Maternal control of haplodiploid sex determination in the wasp Nasonia

How to make a haploid male

Haplodiploidy has evolved repeatedly among invertebrates, and appears to be associated with inbreeding. Evolutionary biologists have long debated the possible benefits for females in diplodiploid species to produce haploid sons-beginning their population's transition to haplodiploidy-and whether inbreeding promotes or inhibits this transition. However, little attention has been given to what makes a haploid individual male rather than female, and whether the mechanism of sex determination may modulate the costs and benefits of male haploidy. We remedy this by performing a theoretical analysis of the origin and invasion of male haploidy across the full range of sex-determination mechanisms and sib-mating rates. We find that male haploidy is facilitated by three different mechanisms of sex determination-all involving male heterogamety-and impeded by the others. We also find that inbreeding does not pose an obvious evolutionary barrier, on account of a previously neglected sex-ratio effect whereby the production of haploid sons leads to an abundance of granddaughters that is advantageous in the context of inbreeding. We find empirical support for these predictions in a survey of sex determination and inbreeding across haplodiploids and their sister taxa.

Evidence for Stabilizing Selection Driving Mutational Turnover of Short Motifs in the Eukaryotic Complementary Sex Determiner (Csd) Protein

Short linear motifs (SLiMs) can play pivotal functional roles in proteins, such as targeting proteins to specific subcellular localizations, modulating the efficiency of translation and tagging proteins for degradation. Until recently we had little knowledge about SLiM evolution. Only a few amino acids in these motifs are functionally important, making them likely to evolve ex nihilo and suggesting that they can play key roles in protein evolution. Several reports now suggest that these motifs can appear and disappear while their function in the protein is preserved, a process sometimes referred to as “turnover”. However, there has been a lack of specific experiments to determine whether independently evolved motifs do indeed have the same function, which would conclusively determine whether the process of turnover actually occurs. In this study, we experimentally detected evidence for such a mutational turnover process for nuclear localization signals (NLS) during the post-duplication divergence of the Complementary sex determiner (Csd) and Feminizer (Fem) proteins in the honeybee (Apis mellifera) lineage. Experiments on the nuclear transport activity of protein segments and those of the most recent common ancestor (MRCA) sequences revealed that three new NLS motifs evolved in the Csd protein during the post-duplication divergence while other NLS motifs were lost that existed before duplication. A screen for essential and newly evolved amino acids revealed that new motifs in the Csd protein evolved by one or two missense mutations coding for lysine. Amino acids that were predating the duplication were also essential in the acquisition of the C1 motif suggesting that the ex nihilo origin was constrained by preexisting amino acids in the physical proximity. Our data support a model in which stabilizing selection maintains the constancy of nuclear transport function but allowed mutational turnover of the encoding NLS motifs.

Evidence for involvement of a transformer paralogue in sex determination of the wasp Leptopilina clavipes

Transformer (tra) is the central gear in many insect sex determination pathways and transduces a wide range of primary signals. Mediated by transformer-2 (tra2) it directs sexual development into the female or male mode. Duplications of tra have been detected in numerous Hymenoptera, but a function in sex determination has been confirmed only in Apis mellifera. We identified a tra2 orthologue (Lc-tra2), a tra orthologue (Lc-tra) and a tra paralogue (Lc-traB) in the genome of Leptopilina clavipes (Hymenoptera: Cynipidae). We compared the sequence and structural conservation of these genes between sexual (arrhenotokous) and asexual all-female producing (thelytokous) individuals. Lc-tra is sex-specifically spliced in adults consistent with its orthologous function. The male-specific regions of Lc-tra are conserved in both reproductive modes. The paralogue Lc-traB lacks the genomic region coding for male-specific exons and can only be translated into a full-length TRA-like peptide sequence. Furthermore, unlike LC-TRA, the LC-TRAB interstrain sequence variation is not differentiated into a sexual and an asexual haplotype. The LC-TRAB protein interacts with LC-TRA as well as LC-TRA2. This suggests that Lc-traB functions as a conserved element in sex determination of sexual and asexual individuals.

Evolutionary and developmental dynamics of sex-biased gene expression in common frogs with proto-Y chromosomes

BACKGROUND

The patterns of gene expression on highly differentiated sex chromosomes differ drastically from those on autosomes, due to sex-specific patterns of selection and inheritance. As a result, X chromosomes are often enriched in female-biased genes (feminization) and Z chromosomes in male-biased genes (masculinization). However, it is not known how quickly sexualization of gene expression and transcriptional degeneration evolve after sex-chromosome formation. Furthermore, little is known about how sex-biased gene expression varies throughout development.

RESULTS

We sample a population of common frogs (Rana temporaria) with limited sex-chromosome differentiation (proto-sex chromosome), leaky genetic sex determination evidenced by the occurrence of XX males, and delayed gonadal development, meaning that XY individuals may first develop ovaries before switching to testes. Using high-throughput RNA sequencing, we investigate the dynamics of gene expression throughout development, spanning from early embryo to froglet stages. Our results show that sex-biased expression affects different genes at different developmental stages and increases during development, reaching highest levels in XX female froglets. Additionally, sex-biased gene expression depends on phenotypic, rather than genotypic sex, with similar expression in XX and XY males; correlates with gene evolutionary rates; and is not localized to the proto-sex chromosome nor near the candidate sex-determining gene Dmrt1.

CONCLUSIONS

The proto-sex chromosome of common frogs does not show evidence of sexualization of gene expression, nor evidence for a faster rate of evolution. This challenges the notion that sexually antagonistic genes play a central role in the initial stages of sex-chromosome evolution.

Do social insects support Haig's kin theory for the evolution of genomic imprinting?

Although numerous imprinted genes have been described in several lineages, the phenomenon of genomic imprinting presents a peculiar evolutionary problem. Several hypotheses have been proposed to explain gene imprinting, the most supported being Haig's kinship theory. This theory explains the observed pattern of imprinting and the resulting phenotypes as a competition for resources between related individuals, but despite its relevance it has not been independently tested. Haig's theory predicts that gene imprinting should be present in eusocial insects in many social scenarios. These lineages are therefore ideal for testing both the theory's predictions and the mechanism of gene imprinting. Here we review the behavioral evidence of genomic imprinting in eusocial insects, the evidence of a mechanism for genomic imprinting and finally we evaluate recent results showing parent of origin allele specific expression in honeybees in the light of Haig's theory.

The Maternal Effect Gene Wds Controls Wolbachia Titer in Nasonia

Maternal transmission of intracellular microbes is pivotal in establishing long-term, intimate symbioses. For germline microbes that exert negative reproductive effects on their hosts, selection can theoretically favor the spread of host genes that counteract the microbe's harmful effects. Here, we leverage a major difference in bacterial (Wolbachia pipientis) titers between closely related wasp species with forward genetic, transcriptomic, and cytological approaches to map two quantitative trait loci that suppress bacterial titers via a maternal effect. Fine mapping and knockdown experiments identify the gene Wolbachia density suppressor (Wds), which dominantly suppresses bacterial transmission from mother to embryo. Wds evolved by lineage-specific non-synonymous changes driven by positive selection. Collectively, our findings demonstrate that a genetically simple change arose by positive Darwinian selection in less than a million years to regulate maternally transmitted bacteria via a dominant, maternal effect gene.

The doublesex gene integrates multi-locus complementary sex determination signals in the Japanese ant, Vollenhovia emeryi

A female diploid, male haploid sex determination system (haplodiploidy) is found in hymenopteran taxa, such as ants, wasps, bees and sawflies. In this system, a single, complementary sex-determination (sl-CSD) locus functions as the primary sex-determination signal. In the taxa that has evolved this system, females and males are heterozygous and hemi/homozygous at the CSD locus, respectively. While the sl-CSD system enables females to alter sex ratios in the nest, it carries a high cost in terms of inbreeding, as individuals that are homozygous at the CSD locus become sterile diploid males. To counter this risk, some of hymenopteran species have evolved a multi-locus CSD (ml-CSD) system, which effectively reduces the proportion of sterile males. However, the mechanism by which these multiple primary signals are integrated and how they affect the terminal sex-differentiation signal of the molecular cascade have not yet been clarified. To resolve these questions, we examined the molecular cascade in the Japanese ant Vollenhovia emeryi, which we previously confirmed has two CSD loci. Here, we showed that the sex-determination gene, doublesex (dsx), which is highly conserved among phylogenetically distant taxa, is responsible for integrating two CSD signals in V. emeryi. After identifying and characterizing dsx, genotypes containing two CSD loci and splicing patterns of dsx were found to correspond to the sexual phenotype, suggesting that two primary signals are integrated into dsx. These findings will facilitate future molecular and functional studies of the sex determination cascade in V. emeryi, and shed light on the evolution and diversification of sex determination systems in insects.

Maternal provision of non-sex-specific transformer messenger RNA in sex determination of the wasp Asobara tabida

In many insect species maternal provision of sex-specifically spliced messenger RNA (mRNA) of sex determination genes is an essential component of the sex determination mechanism. In haplodiploid Hymenoptera, maternal provision in combination with genomic imprinting has been shown for the parasitoid Nasonia vitripennis, known as maternal effect genomic imprinting sex determination (MEGISD). Here, we characterize the sex determination cascade of Asobara tabida, another hymenopteran parasitoid. We show the presence of the conserved sex determination genes doublesex (dsx), transformer (tra) and transformer-2 (tra2) orthologues in As. tabida. Of these, At-dsx and At-tra are sex-specifically spliced, indicating a conserved function in sex determination. At-tra and At-tra2 mRNA is maternally provided to embryos but, in contrast to most studied insects, As. tabida females transmit a non-sex-specific splice form of At-tra mRNA to the eggs. In this respect, As. tabida sex determination differs from the MEGISD mechanism. How the paternal genome can induce female development in the absence of maternal provision of sex-specifically spliced mRNA remains an open question. Our study reports a hitherto unknown variant of maternal effect sex determination and accentuates the diversity of insect sex determination mechanisms.

Embryo Microinjection and Transplantation Technique for Nasonia vitripennis Genome Manipulation

The jewel wasp Nasonia vitripennis has emerged as an effective model system for the study of processes including sex determination, haplo-diploid sex determination, venom synthesis, and host-symbiont interactions, among others. A major limitation of working with this organism is the lack of effective protocols to perform directed genome modifications. An important part of genome modification is delivery of editing reagents, including CRISPR/Cas9 molecules, into embryos through microinjection. While microinjection is well established in many model organisms, this technique is particularly challenging to perform in N. vitripennis primarily due to its small embryo size, and the fact that embryonic development occurs entirely within a parasitized blowfly pupa. The following procedure overcomes these significant challenges while demonstrating a streamlined, visual procedure for effectively removing wasp embryos from parasitized host pupae, microinjecting them, and carefully transplanting them back into the host for continuation and completion of development. This protocol will strongly enhance the capability of research groups to perform advanced genome modifications in this organism.

Maternal provision of transformer-2 is required for female development and embryo viability in the wasp Nasonia vitripennis

In insect sex determination a primary signal starts the genetic sex determination cascade that, in most insect orders, is subsequently transduced down the cascade by a transformer (tra) ortholog. Only a female-specifically spliced tra mRNA yields a functional TRA-protein that forms a complex with TRA2, encoded by a transformer-2 (tra2) ortholog, to act as a sex specific splicing regulator of the downstream transcription factors doublesex (dsx) and fruitless (fru). Here, we identify the tra2 ortholog of the haplodiploid parasitoid wasp N. vitripennis (Nv-tra2) and confirm its function in N. vitripennis sex determination. Knock down of Nv-tra2 by parental RNA interference (pRNAi) results in complete sex reversal of diploid offspring from female to male, indicating the requirement of Nv-tra2 for female sex determination. As Nv-tra2 pRNAi leads to frequent lethality in early developmental stages, maternal provision of Nv-tra2 transcripts is apparently also required for another, non-sex determining function during embryogenesis. In addition, lethality following Nv-tra2 pRNAi appears more pronounced in diploid than in haploid offspring. This diploid lethal effect was also observed following Nv-tra pRNAi, which served as a positive control in our experiments. As diploid embryos from fertilized eggs have a paternal chromosome set in addition to the maternal one, this suggests that either the presence of this paternal chromosome set or the dosage effect resulting from the diploid state is incompatible with the induced male development in N. vitripennis caused by either Nv-tra2 or Nv-tra pRNAi. The role of Nv-tra2 in activating the female sex determination pathway yields more insight into the sex determination mechanism of Nasonia.

Novel approach to heritability detection suggests robustness to paternal genotype in a complex morphological trait

Heritable variation is essential for evolution by natural selection. In Neotropical army ants, the ecological role of a given species is linked intimately to the morphological variation within the sterile worker caste. Furthermore, the army ant Eciton burchellii is highly polyandrous, presenting a unique opportunity to explore heritability of morphological traits among related workers sharing the same colonial environment. In order to exploit the features of this organismal system, we generated a large genetic and morphological dataset and applied our new method that employs geometric morphometrics (GM) to detect the heritability of complex morphological traits. After validating our approach with an existing dataset of known heritability, we simulated our ability to detect heritable variation given our sampled genotypes, demonstrating the method can robustly recover heritable variation of small effect size. Using this method, we tested for genetic caste determination and heritable morphological variation using genetic and morphological data on 216 individuals of E. burchellii. Results reveal this ant lineage (1) has the highest mating frequency known in ants, (2) demonstrates no paternal genetic caste determination, and (3) suggests a lack of heritable morphological variation in this complex trait associated with paternal genotype. We recommend this method for leveraging the increased resolution of GM data to explore and understand heritable morphological variation in nonmodel organisms.

Orientation of Navel Orangeworm (Lepidoptera: Pyralidae) Larvae and Adults Toward Volatiles Associated With Almond Hull Split and Aspergillus flavus

The navel orangeworm Amyelois transitella (Walker, 1863, Lepidoptera: Pyralidae), a pest of California tree nuts, is associated with the fungus Aspergillus flavus, and previous research suggests these species are facultative mutualists. Because navel orangeworm larvae exhibit improved performance on diets containing this fungus, orientation toward hostplants infected with A. flavus may be adaptive. We conducted behavioral assays to determine if larvae respond to chemical cues produced by almond hull split and fungal infection. In petri dish arenas, larvae showed a preference for 1-octen-3-ol and 2-phenylethanol, volatiles characteristic of damaged plants, as well as methanolic extracts of almond meal with 1-octen-3-ol and the fungal volatile conophthorin. In contrast, larvae displayed aversion to ethyl benzoate, an inhibitor of fungal growth. When we assessed oviposition behavior relative to substrates with and without A. flavus, females laid almost twice as many eggs near inoculated surfaces. Moreover, an average of 63% of eggs laid near inoculated substrates were fertilized, compared with 24% of eggs near uninoculated sites. We also tested the hypothesis that unfertilized eggs are laid on nutrient-poor substrates to provide supplemental nutrition for larvae in an assay comparing larval survivorship in the presence and absence of unfertilized eggs. Neonates given eggs survived 2.5 times longer on an average than unprovisioned neonates (208.8 h vs. 85.2 h), indicating that this species may compensate with cannibalism for oviposition on lower-quality food sources. We conclude that larvae orient to probable host plant and fungal volatiles associated with hull split and document a possible strategy for larvae to establish on low-quality hosts.

Promiscuity in post-transcriptional control of gene expression: Drosophila sex-lethal and its regulatory partnerships

The Drosophila RNA‐binding protein Sex‐lethal (Sxl) is a potent post‐transcriptional regulator of gene expression that controls female development. It regulates the expression of key factors involved in sex‐specific differences in morphology, behavior, and dosage compensation. Functional Sxl protein is only expressed in female flies, where it binds to U‐rich RNA motifs present in its target mRNAs to regulate their fate. Sxl is a very versatile regulator that, by shuttling between the nucleus and the cytoplasm, can regulate almost all aspects of post‐transcriptional gene expression including RNA processing, nuclear export, and translation. For these functions, Sxl employs multiple interactions to either antagonize RNA‐processing factors or to recruit various coregulators, thus allowing it to establish a female‐specific gene expression pattern. Here, we summarize the current knowledge about Sxl function and review recent mechanistic and structural studies that further our understanding of how such a seemingly ‘simple’ RNA‐binding protein can exert this plethora of different functions.

Ploidy has little effect on timing early embryonic events in the haplo-diploid wasp Nasonia

The nucleocytoplasmic (N/C) ratio plays a prominent role in the maternal-to-zygotic transition (MZT) in many animals. The effect of the N/C ratio on cell-cycle lengthening and zygotic genome activation (ZGA) has been studied extensively in Drosophila, where haploid embryos experience an additional division prior to completing cellularization and triploid embryos cellularize precociously by one division. In this study, we set out to understand how the obligate difference in ploidy in the haplodiploid wasp, Nasonia, affects the MZT and which aspects of the Drosophila MZT are conserved. While subtle differences in early embryonic development were observed in comparisons among haploid, diploid, and triploid embryos, in all cases embryos cellularize at cell cycle 12. When ZGA was inhibited, both diploid female, and haploid male, embryos went through 12 syncytial divisions and failed to cellularize before dying without further divisions. We also found that key players of the Drosophila MZT are conserved in Nasonia but have novel expression patterns. Our results suggest that zygotically expressed genes have a reduced role in determining the timing of cellularization in Nasonia relative to Drosophila, and that a stronger reliance on a maternal timer is more compatible with species where variations in embryonic ploidy are obligatory.

Generation of heritable germline mutations in the jewel wasp Nasonia vitripennis using CRISPR/Cas9

The revolutionary RNA-guided endonuclease CRISPR/Cas9 system has proven to be a powerful tool for gene editing in a plethora of organisms. Here, utilizing this system we developed an efficient protocol for the generation of heritable germline mutations in the parasitoid jewel wasp, Nasonia vitripennis, a rising insect model organism for the study of evolution, development of axis pattern formation, venom production, haplo-diploid sex determination, and host–symbiont interactions. To establish CRISPR-directed gene editing in N. vitripennis, we targeted a conserved eye pigmentation gene cinnabar, generating several independent heritable germline mutations in this gene. Briefly, to generate these mutants, we developed a protocol to efficiently collect N. vitripennis eggs from a parasitized flesh fly pupa, Sarcophaga bullata, inject these eggs with Cas9/guide RNA mixtures, and transfer injected eggs back into the host to continue development. We also describe a flow for screening mutants and establishing stable mutant strains through genetic crosses. Overall, our results demonstrate that the CRISPR/Cas9 system is a powerful tool for genome manipulation in N. vitripennis, with strong potential for expansion to target critical genes, thus allowing for the investigation of several important biological phenomena in this organism.

The autoregulatory loop: A common mechanism of regulation of key sex determining genes in insects

Sex determination in most insects is structured as a gene cascade, wherein a primary signal is passed through a series of sex-determining genes, culminating in a downstream double-switch known as doublesex that decides the sexual fate of the embryo. From the literature available on sex determination cascades, it becomes apparent that sex determination mechanisms have evolved rapidly. The primary signal that provides the cue to determine the sex of the embryo varies remarkably, not only among taxa, but also within taxa. Furthermore, the upstream key gene in the cascade also varies between species and even among closely related species. The order Insecta alone provides examples of astoundingly complex diversity of upstream key genes in sex determination mechanisms. Besides, unlike key upstream genes, the downstream double-switch gene is alternatively spliced to form functional sex-specific isoforms. This sex-specific splicing is conserved across insect taxa. The genes involved in the sex determination cascade such as Sex-lethal (Sxl) in Drosophila melanogaster, transformer (tra) in many other dipterans, coleopterans and hymenopterans, Feminizer (fem) in Apis mellifera, and IGF-II mRNA-binding protein (Bmimp) in Bombyx mori are reported to be regulated by an autoregulatory positive feedback loop. In this review, by taking examples from various insects, we propose the hypothesis that autoregulatory loop mechanisms of sex determination might be a general strategy. We also discuss the possible reasons for the evolution of autoregulatory loops in sex determination cascades and their impact on binary developmental choices.

Incomplete removal of Wolbachia with tetracycline has two-edged reproductive effects in the thelytokous wasp Encarsia formosa (Hymenoptera: Aphelinidae)

Wolbachia pipientis are intracellular endosymbionts that induce parthenogenesis in the parasitoid Encarsia formosa. Previous studies that focused on effects of Wolbachia on the wasp usually used tetracycline to remove Wolbachia without concern for the joint influences of tetracycline and Wolbachia. Here we treated the wasps (F0 lines) with tetracycline to produce offspring (F1 lines) which were not fed tetracycline to avoid antibiotic influence. The quantitative data and fluorescence in situ hybridization showed that Wolbachia titers were reduced but not totally removed. The Wolbachia that infected the male offspring were unpredictably detected. Low dose tetracycline enhanced the fertility of 2-day-old F0 wasps after 24 h of treatment; however, compared with controls, the oocyte load of 3- to 6-day-old tetracycline-treated wasps decreased day by day, and tetracycline reduced the longevity of the wasps. The fecundity of controls was significantly higher than that of the treated F1-10 and F1-20 generations. Gene expression of vitellogenin reflected the same trend as that of wasp fecundities in both F0 and F1 lines. Moreover, female offspring proportions of F0 and F1 lines were related to the titer of infected Wolbachia, demonstrating that Wolbachia titer affected the sex determination of E. formosa.

Bombyx mori P-element Somatic Inhibitor (BmPSI) Is a Key Auxiliary Factor for Silkworm Male Sex Determination

Manipulation of sex determination pathways in insects provides the basis for a wide spectrum of strategies to benefit agriculture and public health. Furthermore, insects display a remarkable diversity in the genetic pathways that lead to sex differentiation. The silkworm, Bombyx mori, has been cultivated by humans as a beneficial insect for over two millennia, and more recently as a model system for studying lepidopteran genetics and development. Previous studies have identified the B. mori Fem piRNA as the primary female determining factor and BmMasc as its downstream target, while the genetic scenario for male sex determination was still unclear. In the current study, we exploite the transgenic CRISPR/Cas9 system to generate a comprehensive set of knockout mutations in genes BmSxl, Bmtra2, BmImp, BmImp^M, BmPSI and BmMasc, to investigate their roles in silkworm sex determination. Absence of Bmtra2 results in the complete depletion of Bmdsx transcripts, which is the conserved downstream factor in the sex determination pathway, and induces embryonic lethality. Loss of BmImp or BmImp^M function does not affect the sexual differentiation. Mutations in BmPSI and BmMasc genes affect the splicing of Bmdsx and the female reproductive apparatus appeared in the male external genital. Intriguingly, we identify that BmPSI regulates expression of BmMasc, BmImp^M and Bmdsx, supporting the conclusion that it acts as a key auxiliary factor in silkworm male sex determination.

The sex determination system extremely diverse among organisms including insects in which even each order occupy a different manner of sex determination. The silkworm, Bombyx mori, is a lepidopteran model insect with economic importance. The mechanism of the silkworm sex determination has been in mystery for a long time until a Fem piRNA was identified as the primary female sex determinator recently. However, genetic and phenotypic proofs are urgently needed to fully exploit the mechanism, especially of the male sex determination. In the current study, we provided comprehensively genetic evidences by generating CRISPR/Cas9-mediated knockout mutations for those genes BmSxl, Bmtra2, BmImp, BmImp^M, BmPSI and BmMasc, which were considered to be involved in insect sex determination. The results showed that mutations of BmSxl, BmImp and BmImp^M had no physiological and morphological effects on the sexual development while Bmtra2 depletion caused Bmdsx splicing disappeared and induced embryonic lethality. Importantly, the BmPSI regulates expression of BmMasc, BmImp^M and Bmdsx, supporting the conclusion that it acts as a key auxiliary factor to regulate the male sex determination in the silkworm.

Sex Determination, Sex Chromosomes, and Karyotype Evolution in Insects

Insects harbor a tremendous diversity of sex determining mechanisms both within and between groups. For example, in some orders such as Hymenoptera, all members are haplodiploid, whereas Diptera contain species with homomorphic as well as male and female heterogametic sex chromosome systems or paternal genome elimination. We have established a large database on karyotypes and sex chromosomes in insects, containing information on over 13000 species covering 29 orders of insects. This database constitutes a unique starting point to report phylogenetic patterns on the distribution of sex determination mechanisms, sex chromosomes, and karyotypes among insects and allows us to test general theories on the evolutionary dynamics of karyotypes, sex chromosomes, and sex determination systems in a comparative framework. Phylogenetic analysis reveals that male heterogamety is the ancestral mode of sex determination in insects, and transitions to female heterogamety are extremely rare. Many insect orders harbor species with complex sex chromosomes, and gains and losses of the sex-limited chromosome are frequent in some groups. Haplodiploidy originated several times within insects, and parthenogenesis is rare but evolves frequently. Providing a single source to electronically access data previously distributed among more than 500 articles and books will not only accelerate analyses of the assembled data, but also provide a unique resource to guide research on which taxa are likely to be informative to address specific questions, for example, for genome sequencing projects or large-scale comparative studies.

Complementary sex determination, inbreeding depression and inbreeding avoidance in a gregarious sawfly

Although most Hymenoptera reproduce via arrhenotokous haplodiploidy, the underlying genetic mechanisms vary. Of these, the most widespread mechanism appears to be single-locus complementary sex determination (sl-CSD), in which individuals that are diploid and heterozygous at a sex-determining locus are female, and individuals that are homozygous or hemizygous are male. Because inbreeding increases the probability of producing diploid males, which are often sterile or inviable, sl-CSD can generate substantial inbreeding depression. To counteract this, Hymenoptera with traits that promote inbreeding, such as gregariousness, may evolve one or more of the following: inbreeding avoidance, functional diploid males or alternative sex determination mechanisms. Here, we investigate sex determination, inbreeding depression and inbreeding avoidance in Neodiprion lecontei, a gregarious, pine-feeding sawfly in the family Diprionidae. First, via inbreeding experiments and flow cytometry, we demonstrate that this species has CSD. By modeling expected sex ratios under different conditions, we also show that our data are consistent with sl-CSD. Second, via tracking survival in inbred and outbred families, we demonstrate that inbred families have reduced larval survival and that this mortality is partly attributable to the death of diploid males. Third, using a no-choice mating assay, we demonstrate that females are less willing to mate with siblings than nonsiblings. Together, these results suggest that inbreeding depression stemming from CSD has shaped mating behavior in N. lecontei. These results also set the stage for future comparative work that will investigate the interplay between sex determination, ecology and behavior in additional diprionid species that vary in larval gregariousness.

Transcriptome profiling of Diachasmimorpha longicaudata towards useful molecular tools for population management

Background

Diachasmimorpha longicaudata (Hymenoptera: Braconidae) is a solitary parasitoid of Tephritidae (Diptera) fruit flies of economic importance currently being mass-reared in bio-factories and successfully used worldwide. A peculiar biological aspect of Hymenoptera is its haplo-diploid life cycle, where females (diploid) develop from fertilized eggs and males (haploid) from unfertilized eggs. Diploid males were described in many species and recently evidenced in D. longicaudata by mean of inbreeding studies. Sex determination in this parasitoid is based on the Complementary Sex Determination (CSD) system, with alleles from at least one locus involved in early steps of this pathway. Since limited information is available about genetics of this parasitoid species, a deeper analysis on D. longicaudata’s genomics is required to provide molecular tools for achieving a more cost effective production under artificial rearing conditions.

Results

We report here the first transcriptome analysis of male-larvae, adult females and adult males of D. longicaudata using 454-pyrosequencing. A total of 469766 reads were analyzed and 8483 high-quality isotigs were assembled. After functional annotation, a total of 51686 unigenes were produced, from which, 7021 isotigs and 20227 singletons had at least one BLAST hit against the NCBI non-redundant protein database. A preliminary comparison of adult female and male evidenced that 98 transcripts showed differential expression profiles, with at least a 10-fold difference. Among the functionally annotated transcripts we detected four sequences potentially involved in sex determination and three homologues to two known genes involved in the sex determination cascade. Finally, a total of 4674SimpleSequence Repeats (SSRs) were in silico identified and characterized.

Conclusion

The information obtained here will significantly contribute to the development of D. longicaudata functional genomics, genetics and population-based genome studies. Thousands of new microsatellite markers were identified as toolkits for population genetics analysis. The transcriptome characterized here is the starting point to elucidate the molecular bases of the sex determination mechanism in this species.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2759-2) contains supplementary material, which is available to authorized users.

Disentangling a Holobiont - Recent Advances and Perspectives in Nasonia Wasps

The parasitoid wasp genus Nasonia (Hymenoptera: Chalcidoidea) is a well-established model organism for insect development, evolutionary genetics, speciation, and symbiosis. The host-microbiota assemblage which constitutes the Nasonia holobiont (a host together with all of its associated microbes) consists of viruses, two heritable bacterial symbionts and a bacterial community dominated in abundance by a few taxa in the gut. In the wild, all four Nasonia species are systematically infected with the obligate intracellular bacterium Wolbachia and can additionally be co-infected with Arsenophonus nasoniae. These two reproductive parasites have different transmission modes and host manipulations (cytoplasmic incompatibility vs. male-killing, respectively). Pioneering studies on Wolbachia in Nasonia demonstrated that closely related Nasonia species harbor multiple and mutually incompatible Wolbachia strains, resulting in strong symbiont-mediated reproductive barriers that evolved early in the speciation process. Moreover, research on host-symbiont interactions and speciation has recently broadened from its historical focus on heritable symbionts to the entire microbial community. In this context, each Nasonia species hosts a distinguishable community of gut bacteria that experiences a temporal succession during host development and members of this bacterial community cause strong hybrid lethality during larval development. In this review, we present the Nasonia species complex as a model system to experimentally investigate questions regarding: (i) the impact of different microbes, including (but not limited to) heritable endosymbionts, on the extended phenotype of the holobiont, (ii) the establishment and regulation of a species-specific microbiota, (iii) the role of the microbiota in speciation, and (iv) the resilience and adaptability of the microbiota in wild populations subjected to different environmental pressures. We discuss the potential for easy microbiota manipulations in Nasonia as a promising experimental approach to address these fundamental aspects.

Diploid male production correlates with genetic diversity in the parasitoid wasp Venturia canescens: a genetic approach with new microsatellite markers

Sex determination is ruled by haplodiploidy in Hymenoptera, with haploid males arising from unfertilized eggs and diploid females from fertilized eggs. However, diploid males with null fitness are produced under complementary sex determination (CSD), when individuals are homozygous for this locus. Diploid males are expected to be more frequent in genetically eroded populations (such as islands and captive populations), as genetic diversity at the csd locus should be low. However, only a few studies have focused on the relation between population size, genetic diversity, and the proportion of diploid males in the field. Here, we developed new microsatellite markers in order to assess and compare genetic diversity and diploid male proportion (DMP) in populations from three distinct habitat types - mainland, island, or captive -, in the parasitoid wasp Venturia canescens. Eroded genetic diversity and higher DMP were found in island and captive populations, and habitat type had large effect on genetic diversity. Therefore, DMP reflects the decreasing genetic diversity in small and isolated populations. Thus, Hymenopteran populations can be at high extinction risk due to habitat destruction or fragmentation.

Transgenic Expression of the piRNA-Resistant Masculinizer Gene Induces Female-Specific Lethality and Partial Female-to-Male Sex Reversal in the Silkworm, Bombyx mori

In Bombyx mori (B. mori), Fem piRNA originates from the W chromosome and is responsible for femaleness. The Fem piRNA-PIWI complex targets and cleaves mRNAs transcribed from the Masc gene. Masc encodes a novel CCCH type zinc-finger protein and is required for male-specific splicing of B. mori doublesex (Bmdsx) transcripts. In the present study, several silkworm strains carrying a transgene, which encodes a Fem piRNA-resistant Masc mRNA (Masc-R), were generated. Forced expression of the Masc-R transgene caused female-specific lethality during the larval stages. One of the Masc-R strains weakly expressed Masc-R in various tissues. Females heterozygous for the transgene expressed male-specific isoform of the Bombyx homolog of insulin-like growth factor II mRNA-binding protein (ImpM) and Bmdsx. All examined females showed a lower inducibility of vitellogenin synthesis and exhibited abnormalities in the ovaries. Testis-like tissues were observed in abnormal ovaries and, notably, the tissues contained considerable numbers of sperm bundles. Homozygous expression of the transgene resulted in formation of the male-specific abdominal segment in adult females and caused partial male differentiation in female genitalia. These results strongly suggest that Masc is an important regulatory gene of maleness in B. mori.

Role of Bmznf-2, a Bombyx mori CCCH zinc finger gene, in masculinisation and differential splicing of Bmtra-2

Deciphering the regulatory factors involved in Bombyx mori sex determination has been a puzzle, challenging researchers for nearly a century now. The pre-mRNA of B. mori doublesex (Bmdsx), a master regulator gene of sexual differentiation, is differentially spliced, producing Bmdsxm and Bmdsxf transcripts in males and females respectively. The putative proteins encoded by these differential transcripts orchestrate antagonistic functions, which lead to sexual differentiation. A recent study in B. mori illustrated the role of a W-derived fem piRNA in conferring femaleness. In females, the fem piRNA was shown to suppress the activity of a Z-linked CCCH type zinc finger (znf) gene, Masculiniser (masc), which indirectly promotes the Bmdsxm type of splicing. In this study, we report a novel autosomal (Chr 25) CCCH type znf motif encoding gene Bmznf-2 as one of the potential factors in the Bmdsx sex specific differential splicing, and we also provide insights into its role in the alternative splicing of Bmtra2 by using ovary derived BmN cells. Over-expression of Bmznf-2 induced Bmdsxm type of splicing (masculinisation) with a correspondingly reduced expression of Bmdsxf type isoform in BmN cells. Further, the site-directed mutational studies targeting the tandem CCCH znf motifs revealed their indispensability in the observed phenotype of masculinisation. Additionally, the dual luciferase assays in BmN cells using 5' UTR region of the Bmznf-2 strongly implied the existence of a translational repression over this gene. From these findings, we propose Bmznf-2 to be one of the potential factors of masculinisation similar to Masc. From the growing number of Bmdsx splicing regulators, we assume that the sex determination cascade of B. mori is quite intricate in nature; hence, it has to be further investigated for its comprehensive understanding.

Identification and functional analyses of sex determination genes in the sexually dimorphic stag beetle Cyclommatus metallifer

BACKGROUND

Genes in the sex determination pathway are important regulators of sexually dimorphic animal traits, including the elaborate and exaggerated male ornaments and weapons of sexual selection. In this study, we identified and functionally analyzed members of the sex determination gene family in the golden metallic stag beetle Cyclommatus metallifer, which exhibits extreme differences in mandible size between males and females.

RESULTS

We constructed a C. metallifer transcriptomic database from larval and prepupal developmental stages and tissues of both males and females. Using Roche 454 pyrosequencing, we generated a de novo assembled database from a total of 1,223,516 raw reads, which resulted in 14,565 isotigs (putative transcript isoforms) contained in 10,794 isogroups (putative identified genes). We queried this database for C. metallifer conserved sex determination genes and identified 14 candidate sex determination pathway genes. We then characterized the roles of several of these genes in development of extreme sexual dimorphic traits in this species. We performed molecular expression analyses with RT-PCR and functional analyses using RNAi on three C. metallifer candidate genes--Sex-lethal (CmSxl), transformer-2 (Cmtra2), and intersex (Cmix). No differences in expression pattern were found between the sexes for any of these three genes. In the RNAi gene-knockdown experiments, we found that only the Cmix had any effect on sexually dimorphic morphology, and these mimicked the effects of Cmdsx knockdown in females. Knockdown of CmSxl had no measurable effects on stag beetle phenotype, while knockdown of Cmtra2 resulted in complete lethality at the prepupal period. These results indicate that the roles of CmSxl and Cmtra2 in the sex determination cascade are likely to have diverged in stag beetles when compared to Drosophila. Our results also suggest that Cmix has a conserved role in this pathway. In addition to those three genes, we also performed a more complete functional analysis of the C. metallifer dsx gene (Cmdsx) to identify the isoforms that regulate dimorphism more fully using exon-specific RNAi. We identified a total of 16 alternative splice variants of the Cmdsx gene that code for up to 14 separate exons. Despite the variation in RNA splice products of the Cmdsx gene, only four protein isoforms are predicted. The results of our exon-specific RNAi indicated that the essential CmDsx isoform for postembryonic male differentiation is CmDsxB, whereas postembryonic female specific differentiation is mainly regulated by CmDsxD.

CONCLUSIONS

Taken together, our results highlight the importance of studying the function of highly conserved sex determination pathways in numerous insect species, especially those with dramatic and exaggerated sexual dimorphism, because conservation in protein structure does not always translate into conservation in downstream function.

Transcriptome Analysis of Honeybee (Apis Mellifera) Haploid and Diploid Embryos Reveals Early Zygotic Transcription during Cleavage

In honeybees, the haplodiploid sex determination system promotes a unique embryogenesis process wherein females develop from fertilized eggs and males develop from unfertilized eggs. However, the developmental strategies of honeybees during early embryogenesis are virtually unknown. Similar to most animals, the honeybee oocytes are supplied with proteins and regulatory elements that support early embryogenesis. As the embryo develops, the zygotic genome is activated and zygotic products gradually replace the preloaded maternal material. The analysis of small RNA and mRNA libraries of mature oocytes and embryos originated from fertilized and unfertilized eggs has allowed us to explore the gene expression dynamics in the first steps of development and during the maternal-to-zygotic transition (MZT). We localized a short sequence motif identified as TAGteam motif and hypothesized to play a similar role in honeybees as in fruit flies, which includes the timing of early zygotic expression (MZT), a function sustained by the presence of the zelda ortholog, which is the main regulator of genome activation. Predicted microRNA (miRNA)-target interactions indicated that there were specific regulators of haploid and diploid embryonic development and an overlap of maternal and zygotic gene expression during the early steps of embryogenesis. Although a number of functions are highly conserved during the early steps of honeybee embryogenesis, the results showed that zygotic genome activation occurs earlier in honeybees than in Drosophila based on the presence of three primary miRNAs (pri-miRNAs) (ame-mir-375, ame-mir-34 and ame-mir-263b) during the cleavage stage in haploid and diploid embryonic development.

Molecular cloning of the sex-related gene PSI in Bemisia tabaci and its alternative splicing properties

The P-element somatic inhibitor (PSI) is gene known to regulate the transcription of doublesex (dsx) when transformer (tra) is absent in Bombyx mori. In this study, we identified and characterized a PSI homolog in Bemisia tabaci (BtPSI). BtPSI cDNA had a total length of 5700 bp and contained a predicted open reading frame (ORF) of 2208 nucleotides encoding for 735 amino acids. Multiple sequence alignments of the common regions of PSI proteins from B. tabaci and five other insect species revealed a high degree of sequence conservation. BtPSI is expressed in all stages of B. tabaci development, and expression did not significantly differ between female and male adult. A total of 92 BtPSI isoforms (78 in female and 22 in male) were identified, and a marker indicating the female-specific form was found. These results increase the understanding of genes that may determine sex in B. tabaci and provide a foundation for research on the sex determination mechanism in this insect.

The expanding genetic toolbox of the wasp Nasonia vitripennis and its relatives

The parasitoid wasp Nasonia represents a genus of four species that is emerging as a powerful genetic model system that has made and will continue to make important contributions to our understanding of evolutionary biology, development, ecology, and behavior. Particularly powerful are the haplodiploid genetics of the system, which allow some of the advantages of microbial genetics to be applied to a complex multicellular eukaryote. In addition, fertile, viable hybrids can be made among the four species in the genus. This makes Nasonia exceptionally well suited for evolutionary genetics approaches, especially when combined with its haploid genetics and tractability in the laboratory. These features are complemented by an expanding array of genomic, transcriptomic, and functional resources, the application of which has already made Nasonia an important model system in such emerging fields as evolutionary developmental biology and microbiomics. This article describes the genetic and genomic advantages of Nasonia wasps and the resources available for their genetic analysis.

Stable eusociality via maternal manipulation when resistance is costless

In many eusocial species, queens use pheromones to influence offspring to express worker phenotypes. Although evidence suggests that queen pheromones are honest signals of the queen's reproductive health, here I show that queen's honest signalling can result from ancestral maternal manipulation. I develop a mathematical model to study the coevolution of maternal manipulation, offspring resistance to manipulation and maternal resource allocation. I assume that (i) maternal manipulation causes offspring to be workers against offspring's interests; (ii) offspring can resist at no direct cost, as is thought to be the case with pheromonal manipulation; and (iii) the mother chooses how much resource to allocate to fertility and maternal care. In the coevolution of these traits, I find that maternal care decreases, thereby increasing the benefit that offspring obtain from help, which in the long run eliminates selection for resistance. Consequently, ancestral maternal manipulation yields stable eusociality despite costless resistance. Additionally, ancestral manipulation in the long run becomes honest signalling that induces offspring to help. These results indicate that both eusociality and its commonly associated queen honest signalling can be likely to originate from ancestral manipulation.

Genome sequence of the Asian Tiger mosquito, Aedes albopictus, reveals insights into its biology, genetics, and evolution

The Asian tiger mosquito, Aedes albopictus, is a highly successful invasive species that transmits a number of human viral diseases, including dengue and Chikungunya fevers. This species has a large genome with significant population-based size variation. The complete genome sequence was determined for the Foshan strain, an established laboratory colony derived from wild mosquitoes from southeastern China, a region within the historical range of the origin of the species. The genome comprises 1,967 Mb, the largest mosquito genome sequenced to date, and its size results principally from an abundance of repetitive DNA classes. In addition, expansions of the numbers of members in gene families involved in insecticide-resistance mechanisms, diapause, sex determination, immunity, and olfaction also contribute to the larger size. Portions of integrated flavivirus-like genomes support a shared evolutionary history of association of these viruses with their vector. The large genome repertory may contribute to the adaptability and success of Ae. albopictus as an invasive species.

Identification of Genes Uniquely Expressed in the Germ-Line Tissues of the Jewel Wasp Nasonia vitripennis

The jewel wasp Nasonia vitripennis is a rising model organism for the study of haplo-diploid reproduction characteristic of hymenopteran insects, which include all wasps, bees, and ants. We performed transcriptional profiling of the ovary, the female soma, and the male soma of N. vitripennis to complement a previously existing transcriptome of the wasp testis. These data were deposited into an open-access genome browser for visualization of transcripts relative to their gene models. We used these data to identify the assemblies of genes uniquely expressed in the germ-line tissues. We found that 156 protein-coding genes are expressed exclusively in the wasp testis compared with only 22 in the ovary. Of the testis-specific genes, eight are candidates for male-specific DNA packaging proteins known as protamines. We found very similar expression patterns of centrosome associated genes in the testis and ovary, arguing that de novo centrosome formation, a key process for development of unfertilized eggs into males, likely does not rely on large-scale transcriptional differences between these tissues. In contrast, a number of meiosis-related genes show a bias toward testis-specific expression, despite the lack of true meiosis in N. vitripennis males. These patterns may reflect an unexpected complexity of male gamete production in the haploid males of this organism. Broadly, these data add to the growing number of genomic and genetic tools available in N. vitripennis for addressing important biological questions in this rising insect model organism.

Plasticity of gene-regulatory networks controlling sex determination: of masters, slaves, usual suspects, newcomers, and usurpators

Sexual dimorphism is one of the most pervasive and diverse features of animal morphology, physiology, and behavior. Despite the generality of the phenomenon itself, the mechanisms controlling how sex is determined differ considerably among various organismic groups, have evolved repeatedly and independently, and the underlying molecular pathways can change quickly during evolution. Even within closely related groups of organisms for which the development of gonads on the morphological, histological, and cell biological level is undistinguishable, the molecular control and the regulation of the factors involved in sex determination and gonad differentiation can be substantially different. The biological meaning of the high molecular plasticity of an otherwise common developmental program is unknown. While comparative studies suggest that the downstream effectors of sex-determining pathways tend to be more stable than the triggering mechanisms at the top, it is still unclear how conserved the downstream networks are and how all components work together. After many years of stasis, when the molecular basis of sex determination was amenable only in the few classical model organisms (fly, worm, mouse), recently, sex-determining genes from several animal species have been identified and new studies have elucidated some novel regulatory interactions and biological functions of the downstream network, particularly in vertebrates. These data have considerably changed our classical perception of a simple linear developmental cascade that makes the decision for the embryo to develop as male or female, and how it evolves.

Genetic and epigenetic architecture of sex-biased expression in the jewel wasps Nasonia vitripennis and giraulti

There is extraordinary diversity in sexual dimorphism (SD) among animals, but little is known about its epigenetic basis. To study the epigenetic architecture of SD in a haplodiploid system, we performed RNA-seq and whole-genome bisulfite sequencing of adult females and males from two closely related parasitoid wasps, Nasonia vitripennis and Nasonia giraulti. More than 75% of expressed genes displayed significantly sex-biased expression. As a consequence, expression profiles are more similar between species within each sex than between sexes within each species. Furthermore, extremely male- and female-biased genes are enriched for totally different functional categories: male-biased genes for key enzymes in sex-pheromone synthesis and female-biased genes for genes involved in epigenetic regulation of gene expression. Remarkably, just 70 highly expressed, extremely male-biased genes account for 10% of all transcripts in adult males. Unlike expression profiles, DNA methylomes are highly similar between sexes within species, with no consistent sex differences in methylation found. Therefore, methylation changes cannot explain the extensive level of sex-biased gene expression observed. Female-biased genes have smaller sequence divergence between species, higher conservation to other hymenopterans, and a broader expression range across development. Overall, female-biased genes have been recruited from genes with more conserved and broadly expressing "house-keeping" functions, whereas male-biased genes are more recently evolved and are predominately testis specific. In summary, Nasonia accomplish a striking degree of sex-biased expression without sex chromosomes or epigenetic differences in methylation. We propose that methylation provides a general signal for constitutive gene expression, whereas other sex-specific signals cause sex-biased gene expression.

Regulation of transcription factors on sexual dimorphism of fig wasps

Fig wasps exhibit extreme intraspecific morphological divergence in the wings, compound eyes, antennae, body color, and size. Corresponding to this, behaviors and lifestyles between two sexes are also different: females can emerge from fig and fly to other fig tree to oviposit and pollinate, while males live inside fig for all their lifetime. Genetic regulation may drive these extreme intraspecific morphological and behavioral divergence. Transcription factors (TFs) involved in morphological development and physiological activity may exhibit sex-specific expressions. Herein, we detect 865 TFs by using genomic and transcriptomic data of the fig wasp Ceratosolen solmsi. Analyses of transcriptomic data indicated that up-regulated TFs in females show significant enrichment in development of the wing, eye and antenna in all stages, from larva to adult. Meanwhile, TFs related to the development of a variety of organs display sex-specific patterns of expression in the adults and these may contribute significantly to their sexual dimorphism. In addition, up-regulated TFs in adult males exhibit enrichment in genitalia development and circadian rhythm, which correspond with mating and protandry. This finding is consistent with their sex-specific behaviors. In conclusion, our results strongly indicate that TFs play important roles in the sexual dimorphism of fig wasps.

Population-level consequences of complementary sex determination in a solitary parasitoid

Background

Sex determination mechanisms are known to be evolutionarily labile but the factors driving transitions in sex determination mechanisms are poorly understood. All insects of the Hymenoptera are haplodiploid, with males normally developing from unfertilized haploid eggs. Under complementary sex determination (CSD), diploid males can be produced from fertilized eggs that are homozygous at the sex locus. Diploid males have near-zero fitness and thus represent a genetic load, which is especially severe under inbreeding. Here, we study mating structure and sex determination in the parasitoid Cotesia vestalis to investigate what may have driven the evolution of two complementary sex determination loci in this species.

Results

We genotyped Cotesia vestalis females collected from eight fields in four townships in Western Taiwan. 98 SNP markers were developed by aligning Illumina sequence reads of pooled DNA of eight different females against a de novo assembled genome of C. vestalis. This proved to be an efficient method for this non-model species and provides a resource for future use in related species. We found significant genetic differentiation within the sampled population but variation could not be attributed to sampling locations by AMOVA. Non-random mating was detected, with 8.1% of matings between siblings. Diploid males, detected by flow cytometry, were produced at a rate of 1.4% among diploids.

Conclusions

We think that the low rate of diploid male production is best explained by a CSD system with two independent sex loci, supporting laboratory findings on the same species. Fitness costs of diploid males in C. vestalis are high because diploid males can mate with females and produce infertile triploid offspring. This severe fitness cost of diploid males combined with non-random mating may have resulted in evolution from single locus CSD to CSD with two independent loci.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0340-2) contains supplementary material, which is available to authorized users.

Diploid males support a two-step mechanism of endosymbiont-induced thelytoky in a parasitoid wasp

Background

Haplodiploidy, where females develop from diploid, fertilized eggs and males from haploid, unfertilized eggs, is abundant in some insect lineages. Some species in these lineages reproduce by thelytoky that is caused by infection with endosymbionts: infected females lay haploid eggs that undergo diploidization and develop into females, while males are very rare or absent. It is generally assumed that in thelytokous wasps, endosymbionts merely diploidize the unfertilized eggs, which would then trigger female development.

Results

We found that females in the parasitoid wasp Asobara japonica infected with thelytoky-inducing Wolbachia produce 0.7–1.2 % male offspring. Seven to 39 % of these males are diploid, indicating that diploidization and female development can be uncoupled in A. japonica. Wolbachia titer in adults was correlated with their ploidy and sex: diploids carried much higher Wolbachia titers than haploids, and diploid females carried more Wolbachia than diploid males. Data from introgression lines indicated that the development of diploid individuals into males instead of females is not caused by malfunction-mutations in the host genome but that diploid males are most likely produced when the endosymbiont fails to activate the female sex determination pathway. Our data therefore support a two-step mechanism by which endosymbionts induce thelytoky in A. japonica: diploidization of the unfertilized egg is followed by feminization, whereby each step correlates with a threshold of endosymbiont titer during wasp development.

Conclusions

Our new model of endosymbiont-induced thelytoky overthrows the view that certain sex determination mechanisms constrain the evolution of endosymbiont-induced thelytoky in hymenopteran insects. Endosymbionts can cause parthenogenesis through feminization, even in groups in which endosymbiont-diploidized eggs would develop into males following the hosts’ sex determination mechanism. In addition, our model broadens our understanding of the mechanisms by which endosymbionts induce thelytoky to enhance their transmission to the next generation. Importantly, it also provides a novel window to study the yet-poorly known haplodiploid sex determination mechanisms in haplodiploid insects.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0370-9) contains supplementary material, which is available to authorized users.

Beyond sex allocation: the role of mating systems in sexual selection in parasitoid wasps

Despite the diverse array of mating systems and life histories which characterise the parasitic Hymenoptera, sexual selection and sexual conflict in this taxon have been somewhat overlooked. For instance, parasitoid mating systems have typically been studied in terms of how mating structure affects sex allocation. In the past decade, however, some studies have sought to address sexual selection in the parasitoid wasps more explicitly and found that, despite the lack of obvious secondary sexual traits, sexual selection has the potential to shape a range of aspects of parasitoid reproductive behaviour and ecology. Moreover, various characteristics fundamental to the parasitoid way of life may provide innovative new ways to investigate different processes of sexual selection. The overall aim of this review therefore is to re-examine parasitoid biology with sexual selection in mind, for both parasitoid biologists and also researchers interested in sexual selection and the evolution of mating systems more generally. We will consider aspects of particular relevance that have already been well studied including local mating structure, sex allocation and sperm depletion. We go on to review what we already know about sexual selection in the parasitoid wasps and highlight areas which may prove fruitful for further investigation. In particular, sperm depletion and the costs of inbreeding under chromosomal sex determination provide novel opportunities for testing the role of direct and indirect benefits for the evolution of mate choice.

Hybrid incompatibilities are affected by dominance and dosage in the haplodiploid wasp Nasonia

Study of genome incompatibilities in species hybrids is important for understanding the genetic basis of reproductive isolation and speciation. According to Haldane's rule hybridization affects the heterogametic sex more than the homogametic sex. Several theories have been proposed that attribute asymmetry in hybridization effects to either phenotype (sex) or genotype (heterogamety). Here we investigate the genetic basis of hybrid genome incompatibility in the haplodiploid wasp Nasonia using the powerful features of haploid males and sex reversal. We separately investigate the effects of heterozygosity (ploidy level) and sex by generating sex reversed diploid hybrid males and comparing them to genotypically similar haploid hybrid males and diploid hybrid females. Hybrid effects of sterility were more pronounced than of inviability, and were particularly strong in haploid males, but weak to absent in diploid males and females, indicating a strong ploidy level but no sex specific effect. Molecular markers identified a number of genomic regions associated with hybrid inviability in haploid males that disappeared under diploidy in both hybrid males and females. Hybrid inviability was rescued by dominance effects at some genomic regions, but aggravated or alleviated by dosage effects at other regions, consistent with cytonuclear incompatibilities. Dosage effects underlying Bateson–Dobzhansky–Muller (BDM) incompatibilities need more consideration in explaining Haldane's rule in diploid systems.

The evolutionary dynamics of major regulators for sexual development among Hymenoptera species

All hymenopteran species, such as bees, wasps and ants, are characterized by the common principle of haplodiploid sex determination in which haploid males arise from unfertilized eggs and females from fertilized eggs. The underlying molecular mechanism has been studied in detail in the western honey bee Apis mellifera, in which the gene complementary sex determiner (csd) acts as primary signal of the sex determining pathway, initiating female development by csd-heterozygotes. Csd arose from gene duplication of the feminizer (fem) gene, a transformer (tra) ortholog, and mediates in conjunction with transformer2 (tra2) sex-specific splicing of fem. Comparative molecular analyses identified fem/tra and its downstream target doublesex (dsx) as conserved unit within the sex determining pathway of holometabolous insects. In this study, we aim to examine evolutionary differences among these key regulators. Our main hypothesis is that sex determining key regulators in Hymenoptera species show signs of coevolution within single phylogenetic lineages. We take advantage of several newly sequenced genomes of bee species to test this hypothesis using bioinformatic approaches. We found evidences that duplications of fem are restricted to certain bee lineages and notable amino acid differences of tra2 between Apis and non-Apis species propose structural changes in Tra2 protein affecting co-regulatory function on target genes. These findings may help to gain deeper insights into the ancestral mode of hymenopteran sex determination and support the common view of the remarkable evolutionary flexibility in this regulatory pathway.

Double nexus--Doublesex is the connecting element in sex determination

In recent years, our knowledge of the conserved master-switch gene doublesex (dsx) and its function in regulating the development of dimorphic traits in insects has deepened considerably. Here, a comprehensive overview is given on the properties of the male- and female-specific dsx transcripts yielding DSX^F and DSX^M proteins in Drosophila melanogaster, and the many downstream targets that they regulate. As insects have cell-autonomous sex determination, it was assumed that dsx would be expressed in every somatic cell, but recent research showed that dsx is expressed only when a cell is required to show its sexual identity through function or morphology. This spatiotemporal regulation of dsx expression has not only been established in D. melanogaster but in all insect species studied. Gradually, it has been appreciated that dsx could no longer be viewed as the master-switch gene orchestrating sexual development and behaviour in each cell, but instead should be viewed as the interpreter for the sexual identity of the cell, expressing this identity only on request, making dsx the central nexus of insect sex determination.

De novo assembly and transcriptome analysis of the Mediterranean fruit fly Ceratitis capitata early embryos

The agricultural pest Ceratitis capitata, also known as the Mediterranean fruit fly or Medfly, belongs to the Tephritidae family, which includes a large number of other damaging pest species. The Medfly has been the first non-drosophilid fly species which has been genetically transformed paving the way for designing genetic-based pest control strategies. Furthermore, it is an experimentally tractable model, in which transient and transgene-mediated RNAi have been successfully used. We applied Illumina sequencing to total RNA preparations of 8–10 hours old embryos of C. capitata, This developmental window corresponds to the blastoderm cellularization stage. In summary, we assembled 42,614 transcripts which cluster in 26,319 unique transcripts of which 11,045 correspond to protein coding genes; we identified several hundreds of long ncRNAs; we found an enrichment of transcripts encoding RNA binding proteins among the highly expressed transcripts, such as CcTRA-2, known to be necessary to establish and, most likely, to maintain female sex of C. capitata. Our study is the first de novo assembly performed for Ceratitis capitata based on Illumina NGS technology during embryogenesis and it adds novel data to the previously published C. capitata EST databases. We expect that it will be useful for a variety of applications such as gene cloning and phylogenetic analyses, as well as to advance genetic research and biotechnological applications in the Medfly and other related Tephritidae.

Patterns and mechanisms of evolutionary transitions between genetic sex-determining systems

The diversity and patchy phylogenetic distribution of genetic sex-determining mechanisms observed in some taxa is thought to have arisen by the addition, modification, or replacement of regulators at the upstream end of the sex-determining pathway. Here, I review the various evolutionary forces acting on upstream regulators of sexual development that can cause transitions between sex-determining systems. These include sex-ratio selection and pleiotropic benefits, as well as indirect selection mechanisms involving sex-linked sexually antagonistic loci or recessive deleterious mutations. Most of the current theory concentrates on the population-genetic aspects of sex-determination transitions, using models that do not reflect the developmental mechanisms involved in sex determination. However, the increasing availability of molecular data creates opportunities for the development of mechanistic models that can clarify how selection and developmental architecture interact to direct the evolution of sex-determination genes.

Sex determination: why so many ways of doing it?

Sexual reproduction is an ancient feature of life on earth, and the familiar X and Y chromosomes in humans and other model species have led to the impression that sex determination mechanisms are old and conserved. In fact, males and females are determined by diverse mechanisms that evolve rapidly in many taxa. Yet this diversity in primary sex-determining signals is coupled with conserved molecular pathways that trigger male or female development. Conflicting selection on different parts of the genome and on the two sexes may drive many of these transitions, but few systems with rapid turnover of sex determination mechanisms have been rigorously studied. Here we survey our current understanding of how and why sex determination evolves in animals and plants and identify important gaps in our knowledge that present exciting research opportunities to characterize the evolutionary forces and molecular pathways underlying the evolution of sex determination.

Independent evolutionary origin of fem paralogous genes and complementary sex determination in hymenopteran insects

The primary signal of sex determination in the honeybee, the complementary sex determiner (csd) gene, evolved from a gene duplication event from an ancestral copy of the fem gene. Recently, other paralogs of the fem gene have been identified in several ant and bumblebee genomes. This discovery and the close phylogenetic relationship of the paralogous gene sequences led to the hypothesis of a single ancestry of the csd genetic system of complementary sex determination in the Hymenopteran insects, in which the fem and csd gene copies evolved as a unit in concert with the mutual transfers of sequences (concerted evolution). Here, we show that the paralogous gene copies evolved repeatedly through independent gene duplication events in the honeybee, bumblebee, and ant lineage. We detected no sequence tracts that would indicate a DNA transfer between the fem and the fem1/csd genes between different ant and bee species. Instead, we found tracts of duplication events in other genomic locations, suggesting that gene duplication was a frequent event in the evolution of these genes. These and other evidences suggest that the fem1/csd gene originated repeatedly through gene duplications in the bumblebee, honeybee, and ant lineages in the last 100 million years. Signatures of concerted evolution were not detectable, implicating that the gene tree based on neutral synonymous sites represents the phylogenetic relationships and origins of the fem and fem1/csd genes. Our results further imply that the fem1 and csd gene in bumblebees, honeybees, and ants are not orthologs, because they originated independently from the fem gene. Hence, the widely shared and conserved complementary sex determination mechanism in Hymenopteran insects is controlled by different genes and molecular processes. These findings highlight the limits of comparative genomics and emphasize the requirement to study gene functions in different species and major hymenopteran lineages.

Sexually dimorphic body color is regulated by sex-specific expression of yellow gene in ponerine ant, Diacamma sp

Most hymenopteran species exhibit conspicuous sexual dimorphism due to ecological differences between the sexes. As hymenopteran genomes, under the haplodiploid genetic system, exhibit quantitative differences between sexes while remaining qualitatively identical, sexual phenotypes are assumed to be expressed through sex-specific gene usage. In the present study, the molecular basis for expression of sexual dimorphism in a queenless ant, Diacamma sp., which exhibits a distinct color dimorphism, was examined. Worker females of the species appear bluish-black, while winged males exhibit a yellowish-brown body color. Initially, observations of the pigmentation processes during pupal development revealed that black pigmentation was present in female pupae but not in males, suggesting that sex-specific melanin synthesis was responsible for the observed color dimorphism. Therefore, five orthologs of the genes involved in the insect melanin synthesis (yellow, ebony, tan, pale and dopa decarboxylase) were subcloned and their spatiotemporal expression patterns were examined using real-time quantitative RT-PCR. Of the genes examined, yellow, which plays a role in black melanin synthesis in insects, was expressed at higher levels in females than in males throughout the entire body during the pupal stage. RNA interference of yellow was then carried out in order to determine the gene function, and produced females with a more yellowish, brighter body color similar to that of males. It was concluded that transcriptional regulation of yellow was responsible for the sexual color dimorphism observed in this species.