Genetic characterization of ms (3) K81, a paternal effect gene of Drosophila melanogaster

Paternal effects on telomere integrity during the sperm-to-embryo transition

Telomeres are essential nucleoprotein structures that preserve our terminal DNA sequence and protect chromosome ends from fusion. Our vast knowledge of telomeres comes almost entirely from studies of healthy and diseased somatic cells. However, building evidence suggests that the molecules and mechanisms required for telomere integrity in somatic cells are insufficient to preserve telomere integrity during the sperm-to-embryo transition. Here, we review this growing body of work on telomere 'paternal effects', wherein zygotic telomere integrity is determined not by the genotype of the zygote but instead by the genotype of the father. Direct inheritance of sperm-specific proteins establishes paternal telomere epigenetic identity, while direct inheritance of sperm telomere length contributes to telomere length inheritance. Together, these investigations of telomere integrity through the sperm-to-embryo transition reveal potent paternal effects on zygotic telomere functions, with implications for human infertility.

Maintaining Telomeres without Telomerase in Drosophila: Novel Mechanisms and Rapid Evolution to Save a Genus

Telomere maintenance is crucial for preventing the linear eukaryotic chromosome ends from being mistaken for DNA double-strand breaks, thereby avoiding chromosome fusions and the loss of genetic material. Unlike most eukaryotes that use telomerase for telomere maintenance, Drosophila relies on retrotransposable elements-specifically HeT-A, TAHRE, and TART (collectively referred to as HTT)-which are regulated and precisely targeted to chromosome ends. Drosophila telomere protection is mediated by a set of fast-evolving proteins, termed terminin, which bind to chromosome termini without sequence specificity, balancing DNA damage response factors to avoid erroneous repair mechanisms. This unique telomere capping mechanism highlights an alternative evolutionary strategy to compensate for telomerase loss. The modulation of recombination and transcription at Drosophila telomeres offers insights into the diverse mechanisms of telomere maintenance. Recent studies at the population level have begun to reveal the architecture of telomere arrays, the diversity among the HTT subfamilies, and their relative frequencies, aiming to understand whether and how these elements have evolved to reach an equilibrium with the host and to resolve genetic conflicts. Further studies may shed light on the complex relationships between telomere transcription, recombination, and maintenance, underscoring the adaptive plasticity of telomeric complexes across eukaryotes.

Hybridization promotes asexual reproduction in Caenorhabditis nematodes

Although most unicellular organisms reproduce asexually, most multicellular eukaryotes are obligately sexual. This implies that there are strong barriers that prevent the origin or maintenance of asexuality arising from an obligately sexual ancestor. By studying rare asexual animal species we can gain a better understanding of the circumstances that facilitate their evolution from a sexual ancestor. Of the known asexual animal species, many originated by hybridization between two ancestral sexual species. The balance hypothesis predicts that genetic incompatibilities between the divergent genomes in hybrids can modify meiosis and facilitate asexual reproduction, but there are few instances where this has been shown. Here we report that hybridizing two sexual Caenorhabditis nematode species (C. nouraguensis females and C. becei males) alters the normal inheritance of the maternal and paternal genomes during the formation of hybrid zygotes. Most offspring of this interspecies cross die during embryogenesis, exhibiting inheritance of a diploid C. nouraguensis maternal genome and incomplete inheritance of C. becei paternal DNA. However, a small fraction of offspring develop into viable adults that can be either fertile or sterile. Fertile offspring are produced asexually by sperm-dependent parthenogenesis (also called gynogenesis or pseudogamy); these progeny inherit a diploid maternal genome but fail to inherit a paternal genome. Sterile offspring are hybrids that inherit both a diploid maternal genome and a haploid paternal genome. Whole-genome sequencing of individual viable worms shows that diploid maternal inheritance in both fertile and sterile offspring results from an altered meiosis in C. nouraguensis oocytes and the inheritance of two randomly selected homologous chromatids. We hypothesize that hybrid incompatibility between C. nouraguensis and C. becei modifies maternal and paternal genome inheritance and indirectly induces gynogenetic reproduction. This system can be used to dissect the molecular mechanisms by which hybrid incompatibilities can facilitate the emergence of asexual reproduction.

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.

Developmentally regulated H2Av buffering via dynamic sequestration to lipid droplets in Drosophila embryos

Regulating nuclear histone balance is essential for survival, yet in early Drosophila melanogaster embryos many regulatory strategies employed in somatic cells are unavailable. Previous work had suggested that lipid droplets (LDs) buffer nuclear accumulation of the histone variant H2Av. Here, we elucidate the buffering mechanism and demonstrate that it is developmentally controlled. Using live imaging, we find that H2Av continuously exchanges between LDs. Our data suggest that the major driving force for H2Av accumulation in nuclei is H2Av abundance in the cytoplasm and that LD binding slows nuclear import kinetically, by limiting this cytoplasmic pool. Nuclear H2Av accumulation is indeed inversely regulated by overall buffering capacity. Histone exchange between LDs abruptly ceases during the midblastula transition, presumably to allow canonical regulatory mechanisms to take over. These findings provide a mechanistic basis for the emerging role of LDs as regulators of protein homeostasis and demonstrate that LDs can control developmental progression.

The Goddard and Saturn Genes Are Essential for Drosophila Male Fertility and May Have Arisen De Novo

New genes arise through a variety of mechanisms, including the duplication of existing genes and the de novo birth of genes from noncoding DNA sequences. While there are numerous examples of duplicated genes with important functional roles, the functions of de novo genes remain largely unexplored. Many newly evolved genes are expressed in the male reproductive tract, suggesting that these evolutionary innovations may provide advantages to males experiencing sexual selection. Using testis-specific RNA interference, we screened 11 putative de novo genes in Drosophila melanogaster for effects on male fertility and identified two, goddard and saturn, that are essential for spermatogenesis and sperm function. Goddard knockdown (KD) males fail to produce mature sperm, while saturn KD males produce few sperm, and these function inefficiently once transferred to females. Consistent with a de novo origin, both genes are identifiable only in Drosophila and are predicted to encode proteins with no sequence similarity to any annotated protein. However, since high levels of divergence prevented the unambiguous identification of the noncoding sequences from which each gene arose, we consider goddard and saturn to be putative de novo genes. Within Drosophila, both genes have been lost in certain lineages, but show conserved, male-specific patterns of expression in the species in which they are found. Goddard is consistently found in single-copy and evolves under purifying selection. In contrast, saturn has diversified through gene duplication and positive selection. These data suggest that de novo genes can acquire essential roles in male reproduction.

Scaling of cytoskeletal organization with cell size in Drosophila

Actin-rich denticle precursors are regularly distributed in the Drosophila embryo. Cytoskeletal scaling occurs through changes in denticle number and spacing. Denticle spacing scales with cell length over a 10-fold range. Accurate denticle positioning requires the microtubule cytoskeleton.

Spatially organized macromolecular complexes are essential for cell and tissue function, but the mechanisms that organize micron-scale structures within cells are not well understood. Microtubule-based structures such as mitotic spindles scale with cell size, but less is known about the scaling of actin structures within cells. Actin-rich denticle precursors cover the ventral surface of the Drosophila embryo and larva and provide templates for cuticular structures involved in larval locomotion. Using quantitative imaging and statistical modeling, we demonstrate that denticle number and spacing scale with cell length over a wide range of cell sizes in embryos and larvae. Denticle number and spacing are reduced under space-limited conditions, and both features robustly scale over a 10-fold increase in cell length during larval growth. We show that the relationship between cell length and denticle spacing can be recapitulated by specific mathematical equations in embryos and larvae and that accurate denticle spacing requires an intact microtubule network and the microtubule minus end–binding protein, Patronin. These results identify a novel mechanism of micro­tubule-dependent actin scaling that maintains precise patterns of actin organization during tissue growth.

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.

The Deadbeat Paternal Effect of Uncapped Sperm Telomeres on Cell Cycle Progression and Chromosome Behavior in Drosophila melanogaster

Telomere-capping complexes (TCCs) protect the ends of linear chromosomes from illegitimate repair and end-to-end fusions and are required for genome stability. The identity and assembly of TCC components have been extensively studied, but whether TCCs require active maintenance in nondividing cells remains an open question. Here we show that Drosophila melanogaster requires Deadbeat (Ddbt), a sperm nuclear basic protein (SNBP) that is recruited to the telomere by the TCC and is required for TCC maintenance during genome-wide chromatin remodeling, which transforms spermatids to mature sperm. Ddbt-deficient males produce sperm lacking TCCs. Their offspring delay the initiation of anaphase as early as cycle 1 but progress through the first two cycles. Persistence of uncapped paternal chromosomes induces arrest at or around cycle 3. This early arrest can be rescued by selective elimination of paternal chromosomes and production of gynogenetic haploid or haploid mosaics. Progression past cycle 3 can also occur if embryos have reduced levels of the maternally provided checkpoint kinase Chk2. The findings provide insights into how telomere integrity affects the regulation of the earliest embryonic cell cycles. They also suggest that other SNBPs, including those in humans, may have analogous roles and manifest as paternal effects on embryo quality.

Cross-Species Interaction between Rapidly Evolving Telomere-Specific Drosophila Proteins

Telomere integrity in Drosophila melanogaster is maintained by a putative multisubunit complex called terminin that is believed to act in analogy to the mammalian shelterin complex in protecting chromosome ends from being recognized as sites of DNA damage. The five proteins supposed to form the terminin complex are HP1-ORC associated protein, HP1-HOAP interacting protein, Verrocchio, Drosophila Telomere Loss/Modigliani and Heterochromatic Protein 1. Four of these proteins evolve rapidly within the Drosophila genus. The accelerated evolution of terminin components may indicate the involvement of these proteins in the process by which new species arise, as the resulting divergence of terminin proteins might prevent hybrid formation, thus driving speciation. However, terminin is not an experimentally proven entity, and no biochemical studies have been performed to investigate its assembly and action in detail. Motivated by these facts in order to initiate biochemical studies on terminin function, we attempted to reconstitute terminin by co-expressing its subunits in bacteria and investigated the possible role of the fast-evolving parts of terminin components in complex assembly. Our results suggest formation of stable subcomplexes of terminin, but not of the whole complex in vitro. We found that the accelerated evolution is restricted to definable regions of terminin components, and that the divergence of D. melanogaster Drosophila Telomere Loss and D. yakuba Verrocchio proteins does not preclude their stable interaction.

Protection of Drosophila chromosome ends through minimal telomere capping

In Drosophila, telomere-capping proteins have the remarkable capacity to recognize chromosome ends in a sequence-independent manner. This epigenetic protection is essential to prevent catastrophic ligations of chromosome extremities. Interestingly, capping proteins occupy a large telomere chromatin domain of several kilobases; however, the functional relevance of this to end protection is unknown. Here, we investigate the role of the large capping domain by manipulating HOAP (encoded by caravaggio) capping-protein expression in the male germ cells, where telomere protection can be challenged without compromising viability. We show that the exhaustion of HOAP results in a dramatic reduction of other capping proteins at telomeres, including K81 [encoded by ms(3)K81], which is essential for male fertility. Strikingly however, we demonstrate that, although capping complexes are barely detected in HOAP-depleted male germ cells, telomere protection and male fertility are not dramatically affected. Our study thus demonstrates that efficient protection of Drosophila telomeres can be achieved with surprisingly low amounts of capping complexes. We propose that these complexes prevent fusions by acting at the very extremity of chromosomes, reminiscent of the protection conferred by extremely short telomeric arrays in yeast or mammalian systems.

Sperm should evolve to make female meiosis fair

Genomic conflicts arise when an allele gains an evolutionary advantage at a cost to organismal fitness. Oögenesis is inherently susceptible to such conflicts because alleles compete for inclusion into the egg. Alleles that distort meiosis in their favor (i.e., meiotic drivers) often decrease organismal fitness, and therefore indirectly favor the evolution of mechanisms to suppress meiotic drive. In this light, many facets of oögenesis and gametogenesis have been interpreted as mechanisms of protection against genomic outlaws. That females of many animal species do not complete meiosis until after fertilization, appears to run counter to this interpretation, because this delay provides an opportunity for sperm-acting alleles to meddle with the outcome of female meiosis and help like alleles drive in heterozygous females. Contrary to this perceived danger, the population genetic theory presented herein suggests that, in fact, sperm nearly always evolve to increase the fairness of female meiosis in the face of genomic conflicts. These results are consistent with the apparent sperm dependence of the best characterized female meiotic driversin animals. Rather than providing an opportunity for sperm collaboration in female meiotic drive, the "fertilization requirement" indirectly protects females from meiotic drivers by providing sperm an opportunity to suppress drive.

New genes as drivers of phenotypic evolution

During the course of evolution, genomes acquire novel genetic elements as sources of functional and phenotypic diversity, including new genes that originated in recent evolution. In the past few years, substantial progress has been made in understanding the evolution and phenotypic effects of new genes. In particular, an emerging picture is that new genes, despite being present in the genomes of only a subset of species, can rapidly evolve indispensable roles in fundamental biological processes, including development, reproduction, brain function and behaviour. The molecular underpinnings of how new genes can develop these roles are starting to be characterized. These recent discoveries yield fresh insights into our broad understanding of biological diversity at refined resolution.

Mixed asexual and sexual reproduction in the Indo-Pacific reef coral Pocillopora damicornis

Pocillopora damicornis is one of the best studied reef-building corals, yet it's somewhat unique reproductive strategy remains poorly understood. Genetic studies indicate that P. damicornis larvae are produced almost exclusively parthenogenetically, and yet population genetic surveys suggest frequent sexual reproduction. Using microsatellite data from over 580 larvae from 13 colonies, we demonstrate that P. damicornis displays a mixed reproductive strategy where sexual and asexual larvae are produced simultaneously within the same colony. The majority of larvae were parthenogenetic (94%), but most colonies (10 of the 13) produced a subset of their larvae sexually. Logistic regression indicates that the proportion of sexual larvae varied significantly with colony size, cycle day, and calendar day. In particular, the decrease in sexual larvae with colony size suggests that the mixed reproductive strategy changes across the life of the coral. This unique shift in reproductive strategy leads to increasingly asexual replications of successful genotypes, which (in contrast to exclusive parthenogens) have already contributed to the recombinant gene pool.

Repeated evolution of testis-specific new genes: the case of telomere-capping genes in Drosophila

Comparative genome analysis has allowed the identification of various mechanisms involved in gene birth. However, understanding the evolutionary forces driving new gene origination still represents a major challenge. In particular, an intriguing and not yet fully understood trend has emerged from the study of new genes: many of them show a testis-specific expression pattern, which has remained poorly understood. Here we review the case of such a new gene, which involves a telomere-capping gene family in Drosophila. hiphop and its testis-specific paralog K81 are critical for the protection of chromosome ends in somatic cells and male gametes, respectively. Two independent functional studies recently proposed that these genes evolved under a reproductive-subfunctionalization regime. The 2011 release of new Drosophila genome sequences from the melanogaster group of species allowed us to deepen our phylogenetic analysis of the hiphop/K81 family. This work reveals an unsuspected dynamic of gene birth and death within the group, with recurrent duplication events through retroposition mechanisms. Finally, we discuss the plausibility of different evolutionary scenarios that could explain the diversification of this gene family.

Circumventing heterozygosity: sequencing the amplified genome of a single haploid Drosophila melanogaster embryo

Heterozygosity is a major challenge to efficient, high-quality genomic assembly and to the full genomic survey of polymorphism and divergence. In Drosophila melanogaster lines derived from equatorial populations are particularly resistant to inbreeding, thus imposing a major barrier to the determination and analyses of genomic variation in natural populations of this model organism. Here we present a simple genome sequencing protocol based on the whole-genome amplification of the gynogenetically derived haploid genome of a progeny of females mated to males homozygous for the recessive male sterile mutation, ms(3)K81. A single “lane” of paired-end sequences (2 × 76 bp) provides a good syntenic assembly with >95% high-quality coverage (more than five reads). The amplification of the genomic DNA moderately inflates the variation in coverage across the euchromatic portion of the genome. It also increases the frequency of chimeric clones. But the low frequency and random genomic distribution of the chimeric clones limits their impact on the final assemblies. This method provides a solid path forward for population genomic sequencing and offers applications to many other systems in which small amounts of genomic DNA have unique experimental relevance.

Paternal imprint essential for the inheritance of telomere identity in Drosophila

Chromatin remodeling during sperm maturation could erase epigenetic landmarks on the paternal genome, creating a challenge for its reestablishment on fertilization. Here, we show that selective retention of a chromosomal protein in mature sperm protects the identity of paternal telomeres in Drosophila. The ms(3)k81 (k81) gene is a duplication of hiphop that encodes a telomeric protein. Although HipHop protects telomeres in somatic cells, K81 is produced exclusively in males and localizes to telomeres in postmitotic cells, including mature sperm. In embryos fathered by k81 mutants, the maternal supplies fail to reestablish a protective cap on paternal telomeres, leading to their fusions. These fusions hinder the segregation of the paternal genome and result in haploid embryos with maternal chromosomes. The functional divergence between hiphop and k81 manifests not only in their expression patterns but also in the protein functions that they encode. By swapping the two coding regions, we show that K81 can replace HipHop for somatic protection; however, HipHop cannot replace K81 in the germ line to specify telomere identity, because HipHop ectopically expressed in the testis is removed from chromatin during sperm maturation. HipHop lacks a short motif in K81 that is essential for K81 to survive the remodeling process. We show that the combined functions of HipHop and K81 are likely fulfilled by the single ancestral hiphop locus in other Drosophila species, supporting the hypothesis that the evolutionary process of subfunctionalization was responsible for the preservation of the hiphop-k81 duplicate.

Telomeres: a new means to an end

Gene duplication provides an important evolutionary mechanism for functional diversification. A new study in Drosophila indicates that gene duplication has allowed telomere protection to be partitioned between the soma and the specialized chromatin environment of sperm.

Specialization of a Drosophila capping protein essential for the protection of sperm telomeres

BACKGROUND

A critical function of telomeres is to prevent fusion of chromosome ends by the DNA repair machinery. In Drosophila somatic cells, assembly of the protecting capping complex at telomeres notably involves the recruitment of HOAP, HP1, and their recently identified partner, HipHop. We previously showed that the hiphop gene was duplicated before the radiation of the melanogaster subgroup of species, giving birth to K81, a unique paternal effect gene specifically expressed in the male germline.

RESULTS

Here we show that K81 specifically associates with telomeres during spermiogenesis, along with HOAP and HP1, and is retained on paternal chromosomes until zygote formation. In K81 mutant testes, capping proteins are not maintained at telomeres in differentiating spermatids, resulting in the transmission of uncapped paternal chromosomes that fail to properly divide during the first zygotic mitosis. Despite the apparent similar capping roles of K81 and HipHop in their respective domain of expression, we demonstrate by in vivo reciprocal complementation analyses that they are not interchangeable. Strikingly, HipHop appeared to be unable to maintain capping proteins at telomeres during the global chromatin remodeling of spermatid nuclei.

CONCLUSIONS

Our data demonstrate that K81 is essential for the maintenance of capping proteins at telomeres in postmeiotic male germ cells. In species of the melanogaster subgroup, HipHop and K81 have not only acquired complementary expression domains, they have also functionally diverged following the gene duplication event. We propose that K81 specialized in the maintenance of telomere protection in the highly peculiar chromatin environment of differentiating male gametes.

Drosophila I-R hybrid dysgenesis is associated with catastrophic meiosis and abnormal zygote formation

The Drosophila I-R type of hybrid dysgenesis is a sterility syndrome (SF sterility) associated with the mobilization of the I retrotransposon in female germ cells. SF sterility results from a maternal-effect embryonic lethality whose origin has remained unclear since its discovery about 40 years ago. Here, we show that meiotic divisions in SF oocytes are catastrophic and systematically fail to produce a functional female pronucleus at fertilization. As a consequence, most embryos from SF females rapidly arrest their development with aneuploid or damaged nuclei, whereas others develop as non-viable, androgenetic haploid embryos. Finally, we show that, in contrast to mutants affecting the biogenesis of piRNAs, SF egg chambers do not accumulate persistent DNA double-strand breaks, suggesting that I-element activity might perturb the functional organization of meiotic chromosomes without triggering an early DNA damage response.

Recent origins of sperm genes in Drosophila

Newly created genes often acquire testis-specific or enhanced expression but neither the mechanisms responsible for this specificity nor the functional consequences of these evolutionary processes are well understood. Genomic analyses of the Drosophila melanogaster sperm proteome has identified 2 recently evolved gene families on the melanogaster lineage and 4 genes created by retrotransposition during the evolution of the melanogaster group that encode novel sperm components. The expanded Mst35B (protamine) and tektin gene families are the result of tandem duplication events with all family members displaying testis-specific expression. The Mst35B family encodes rapidly evolving protamines that display a robust signature of positive selection within the DNA-binding high-mobility group box consistent with functional diversification in genome repackaging during sperm nuclear remodeling. The Mst35B paralogs also reside in a significant regional cluster of testis-overexpressed genes. Tektins, known components of the axoneme, are encoded by 3 nearly identical X-linked genes, a finding consistent with very recent gene family expansion. In addition to localized duplication events, the evolution of the sperm proteome has also been driven by recent retrotransposition events resulting in Cdlc2, CG13340, Vha36, and CG4706. Cdlc2, CG13340, and Vha36 all display high levels of overexpression in the testis, and Cdlc2 and CG13340 reside within testis-overexpressed gene clusters. Thus, gene creation is a dynamic force in the evolution of sperm composition and possibly function, which further suggests that acquisition of molecular functionality in sperm may be an influential pathway in the fixation of new genes.

Good genes, genetic compatibility and the evolution of polyandry: use of the diallel cross to address competing hypotheses

Genetic benefits can enhance the fitness of polyandrous females through the high intrinsic genetic quality of females' mates or through the interaction between female and male genes. I used a full diallel cross, a quantitative genetics design that involves all possible crosses among a set of genetically homogeneous lines, to determine the mechanism through which polyandrous female decorated crickets (Gryllodes sigillatus) obtain genetic benefits. I measured several traits related to fitness and partitioned the phenotypic variance into components representing the contribution of additive genetic variance ('good genes'), nonadditive genetic variance (genetic compatibility), as well as maternal and paternal effects. The results reveal a significant variance attributable to both nonadditive and additive sources in the measured traits, and their influence depended on which trait was considered. The lack of congruence in sources of phenotypic variance among these fitness-related traits suggests that the evolution and maintenance of polyandry are unlikely to have resulted from one selective influence, but rather are the result of the collective effects of a number of factors.

Induced paternal effects mimic cytoplasmic incompatibility in Drosophila

Wolbachia is an intracellular microbe found in a wide diversity of arthropod and filarial nematode hosts. In arthropods these common bacteria are reproductive parasites that manipulate central elements of their host's reproduction to increase their own maternal transmission in one of several ways. Cytoplasmic incompatibility (CI) is one such manipulation where sperm are somehow modified in infected males and this modification must be rescued by the presence of the same bacterial strain in the egg for normal development to proceed. The molecular mechanisms involved in the expression of CI are unknown. Here we show that Wolbachia infection results in increased mRNA and protein expression of the Drosophila simulans nonmuscle myosin II gene zipper. Induced overexpression of zipper in Wolbachia-free transgenic D. melanogaster males results in paternal-effect lethality that mimics the fertilization defects associated with CI. Likewise, overexpression of the tumor suppressor gene, lethal giant larvae [l(2)gl], results in egg lethality and a CI phenotype. Stoichiometric levels of zipper and l(2)gl are required for proper segregation of cellular determinants during neuroblast stem cell division. Taken together these results form the basis of a working hypothesis whereby Wolbachia induces paternal effects in sperm by manipulating the expression of key regulators of cytoskeletal activity during spermatogenesis.

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).

Evolutionary Genomics: New Genes for New Jobs

Whole genome sequence analyses have confirmed that gene duplication and divergence play major roles in genome evolution. But the details of how young, functionally redundant gene duplicates escape mutational degradation have remained elusive. Several recent studies show that new genes survive because they evolve new, and sometimes essential, functions.

The generation of cloned Drosophila melanogaster

We report here the first successful use of embryonic nuclear transfer to create viable adult Drosophila melanogaster clones. Given the generation time, cost effectiveness, and relative ease of embryonic nuclear transplant in Drosophila, this method can provide an opportunity to further study the constraints on development imposed by transplanting determined or differentiated nuclei.

Sperm chromatin remodelling and Wolbachia-induced cytoplasmic incompatibility in Drosophila

Wolbachia pipientis is an obligate bacterial endosymbiont, which has successfully invaded approximately 20% of all insect species by manipulating their normal developmental patterns. Wolbachia-induced phenotypes include parthenogenesis, male killing, and, most notably, cytoplasmic incompatibility. In the future these phenotypes might be useful in controlling or modifying insect populations but this will depend on our understanding of the basic molecular processes underlying insect fertilization and development. Wolbachia-infected Drosophila simulans express high levels of cytoplasmic incompatibility in which the sperm nucleus is modified and does not form a normal male pronucleus when fertilizing eggs from uninfected females. The sperm modification is somehow rescued in eggs infected with the same strain of Wolbachia. Thus, D. simulans has become an excellent model organism for investigating the manner in which endosymbionts can alter reproductive programs in insect hosts. This paper reviews the current knowledge of Drosophila early development and particularly sperm function. Developmental mutations in Drosophila that are known to affect sperm function will also be discussed.incompatibility.

The Drosophila misfire gene has an essential role in sperm activation during fertilization

The male sterile mutation, misfire (mfr), of Drosophila melanogaster is a novel paternal effect, fertilization defective mutant that effects sperm head decondensation. mfr sperm were motile, appeared normal morphologically and were transferred to the female during copulation. However, less than 0.1% of eggs laid by females mated to mfr males hatched. Although mfr sperm entered eggs at a high frequency (93%), 99% of the inseminated eggs did not initiate the first nuclear division. Unlike wild type fertilizing sperm, the position and shape of mfr sperm tails within the egg were not constant, but varied in a seemingly random manner. The heads of inseminating mutant sperm were always located near the surface of eggs just underlying the egg plasma membrane, and maintained their needle-like shape indicating the failure of nuclear decondensation. Further observations revealed that plasma membrane of inseminating sperm appeared intact, including the head region. These phenotypes were equivalent to those of sneaky (snky), another fertilization defective male sterile mutation. Our observations strongly suggest that mfr mutant males are sterile because their inseminating sperm fail to form a male pronucleus due to the inability of the sperm to properly respond to egg factors responsible for the breakdown of the plasma membrane. Although mfr and snky mutations were phenotypically identical, they mapped to cytologically distinct genetic loci and no genetic interactions were observed, suggesting that at least two distinct paternal gene products are involved in the early stages of pronuclear formation.

Evolutionary consequences of Wolbachia infections

The past decade has revealed the bacterium Wolbachia as the most widespread symbiont of arthropods and nematodes. Behind this evolutionary success is an remarkable variety of effects on host biology, ranging from manipulation of reproduction in favor of females to more classical mutualistic interactions. Here we discuss the potential of Wolbachia for promoting evolutionary changes in its hosts.

The Drosophila nuclear lamina protein YA binds to DNA and histone H2B with four domains

Dramatic changes occur in nuclear organization and function during the critical developmental transition from meiosis to mitosis. The Drosophila nuclear lamina protein YA binds to chromatin and is uniquely required for this transition. In this study, we dissected YA's binding to chromatin. We found that YA can bind to chromatin directly and specifically. It binds to DNA but not RNA, with a preference for double-stranded DNA (linear or supercoiled) over single-stranded DNA. It also binds to histone H2B. YA's binding to DNA and histone H2B is mediated by four domains distributed along the length of the YA molecule. A model for YA function at the end of Drosophila female meiosis is proposed.

Paternal chromosome incorporation into the zygote nucleus is controlled by maternal haploid in Drosophila

maternal haploid (mh) is a strict maternal effect mutation that causes the production of haploid gynogenetic embryos (eggs are fertilized but only maternal chromosomes participate in development). We conducted a cytological analysis of fertilization and early development in mh eggs to elucidate the mechanism of paternal chromosome elimination. In mh eggs, as in wild-type eggs, male and female pronuclei migrate and appose, the first mitotic spindle forms, and both parental sets of chromosomes congress on the metaphase plate. In contrast to control eggs, mh paternal sister chromatids fail to separate in anaphase of the first division. As a consequence the paternal chromatin stretches and forms a bridge in telophase. During the first three embryonic divisions, damaged paternal chromosomes are progressively eliminated from the spindles that organize around maternal chromosomes. A majority of mh embryos do not survive the deleterious presence of aneuploid nuclei and rapidly arrest their development. The rest of mh embryos develop as haploid gynogenetic embryos and die before hatching. The mh phenotype is highly reminiscent of the early developmental defects observed in eggs fertilized by ms(3)K81 mutant males and in eggs produced in incompatible crosses of Drosophila harboring the endosymbiont bacteria Wolbachia.

A genetic test of the mechanism of Wolbachia-induced cytoplasmic incompatibility in Drosophila

Cytoplasmic bacteria of the genus Wolbachia are best known as the cause of cytoplasmic incompatibility (CI): many uninfected eggs fertilized by Wolbachia-modified sperm from infected males die as embryos. In contrast, eggs of infected females rescue modified sperm and develop normally. Although Wolbachia cause CI in at least five insect orders, the mechanism of CI remains poorly understood. Here I test whether the target of Wolbachia-induced sperm modification is the male pronucleus (e.g., DNA or pronuclear proteins) or some extranuclear factor from the sperm required for embryonic development (e.g., the paternal centrosome). I distinguish between these hypotheses by crossing gynogenetic Drosophila melanogaster females to infected males. Gynogenetic females produce diploid eggs whose normal development requires no male pronucleus but still depends on extranuclear paternal factors. I show that when gynogenetic females are crossed to infected males, uniparental progeny with maternally derived chromosomes result. This finding shows that Wolbachia impair the male pronucleus but no extranuclear component of the sperm.

Genetic analysis of viable Hsp90 alleles reveals a critical role in Drosophila spermatogenesis

The Hsp90 chaperone protein maintains the activities of a remarkable variety of signal transducers, but its most critical functions in the context of the whole organism are unknown. Point mutations of Hsp83 (the Drosophila Hsp90 gene) obtained in two different screens are lethal as homozygotes. We report that eight transheterozygous mutant combinations produce viable adults. All exhibit the same developmental defects: sterile males and sterile or weakly fertile females. We also report that scratch, a previously identified male-sterile mutation, is an allele of Hsp82 with a P-element insertion in the intron that reduces expression. Thus, it is a simple reduction in Hsp90 function, rather than possible altered functions in the point mutants, that leads to male sterility. As shown by light and electron microscopy, all stages of spermatogenesis involving microtubule function are affected, from early mitotic divisions to later stages of sperm maturation, individualization, and motility. Aberrant microtubules are prominent in yeast cells carrying mutations in HSP82 (the yeast Hsp90 gene), confirming that Hsp90 function is connected to microtubule dynamics and that this connection is highly conserved. A small fraction of Hsp90 copurifies with taxol-stabilized microtubule proteins in Drosophila embryo extracts, but Hsp90 does not remain associated with microtubules through repeated temperature-induced assembly and disassembly reactions. If the spermatogenesis phenotypes are due to defects in microtubule dynamics, we suggest these are indirect, reflecting a role for Hsp90 in maintaining critical signal transduction pathways and microtubule effectors, rather than a direct role in the assembly and disassembly of microtubules themselves.

The paternal effect gene ms(3)sneaky is required for sperm activation and the initiation of embryogenesis in Drosophila melanogaster

Although a large number of maternal factors are known to be essential for fertilization or the earliest stages of embryogenesis in Drosophila melanogaster, the role of paternally supplied products is not clearly understood. Paternal effect mutations provide a means to identify factors specifically required by the sperm after its entry into the egg. Here we describe the third strict paternal effect gene to be identified in Drosophila ms(3)sneaky(snky), which defines the earliest developmental arrest phenotype so far described. Characterization of two independently isolated snky mutations showed that they affected male fertility, but not viability or female fertility. Cytological analyses showed that spermatogenesis proceeded normally in snky males. However, the snky defect was evident after sperm entry into the egg; snky sperm did not undergo nuclear decondensation, form a functional male pronucleus, or initiate mitotic divisions in the egg. Immunolocalization of tubulin and Drosophila Centrosomin, a known centrosomal component, showed that snky-inseminated eggs failed to reconstitute a microtubule-organizing center. In addition, snky sperm chromatin retained the histochemical properties of mature sperm chromatin for several hours after sperm entry, showed reduced staining with membrane-impermeant nuclear dyes, and failed to replicate. We conclude that the snky+ product is required for the initial response of the sperm to cytoplasmic cues in the egg and for the subsequent initiation of embryogenesis in Drosophila. We suggest that all of the snky defects can be explained by the failure of the sperm plasma membrane to break down after entry into the egg.

Paternal products and by-products in Drosophila development

Tails of fertilizing spermatozoa persist throughout embryogenesis in Drosophila species and can be observed within the midguts of larvae after hatching. Throughout development, sperm proteins slowly diffuse or are stripped from the giant sperm tail residing within the embryo's anterior end. The shape and position of the sperm within the embryo are regulated such that, during organ formation, the unused portion of the sperm is enveloped by the developing midgut. This persistent, paternally derived structure is composed of the sperm's mitochondrial derivatives and appears to be defecated by the larva soon after hatching. These complex sperm-egg interactions may represent mechanisms to avoid intragenomic conflict by ensuring strictly maternal inheritance of mitochondrial DNA (mtDNA).

Paternal effects in Drosophila: implications for mechanisms of early development

The study of paternal effects on development provides a means to identify sperm-supplied products required for fertilization and the initiation of embryogenesis. This review describes paternal effects on animal development and discusses their implications for the role of the sperm in egg activation, centrosome activity, and biparental inheritance in different animal species. Paternal effects observed in Caenorhabditis elegans and in mammals are briefly reviewed. Emphasis is placed on paternal effects in Drosophila melanogaster. Genetic and cytologic evidence for paternal imprinting on chromosome behavior and gene expression in Drosophila are summarized. These effects are compared to chromosome imprinting that leads to paternal chromosome loss in sciarid and coccid insects and mammalian gametic imprinting that results in differential expression of paternal and maternal loci. The phenotypes caused by several early-acting maternal effect mutations identify specific maternal factors that affect the behavior of paternal components during fertilization and the early embryonic mitotic divisions. In addition, maternal effect defects suggest that two types of regulatory mechanisms coordinate parental components and synchronize their progression through mitosis. Some activities are coordinated by independent responses of parental components to shared regulatory factors, while others require communication between paternal and maternal components. Analyses of the paternal effects mutations sneaky, K81, paternal loss, and Horka have identified paternal products that play a role in mediating the initial response of the sperm to the egg cytoplasm, participation of the male pronucleus in the first mitosis, and stable inheritance of the paternal chromosomes in the early embryo.

Formation of the male pronuclear lamina in Drosophila melanogaster

Upon fertilization, a sperm nucleus reorganizes to become a male pronucleus. This reorganization includes breakdown and reformation of the nuclear envelope of the male pronucleus. In this study, we used a maternally encoded nuclear lamina protein, YA, in parallel with another lamina protein, lamin Dm, as probes to study the formation of the male pronuclear lamina in Drosophila melanogaster. Ectopically expressed YA is present in the nuclear envelopes of spermatocytes, but not in mature sperm, similar to endogenous lamin Dm. This suggests that the nuclear envelope of Drosophila sperm differs from that of somatic cells. Upon fertilization, YA and lamin Dm are recruited to the periphery of the male-derived nucleus before or during the early stages of migration by the male pronucleus. Using a paternal effect mutation, snky, we found that recruitment of lamina proteins to the male pronucleus requires, and probably accompanies, reorganization of the sperm nucleus. In order to identify factors that affect the recruitment of nuclear lamina proteins to the male pronucleus, we examined the subcellular localization of YA and lamin Dm in mutant embryos defective for the function of either the male pronucleus (mh, K81, and pal or both pronuclei (gnu, png, and plu). None of these mutations affect the recruitment of YA or lamin Dm to the male pronuclear envelope, suggesting that the mutations affect processes independent of, or after, reorganization of the nuclear envelope. Double mutant analyses between Ya and gnu suggest that YA plays a role in the nuclear envelope permissive for rounds of DNA replication.

Spontaneous mutational variances and covariances for fitness-related traits in Drosophila melanogaster

Starting from a completely homozygous population of Drosophila melanogaster, 176 lines were derived and independently maintained by a single brother-sister mating per generation. Three fitness-related traits were considered (fecundity, egg-to-pupa and pupa-to-adult viabilities). Mutational heritabilities of these traits and genetic correlations between all possible pairs were calculated from the between line divergence (codivergence), after 104-106 generations of mutation accumulation. Mutational heritabilities ranged from 0.60 x 10(-3) to 0.82 x 10(-3) and correlations from -0.11 to 0.25. These values are likely to be underestimates due to selection against deleterious mutations. The distribution of the means of the lines was asymmetric, positive for fecundity and negative for both viability components. The coefficients of asymmetry are also likely to be biased, again due to selection. Extreme lines from the two tails of the distribution were examined in detail. Homozygous line effects were all negative for viability traits but predominantly positive for fecundity, indicating the fixation of mutations with positive effects on the latter. Corresponding heterozygous line effects showed a variable degree of dominance.

Cytological analysis of fertilization and early embryonic development in incompatible crosses of Drosophila simulans

Cytoplasmic incompatibility (CI) is a unique form of male sterility found in numerous insect species that harbor a bacterial endosymbiont Wolbachia. CI is characterized by severe reduction in the progeny produced when infected males are crossed to uninfected females. The reduction in progeny correlates with developmental defects that arise during and immediately following fertilization, suggesting that sperm function is disrupted. To investigate the nature of the cellular defects associated with CI, fertilization and early embryonic development were examined in normal and incompatible crosses of Drosophila simulans using anti-sperm, anti-tubulin and anti-chromatin antibodies. Although pleiotropic, defects associated with CI can be classified into five broad categories: (1) sperm defects in the egg; (2) aberrant morphology of the mitotic apparatus; (3) defects in chromatin structure; (4) proliferation of centrosomes in the absence of nuclear division; and (5) loss of mitotic synchrony. Although mitosis and chromosome behavior are severely disrupted in CI crosses during early development, centrosome duplication and migration appear to continue unabated. The available cytological data suggest that the primary defects observed in incompatible crosses are due to defects in chromosome replication/segregation and in associated centrosome/microtubule-based processes.

A sperm-supplied factor required for embryogenesis in C. elegans

The paternal-effect embryonic-lethal gene, spe-11, is required for normal development of early C. elegans embryos. Spe-11 embryos fail to complete meiosis, form a weak eggshell, fail to orient properly the first mitotic spindle, and fail to undergo cytokinesis. Here we report cloning and sequencing of the spe-11 gene, which encodes a novel protein. As predicted by the paternal-effect mutant phenotype, the gene is expressed during spermatogenesis but is not detectable in females undergoing oogenesis, and the protein is present in mature sperm. To investigate whether SPE-11's essential function is during spermatogenesis or whether sperm-delivered SPE-11 functions in the newly fertilized embryo, we engineered animals to supply SPE-11 to the embryo through the oocyte rather than through the sperm. We found that maternal expression is sufficient for embryonic viability. This result demonstrates that SPE-11 is not required during spermatogenesis, and suggests that SPE-11 is a sperm-supplied factor that participates directly in development of the early embryo. In contrast to the many known maternal factors required for embryogenesis, SPE-11 is the first paternally contributed factor to be genetically identified and molecularly characterized.