent protein (eGFP), ProtamineB-eGFP, and Mst77F-eGFP carrying Drosophila lines show that these proteins become the important chromosomal protein components of elongating spermatids, and His2AvDGFP vanishes. Mst77F mutants [ms(3)nc3] are characterized by small round nuclei and are sterile as males. These data suggest the major features of chromatin condensation in Drosophila spermatogenesis correspond to those in mammals. During early fertilization steps, the paternal pronucleus still contains protamines and Mst77F but regains a nucleosomal conformation before zygote formation. In eggs laid by sesame-deficient females, the paternal pronucleus remains in a protamine-based chromatin status but Mst77F-eGFP is removed, suggesting that the sesame gene product is essential for removal of protamines while Mst77F removal is independent of Sesame.

Origin and neofunctionalization of a Drosophila paternal effect gene essential for zygote viability

BACKGROUND

Although evolutionary novelty by gene duplication is well established, the origin and maintenance of essential genes that provide entirely new functions (neofunctionalization) is still largely unknown. Drosophila is a good model for the search of genes that are young enough to allow deciphering the molecular details of their evolutionary history. Recent years have seen increased interest in genes specifically required for male fertility because they often evolve rapidly. A special class of genes affecting male fertility, the paternal effect genes, have also become a focus of study to geneticists and reproductive biologists interested in fertilization and sperm-egg interactions.

RESULTS

Using molecular genetics and the annotated Drosophila melanogaster genome, we identified CG14251 as the Drosophila paternal effect gene, ms(3)K81 (K81). This assignment was subsequently confirmed by P-element rescue of K81. A search for orthologous K81 sequences revealed that the distribution of K81 is surprisingly restricted to the 9 species comprising the melanogaster subgroup. Phylogenetic analyses indicate that K81 arose through duplication, most likely retroposition, of a ubiquitously expressed gene before the radiation of the melanogaster subgroup, followed by a period of rapid divergence and acquisition of a critical male germline-specific function. Interestingly, K81 has adopted the expression profile of a flanking gene suggesting that transcriptional coregulation may have been important in the neofunctionalization of K81.

CONCLUSION

We present a detailed case history of the origin and evolution of a new essential gene and, in so doing, provide the first molecular identification of a Drosophila paternal effect gene, ms(3)K81 (K81).

Chromatin dynamics and Arabidopsis development

The plant life cycle involves a series of developmental phase transitions. These transitions require the regulation and highly co-ordinated expression of many genes. Epigenetic controls have now been shown to be a key element of this mechanism of regulation. In the model plant Arabidopsis, recent genetic and molecular studies on chromatin have begun to dissect the molecular basis of these epigenetic controls. Chromatin dynamics represent the emerging and exciting field of gene regulation notably involved in plant developmental transitions. By comparing plant and animal systems, new insights into the molecular complexes and mechanisms governing development can be delineated. We are now beginning to identify the components of chromatin complexes and their functions.

HAPLODIPLOIDY AS AN OUTCOME OF COEVOLUTION BETWEEN MALE-KILLING CYTOPLASMIC ELEMENTS AND THEIR HOSTS

Haplodiploidy (encompassing both arrhenotoky and paternal genome elimination) could have originated from coevolution between male-killing endosymbiotic bacteria and their hosts. In insects, haplodiploidy tends to arise in lineages that rely on maternally transmitted bacteria for nutrition and that have gregarious broods in which competition between siblings may occur. When siblings compete, there is strong selection on maternally transmitted elements to kill males. I consider a hypothetical bacterial phenotype that renders male zygotes effectively haploid by preventing chromosome decondensation in male-determining sperm nuclei. By causing high male mortality, such a phenotype can be advantageous to the bacterial lineage. By eliminating paternal genes, it can also be advantageous to the host female. A simple model shows that the host female will benefit under a wide range of values for the efficiency of resource re-allocation, the efficiency of transmission, and the viability of haploid males. This hypothesis helps to explain the ecological correlates of the origins of haplodiploidy, as well as such otherwise puzzling phenomena as obligate cannibalism by male Micromalthus beetles, reversion to diploidy by aposymbiotic male stictococcid scale insects, and the bizarre genomic constitution of scale insect bacteriomes.