Evolutionary History of Sexual Differentiation Mechanism in Insects

Efficient transgenic system for the firebrat Thermobia domestica utilizing hyPBase and G0 founder prescreening

The firebrat, Thermobia domestica, is an apterygote model insect with favorable traits, including rapid generation turnover, high fecundity, and ease of laboratory rearing. We previously developed a method for embryo injection and CRISPR/Cas9-based genome editing in Thermobia. In the present study, we established a highly efficient transgenesis system using the hyperactive piggyBac transposase (hyPBase) to expand genetic manipulation techniques in Thermobia. By injecting embryos with a mixture of hyPBase mRNA and a donor plasmid expressing GFP under the control of an eye enhancer, we achieved the first successful transgenesis in Thermobia. Eye-specific GFP expression was observed in 5.7 % of G0 individuals hatched from injected eggs. Notably, these GFP-positive G0 founders exhibited significantly elevated germline transmission rates (53.3 %) compared with GFP-negative G0 founders (19.0 %). Additionally, a significant difference in the proportion of G1 transgenic progeny emerged between the GFP-positive and GFP-negative G0 groups (20.0 % vs. 2.7 %), highlighting the utility of GFP expression as a predictor of transgenic G1 offspring from injected G0 founders. Furthermore, multiple transgene insertions mediated by hyPBase contributed to the increased transformation efficiency observed in G0 founders with high transmission rates. Our findings offer valuable genetic toolkits for Thermobia that will facilitate advanced research on fundamental biological processes, such as the evolution of wings and metamorphosis.

Epigenomic Changes in Ostrinia Moths Under Elevated Pupal and Adult Temperature

Epigenetic changes in the methylation of DNA may occur in response to environmental stressors, including warming climates. DNA methylation may also play an important role in regulating gene expression during both male and female reproduction in many insect species. However, it is currently unknown how DNA methylation shifts when individuals are reproducing under warmer temperatures. We exposed European corn borer moths (Ostrinia nubilalis) to heat during the pupal and adult life stages then investigated changes in DNA methylation across the genome using enzymatic methyl-seq (EM-seq). We compared methylation patterns in reproductive males and females exposed to heat (28°C) to those that experienced an ambient temperature (23°C). We found that heat exposure led to a small but significant increase in the percentage of methylated CpG sites throughout the genome in both sexes. However, DNA methylation rates were higher in females and differential methylation following heat exposure localised to unique regions in each sex. In males, methylation shifted within genes belonging to pathways including Hippo signalling, ubiquitin-mediated proteolysis, DNA damage repair and spermatogenesis. In females, differential methylation occurred in genes related to histone modification and oogenesis. Our results suggest that DNA methylation patterns respond to moderate heat exposure in Lepidoptera and provide insight into epigenetic responses to heatwaves, suggesting novel pathways that may be involved in responding to heat stress during metamorphosis and reproduction.

Evolution and regulation of animal sex chromosomes

Animal sex chromosomes typically carry the upstream sex-determining gene that triggers testis or ovary development and, in some species, are regulated by global dosage compensation in response to functional decay of the Y chromosome. Despite the importance of these pathways, they exhibit striking differences across species, raising fundamental questions regarding the mechanisms underlying their evolutionary turnover. Recent studies of non-model organisms, including insects, reptiles and teleosts, have yielded a broad view of the diversity of sex chromosomes that challenges established theories. Moreover, continued studies in model organisms with recently developed technologies have characterized the dynamics of sex determination and dosage compensation in three-dimensional nuclear space and at single-cell resolution. Here, we synthesize recent insights into sex chromosomes from a variety of species to review their evolutionary dynamics with respect to the canonical model, as well as their diverse mechanisms of regulation.

Dedicated developmental programing for group-supporting behaviors in eusocial honeybees

The evolutionary changes from solitary to eusocial living in vertebrates and invertebrates are associated with the diversification of social interactions and the development of queen and worker castes. Despite strong innate patterns, our understanding of the mechanisms manifesting these sophisticated behaviors is still rudimentary. Here, we show that doublesex (dsx) manifests group-supporting behaviors in the honeybee (Apis mellifera) worker caste. Computer-based individual behavioral tracking of worker bees with biallelic stop mutations in colonies revealed that the dsx gene is required for the rate and duration of group-supporting behavior that scales the relationship between bees and their work. General sensorimotor functions remained unaffected. Unexpectedly, unlike in other insects, the dsx gene is required for the neuronal wiring of the mushroom body in which the gene is spatially restricted expressed. Together, our study establishes dedicated programming for group-supporting behaviors and provides insight into the connection between development in the neuronal circuitry and behaviors regulating the formation of a eusocial society.

Honeybees' novel complementary sex-determining system: function and origin

Complementary sex determination regulates female and male development in honeybees (Apis mellifera) via heterozygous versus homo-/hemizygous genotypes of the csd (complementary sex determiner) gene involving numerous naturally occurring alleles. This lineage-specific function offers a rare opportunity to understand an undescribed regulatory mechanism and the molecular evolutionary path leading to this mechanism. We reviewed recent advances in understanding how Csd recognizes different versus identical protein variants, how these variants regulate downstream pathways and sexual differentiation, and how this mechanism has evolved and been shaped by evolutionary forces. Finally, we highlighted the shared regulatory principles of sex determination despite the diversity of primary signals and demonstrated that lineage-specific mutations are very informative for characterizing newly evolved functions.

Evolution of the sex-determination gene Doublesex within the termite lineage

The molecular mechanism of sex determination has long been considered conserved in insects. However, recent studies of hemimetabolous insects have challenged this notion. One notable example is termites. In Reticulitermes speratus, a homolog of sex determination gene, Doublesex (RsDsx), exhibits characteristics that are distinct from those of other insects, including sister-group cockroaches. It comprises a single exon, contains only doublesex/mab-3 DNA-binding domain (DM) but lacks a conserved oligomerization domain (OD), and exhibits transcriptional activity only in males. To investigate whether these characteristics are widespread within the termite lineage, we identified Dsx homologs in three different families. The absence of the conserved OD sequences was observed in all termite species examined, whereas the number of exons and expression patterns between sexes varied among families. Particularly, distinctive differences in Dsx were found in species from the Archotermopsidae and Kalotermitidae, both of which have a linear caste developmental pathway. Our findings indicate that diversification of Dsx structure and expression patterns may have contributed to ecological diversification, such as caste developmental pathways, within the termite lineage.

Doublesex is essential for masculinization but not feminization in Lygus hesperus

In most holometabolous insects, sex differentiation occurs via a hierarchical cascade of transcription factors, with doublesex (dsx) regulating genes that control sex-specific traits. Although less is known in hemimetabolous insects, early evidence suggests that substantial differences exist from more evolutionarily advanced insects. Here, we identified and characterized dsx in Lygus hesperus (western tarnished plant bug), a hemipteran pest of many agricultural crops in western North America. The full-length transcript for L. hesperus dsx (Lhdsx) and several variants encode proteins with conserved DNA binding and oligomerization domains. Transcript profiling revealed that Lhdsx is ubiquitously expressed, likely undergoes alternative pre-mRNA splicing, and, unlike several model insects, is sex-biased rather than sex-specific. Embryonic RNA interference (RNAi) of Lhdsx only impacted sex development in adult males, which lacked both internal reproductive organs and external genitalia. No discernible impacts on adult female development or reproductivity were observed. RNAi knockdown of Lhdsx in nymphs likewise only affected adult males, which lacked the characteristic dimorphic coloration but had dramatically elevated vitellogenin transcripts. Gene knockout of Lhdsx by CRISPR/Cas9 editing yielded only females in G0 and strongly biased heterozygous G1 offspring to females with the few surviving males showing severely impaired genital development. These results indicate that L. hesperus male development requires Lhdsx, whereas female development proceeds via a basal pathway that functions independently of dsx. A fundamental understanding of sex differentiation in L. hesperus could be important for future gene-based management strategies of this important agricultural pest.

Candidate target genes of the male-specific expressed Doublesex in the termite Reticulitermes speratus

Eusocial insects such as termites, ants, bees, and wasps exhibit a reproductive division of labor. The developmental regulation of reproductive organ (ovaries and testes) is crucial for distinguishing between reproductive and sterile castes. The development of reproductive organ in insects is regulated by sex-determination pathways. The sex determination gene Doublesex (Dsx), encoding transcription factors, plays an important role in this pathway. Therefore, clarifying the function of Dsx in the developmental regulation of sexual traits is important to understand the social evolution of eusocial insects. However, no studies have reported the function of Dsx in hemimetabolous eusocial group termites. In this study, we searched for binding sites and candidate target genes of Dsx in species with available genome information as the first step in clarifying the function of Dsx in termites. First, we focused on the Reticulitermes speratus genome and identified 101 candidate target genes of Dsx. Using a similar method, we obtained 112, 39, and 76 candidate Dsx target genes in Reticulitermes lucifugus, Coptotermes formosanus, and Macrotermes natalensis, respectively. Second, we compared the candidate Dsx target genes between species and identified 37 common genes between R. speratus and R. lucifugus. These included several genes probably involved in spermatogenesis and longevity. However, only a few common target genes were identified between R. speratus and the other two species. Finally, Dsx dsRNA injection resulted in the differential expression of several target genes, including piwi-like protein and B-box type zinc finger protein ncl-1 in R. speratus. These results provide valuable resource data for future functional analyses of Dsx in termites.

Recognition of polymorphic Csd proteins determines sex in the honeybee

Sex in honeybees, Apis mellifera, is genetically determined by heterozygous versus homo/hemizygous genotypes involving numerous alleles at the single complementary sex determination locus. The molecular mechanism of sex determination is however unknown because there are more than 4950 known possible allele combinations, but only two sexes in the species. We show how protein variants expressed from complementary sex determiner (csd) gene determine sex. In females, the amino acid differences between Csd variants at the potential-specifying domain (PSD) direct the selection of a conserved coiled-coil domain for binding and protein complexation. This recognition mechanism activates Csd proteins and, thus, the female pathway. In males, the absence of polymorphisms establishes other binding elements at PSD for binding and complexation of identical Csd proteins. This second recognition mechanism inactivates Csd proteins and commits male development via default pathway. Our results demonstrate that the recognition of different versus identical variants of a single protein is a mechanism to determine sex.

The identification and expression pattern of the sex determination genes and their sex-specific variants in the egg parasitoid Trichogramma dendrolimi Matsumura (Hymenoptera: Trichogrammatidae)

Introduction: Trichogramma wasps are egg parasitoids of agricultural lepidopteran pests. The sex of Trichogramma is determined by its ploidy as well as certain sex ratio distorters, such as the endosymbiotic bacteria Wolbachia spp. and the paternal sex ratio (PSR) chromosome. The sex determination systems of hymenopterans, such as Trichogramma spp., involve cascades of the genes transformer (tra), transformer-2 (tra2), and doublesex (dsx) and are associated with sex-specific tra and dsx splicing. First, these genes and their sex-specific variants must be identified to elucidate the interactions between the sex ratio disorders and the sex determination mechanism of Trichogramma. Methods: Here, we characterized the sex determination genes tra, tra2, and dsx in Trichogramma dendrolimi. Sex-specific tra and dsx variants were detected in cDNA samples obtained from both male and female Trichogramma wasps. They were observed in the early embryos (1-10 h), late embryos (12-20 h), larvae (32 h and 48 h), pre-pupae (96 h), and pupae (144 h, 168 h, 192 h, and 216 h) of both male and female T. dendrolimi offspring. Results: We detected female-specific tra variants throughout the entire early female offspring stage. The male-specific variant began to express at 9-10 h as the egg was not fertilized. However, we did not find any maternally derived, female-specific tra variant in the early male embryo. This observation suggests that the female-specific tra variant expressed in the female embryo at 1-9 h may not have originated from the maternal female wasp. Discussion: The present study might be the first to identify the sex determination genes and sex-specific gene splicing in Trichogramma wasps. The findings of this study lay the foundation for investigating the sex determination mechanisms of Trichogramma and other wasps. They also facilitate sex identification in immature T. dendrolimi and the application of this important egg parasitoid in biological insect pest control programs.

Identification and characterization of the Doublesex gene and its mRNA isoforms in the brine shrimp Artemia franciscana

Doublesex (DSX) proteins are members of the Doublesex/mab-3-related (DMRT) protein family and play crucial roles in sex determination and differentiation among the animal kingdom. In the present study, we identified two Doublesex (Dsx)-like mRNA isoforms in the brine shrimp Artemia franciscana (Kellogg 1906), which are generated by the combination of alternative promoters, alternative splicing and alternative polyadenylation. The two transcripts exhibited sex-biased enrichment, which we termed AfrDsxM and AfrDsxF. They share a common region which encodes an identical N-terminal DNA-binding (DM) domain. RT-qPCR analyses showed that AfrDsxM is dominantly expressed in male Artemia while AfrDsxF is specifically expressed in females. Expression levels of both isoforms increased along with the developmental stages of their respective sexes. RNA interference with dsRNA showed that the knockdown of AfrDsxM in male larvae led to the appearance of female traits including an ovary-like structure in the original male reproductive system and an elevated expression of vitellogenin. However, silencing of AfrDsxF induced no clear phenotypic change in female Artemia. These results indicated that the male AfrDSXM may act as inhibiting regulator upon the default female developmental mode in Artemia. Furthermore, electrophoretic mobility shift assay analyses revealed that the unique DM domain of AfrDSXs can specifically bind to promoter segments of potential downstream target genes like AfrVtg. These data show that AfrDSXs play crucial roles in regulating sexual development in Artemia, and further provide insight into the evolution of sex determination/differentiation in sexual organisms.

The function and evolution of a genetic switch controlling sexually dimorphic eye differentiation in honeybees

Animals develop sex-specific morphological structures that are diverse between organisms. However, understanding the developmental and evolutionary mechanisms governing these traits is still limited and largely restricted to DM domain genes, which are conserved, sex-specific developmental regulators identified in genetic models. Here, we report a sex-specific developmental regulator gene, glubschauge (glu) that selectively regulates sexually dimorphic eye differentiation in honeybees. We found that the sex determination gene feminizer (fem) controls sex-specific splicing of glu transcripts, establishing a genetic switch in which Glu proteins with a zinc finger (ZnF) domain are only expressed in females. We showed that female coding sequence was essential and sufficient for partial feminization. Comparative sequence and functional studies revealed that the evolutionary origination of the genetic switch was followed by the mutational origin of the essential ZnF domain. Our results demonstrate that glu is a newly evolved sex-specific genetic switch for region-specific regulation of a dimorphic character.

A history of the genetic and molecular identification of genes and their functions controlling insect sex determination

The genetics of the sex determination regulatory cascade in Drosophila melanogaster has a fascinating history, interlinked with the foundation of the Genetics discipline itself. The discovery that alternative splicing rather than differential transcription is the molecular mechanism underlying the upstream control of sex differences in the Drosophila model system was surprising. This notion is now fully integrated into the scientific canon, appearing in many genetics textbooks and online education resources. In the last three decades, it was a key reference point for starting evolutionary studies in other insect species by using homology-based approaches. This review will introduce a very brief history of Drosophila genetics. It will describe the genetic and molecular approaches applied for the identifying and cloning key genes involved in sex determination in Drosophila and in many other insect species. These comparative analyses led to supporting the idea that sex-determining pathways have evolved mainly by recruiting different upstream signals/genes while maintaining widely conserved intermediate and downstream regulatory genes. The review also provides examples of the link between technological advances and research achievements, to stimulate reflections on how science is produced. It aims to hopefully strengthen the related historical and conceptual knowledge of general readers of other disciplines and of younger geneticists, often focused on the latest technical-molecular approaches.