The mechanism of sex-specific splicing at the doublesex gene is different between Drosophila melanogaster and Bombyx mori

Complete characterization of the yak testicular development using accurate full-length transcriptome sequencing

Alternative splicing is a prevalent phenomenon in testicular tissues. Due to the low assembly accuracy of short-read RNA sequencing technology in analyzing post-transcriptional regulatory events, full-length (FL) transcript sequencing is highly demanded to accurately determine FL splicing variants. In this study, we performed FL transcriptome sequencing of testicular tissues from 0.5, 1.5, 2.5, and 4-year-old yaks and 4-year-old cattle-yaks using Oxford Nanopore Technologies. The obtained sequencing data were predicted to have 47,185 open reading frames (ORFs), including 26,630 complete ORFs, detected 7645 fusion transcripts, 15,355 alternative splicing events, 25,798 simple sequence repeats, 7628 transcription factors, and 35,503 long non-coding RNAs. A total of 40,038 novel transcripts were obtained from the sequencing data, and the proportion was almost close to the number of known transcripts identified. Structural analysis and functional annotation of these novel transcripts resulted in the successful annotation of 9568 transcripts, with the highest and lowest annotation numbers in the Nr and KOG databases, respectively. Weighted gene co-expression network analysis revealed the key regulatory pathways and hub genes at various stages of yak testicular development. Our findings enhance our comprehension of transcriptome complexity, contribute to genome annotation refinement, and provide foundational data for further investigations into male sterility in cattle-yaks.

Zygosity-based sex determination in a butterfly drives hypervariability of Masculinizer

Nature has devised many ways of producing males and females. Here, we report on a previously undescribed mechanism for Lepidoptera that functions without a female-specific gene. The number of alleles or allele heterozygosity in a single Z-linked gene (BaMasc) is the primary sex-determining switch in Bicyclus anynana butterflies. Embryos carrying a single BaMasc allele develop into WZ (or Z0) females, those carrying two distinct alleles develop into ZZ males, while (ZZ) homozygotes initiate female development, have mismatched dosage compensation, and die as embryos. Consequently, selection against homozygotes has favored the evolution of spectacular allelic diversity: 205 different coding sequences of BaMasc were detected in a sample of 246 females. The structural similarity of a hypervariable region (HVR) in BaMasc to the HVR in Apis mellifera csd suggests molecular convergence between deeply diverged insect lineages. Our discovery of this primary switch highlights the fascinating diversity of sex-determining mechanisms and underlying evolutionary drivers.

Identification and functional analysis of the female determiner gene in the bean bug, Riptortus pedestris

BACKGROUND

Homing-based gene drives targeting sex-specific lethal genes have been used for genetic control. Additionally, understanding insect sex determination provides new targets for managing insect pests. While sex determination mechanisms in holometabolous insects have been thoroughly studied and employed in pest control, the study of the sex determination pathway in hemimetabolous insects is limited to only a few species. Riptortus pedestris (Fabricius; Hemiptera: Heteroptera), commonly known as the bean bug, is a significant pest for soybeans. Nonetheless, the mechanism of its sex determination and the target gene for genetic control are not well understood.

RESULTS

We identified Rpfmd as the female determiner gene in the sex determination pathway of R. pedestris. Rpfmd encodes a female-specific serine/arginine-rich protein of 436 amino acids and one non-sex-specific short protein of 98 amino acids. Knockdown of Rpfmd in R. pedestris nymphs caused death of molting females with masculinized somatic morphology but did not affect male development. Knockdown of Rpfmd in newly emerged females inhibited ovary development, while maternal-mediated RNA interference (RNAi) knockdown of Rpfmd expression resulted in male-only offspring. Transcriptome sequencing revealed that Rpfmd regulates X chromosome dosage compensation and influences various biological processes in females but has no significant effect on males. Moreover, RNAi mediated knockdown of Rpfmd-C had no influence on the development of R. pedestris, suggesting that Rpfmd regulates sex determination through female-specific splicing isoforms. We also found that Rpfmd pre-mRNA alternative splicing regulation starts at the 24-h embryo stage, indicating the activation of sex differentiation.

CONCLUSION

Our study confirms that Rpfmd, particularly its female-specific isoform (Rpfmd-F), is the female determiner gene that regulates sex differentiation in R. pedestris. Knockdown of Rpfmd results in female-specific lethality without affecting males, making it a promising target for genetic control of this soybean pest throughout its development stages. Additionally, our findings improve the understanding of the sex-determination mechanism in hemimetabolous insects. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Masculinizer is not post-transcriptionally regulated by female-specific piRNAs during sex determination in the Asian corn borer, Ostrinia furnacalis

Lepidopteran insects are heterogametic in females, although most insect species are heterogametic in males. In a lepidopteran model species, the silkworm Bombyx mori (Bombycoidea), the uppermost sex determinant Feminizer (Fem) has been identified on the female-specific W chromosome. Fem is a precursor of PIWI-interacting small RNA (piRNA). Fem piRNA forms a complex with Siwi, one of the two B. mori PIWI-clade Argonaute proteins. In female embryos, Fem piRNA-Siwi complex cleaves the mRNA of the male-determining gene Masculinizer (Masc), directing the female-determining pathway. In male embryos, Masc activates the male-determining pathway in the absence of Fem piRNA. Recently, W chromosome-derived piRNAs complementary to Masc mRNA have also been identified in the diamondback moth Plutella xylostella (Yponomeutoidea), indicating the convergent evolution of piRNA-dependent sex determination in Lepidoptera. Here, we show that this is not the case in the Asian corn borer, Ostrinia furnacalis (Pyraloidea). Although our previous studies demonstrated that O. furnacalis Masc (OfMasc) has a masculinizing function in the embryonic stage, the expression level of OfMasc was indistinguishable between the sexes at the timing of sex determination. Deep sequencing analysis identified no female-specific small RNAs mapped onto OfMasc mRNA. Embryonic knockdown of two PIWI genes did not affect the expression level of OfMasc in either sex. These results demonstrated that piRNA-dependent reduction of Masc mRNA in female embryos is not a common strategy of sex determination, which suggests the possibility of divergent evolution of sex determinants across the order Lepidoptera.

Theme and variation in the evolution of insect sex determination

The development of dimorphic adult sexes is a critical process for most animals, one that is subject to intense selection. Work in vertebrate and insect model species has revealed that sex determination mechanisms vary widely among animal groups. However, this variation is not uniform, with a limited number of conserved factors. Therefore, sex determination offers an excellent context to consider themes and variations in gene network evolution. Here we review the literature describing sex determination in diverse insects. We have screened public genomic sequence databases for orthologs and duplicates of 25 genes involved in insect sex determination, identifying patterns of presence and absence. These genes and a 3.5 reference set of 43 others were used to infer phylogenies and compared to accepted organismal relationships to examine patterns of congruence and divergence. The function of candidate genes for roles in sex determination (virilizer, female-lethal-2-d, transformer-2) and sex chromosome dosage compensation (male specific lethal-1, msl-2, msl-3) were tested using RNA interference in the milkweed bug, Oncopeltus fasciatus. None of these candidate genes exhibited conserved roles in these processes. Amidst this variation we wish to highlight the following themes for the evolution of sex determination: (1) Unique features within taxa influence network evolution. (2) Their position in the network influences a component's evolution. Our analyses also suggest an inverse association of protein sequence conservation with functional conservation.

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.

Transcriptome analysis in the silkworm Bombyx mori overexpressing piRNA-resistant Masculinizer gene

Dosage compensation is a process that produces a similar expression of sex-linked and autosomal genes. In the silkworm Bombyx mori with a WZ sex-determination system, the expression from the single Z in WZ females matches that of ZZ males due to the suppression of Z-linked genes in males. A primary maleness determinant gene, Masculinizer (Masc), is also required for dosage compensation. In females, silkworm Piwi is complexed with the W chromosome-derived female-specific Feminizer (Fem) PIWI-interacting RNA (piRNA) and cleaves Masc mRNA. When Fem piRNA-resistant Masc cDNA (Masc-R) is overexpressed in both sexes, only female larvae are dead during the larval stage. In this study, transcriptome analysis was performed in neonate larvae to examine the effects of Masc-R overexpression on a global gene expression profile. Z-linked genes were globally repressed in Masc-R-overexpressing females due to force-driven dosage compensation. In contrast, Masc-R overexpression had little effect on the expression of Z-linked genes and the male-specific isoform of B. mori insulin-like growth factor II mRNA-binding protein in males, indicating that excessive Masc expression strengthens neither dosage compensation nor maleness in males. Fourteen genes were differentially expressed between Masc-R-overexpressing and control neonate larvae in both sexes, suggesting Masc functions other than dosage compensation and masculinization.

Long Non-Coding RNA Bmdsx-AS1 Effects on Male External Genital Development in Silkworm

Simple Summary

LncRNAs are a class of non-coding RNAs longer than 200 nt that are involved in a variety of biological processes. Studies on lncRNAs in Bombyx mori have shown that some lncRNAs are involved in brain development, silk production and the response to virus infection of the host. However, the roles of lncRNAs are still largely unknown in the silkworm. In this study, we analyzed the function of lncRNAs Bmdsx-AS1 in silkworm by transgenic overexpression, which not only affects the development of male silkworm external genitalia, but also participates in the regulation of EGFR signaling pathway. Moreover, we studied the upstream promoter of Bmdsx-AS1 and found that the BmAbd-B transcription factor of the Hox gene family can negatively regulate the expression of Bmdsx-AS1. These results laid a substantial foundation for in-depth study of the function of lncRNAs in the silkworm.

Abstract

Long non-coding RNAs (lncRNAs) have been suggested to play important roles in some biological processes. However, the detailed mechanisms are not fully understood. We previously identified an antisense lncRNA, Bmdsx-AS1, that is involved in pre-mRNA splicing of the sex-determining gene Bmdsx in the silkworm. In this study, we analyzed the changes in the male external genitalia of transgenic overexpressed Bmdsx-AS1 silkworm lines and analyzed downstream and upstream responses. We found that Bmdsx-AS1 transgenic silkworms, compared with wild type, showed more claspers in the male external genitalia. Quantitative real-time PCR (qPCR) results indicated that overexpression of Bmdsx-AS1 decreased the expression of genes in the EGFR signaling pathway. Knockdown of Bmdsx-AS1 increased the activity of the EGFR pathway. Through promoter prediction, promoter truncation and electrophoretic mobility shift assay (EMSA) analyses, we found that the protein encoded by the Hox gene BmAbd-B specifically binds to the promoter of Bmdsx-AS1. Moreover, overexpression of BmAbd-B in the silkworm BmE cell line indicated that BmAbd-B negatively regulates the mRNA expression of Bmdsx-AS1. Our study provides insights into the regulatory mechanism of the lncRNA in the silkworm.

Self-limiting fall armyworm: a new approach in development for sustainable crop protection and resistance management

BACKGROUND

The fall armyworm, Spodoptera frugiperda, is a significant and widespread pest of maize, sorghum, rice, and other economically important crops. Successful management of this caterpillar pest has historically relied upon application of synthetic insecticides and through cultivation of genetically engineered crops expressing insecticidal proteins (Bt crops). Fall armyworm has, however, developed resistance to both synthetic insecticides and Bt crops, which risks undermining the benefits delivered by these important crop protection tools. Previous modelling and empirical studies have demonstrated that releases of insecticide- or Bt-susceptible insects genetically modified to express conditional female mortality can both dilute insecticide resistance and suppress pest populations.

RESULTS

Here, we describe the first germline transformation of the fall armyworm and the development of a genetically engineered male-selecting self-limiting strain, OX5382G, which exhibits complete female mortality in the absence of an additive in the larval diet. Laboratory experiments showed that males of this strain are competitive against wild-type males for copulations with wild-type females, and that the OX5382G self-limiting transgene declines rapidly to extinction in closed populations following the cessation of OX5382G male releases. Population models simulating the release of OX5382G males in tandem with Bt crops and non-Bt 'refuge' crops show that OX5382G releases can suppress fall armyworm populations and delay the spread of resistance to insecticidal proteins.

CONCLUSIONS

This article describes the development of self-limiting fall armyworm designed to control this pest by suppressing pest populations, and population models that demonstrate its potential as a highly effective method of managing resistance to Bt crops in pest fall armyworm populations. Our results provide early promise for a potentially valuable future addition to integrated pest management strategies for fall armyworm and other pests for which resistance to existing crop protection measures results in damage to crops and impedes sustainable agriculture.

Alternative splicing patterns of doublesex reveal a missing link between Nix and doublesex in the sex determination cascade of Aedes albopictus

Sexual development in insects is regulated by a complicated hierarchical cascade of sex determination. The primary signals are diverse, whereas the central nexus doublesex (dsx) gene is relatively conserved within the pathway. Aedes (Stegomyia) albopictus is an important vector with an extensive worldwide distribution. We previously reported that Ae. albopictus dsx (Aalbdsx) yields one male- (Aalbdsx^M ) and three female-specific isoforms (Aalbdsx^F1-3 ); however, the spatiotemporal expression profiles and mechanisms regulating sex-specific alternative splicing require further investigation. In this study, we demonstrated that the Aalbdsx^M messenger RNA (mRNA) represents the default pattern when analyzed in human foreskin fibroblasts and HeLa cells. We combined reverse transcription polymerase chain reaction with RNA immunoprecipitation using specific antibodies against tagged Ae. albopictus male-determining factor AalNix and confirmed that AalNix indirectly regulates dsx pre-mRNA and regulates its alternative splicing. During the early embryo stage (0-2 and 4-8 h), maternal dsx^F and default splicing dsx^M were detected in both sexes; the expression of dsx^M then decreased until sufficient AalNix transcripts accumulated in male embryos at 20-24 h. These findings suggest that one or more potential dsx splicing enhancers can shift dsx^M to dsx^F in both sexes; however, the presence of Nix influences the function of this unknown splicing enhancer and ultimately leads to the formation of dsx^M in males. Finally, our results provide important insight into the regulatory mechanism of dsx alternative splicing in the mosquito.

Effects of Functional Depletion of Doublesex on Male Development in the Sawfly, Athalia rosae

Simple Summary

The sawfly, Athalia rosae, exploits a haplodiploid mode of reproduction, in which fertilized eggs develop into diploid females, whereas unfertilized eggs parthenogenetically develop into haploid males. The doublesex (dsx) gene is a well-conserved transcription factor that regulates sexual differentiation in insects. In the present study, we knocked down the A. rosae ortholog of dsx (Ardsx) during several developmental stages with repeated double-stranded RNA (dsRNA) injections. As a result, knockdown of Ardsx in haploid males caused almost complete male-to-female sex reversal, but the resulting eggs were infertile. The same knockdown approach using diploid males caused complete male-to-female sex reversal; they were able to produce fertile eggs and exhibited female behaviors. The same RNAi treatment did not affect female differentiation. These results demonstrated that dsx in the sawfly is essential for male development and its depletion caused complete male-to-female sex reversal. This is the first demonstration of functional depletion of dsx not causing intersexuality but inducing total sex reversal in males instead.

Abstract

The doublesex (dsx) gene, which encodes a transcription factor, regulates sexual differentiation in insects. Sex-specific splicing of dsx occurs to yield male- and female-specific isoforms, which promote male and female development, respectively. Thus, functional disruption of dsx leads to an intersexual phenotype in both sexes. We previously identified a dsx ortholog in the sawfly, Athalia rosae. Similar to dsx in other insects, dsx in the sawfly yields different isoforms in males and females as a result of alternative splicing. The sawfly exploits a haplodiploid mode of reproduction, in which fertilized eggs develop into diploid females, whereas unfertilized eggs parthenogenetically develop into haploid males. In the present study, we knocked down the A. rosae ortholog of dsx (Ardsx) during several developmental stages with repeated double-stranded RNA (dsRNA) injections. Knockdown of Ardsx via parental RNA interference (RNAi), which enables knockdown of genes in offspring embryos, led to a lack of internal and external genitalia in haploid male progeny. Additional injection of dsRNA targeting Ardsx in these animals caused almost complete male-to-female sex reversal, but the resulting eggs were infertile. Notably, the same knockdown approach using diploid males obtained by sib-crossing caused complete male-to-female sex reversal; they were morphologically and behaviorally females. The same RNAi treatment did not affect female differentiation. These results indicate that dsx in the sawfly is essential for male development and its depletion caused complete male-to-female sex reversal. This is the first demonstration of functional depletion of dsx not causing intersexuality but inducing total sex reversal in males instead.

A conserved role of the duplicated Masculinizer gene in sex determination of the Mediterranean flour moth, Ephestia kuehniella

Sex determination in the silkworm, Bombyx mori, is based on Feminizer (Fem), a W-linked Fem piRNA that triggers female development in WZ individuals, and the Z-linked Masculinizer (Masc), which initiates male development and dosage compensation in ZZ individuals. While Fem piRNA is missing in a close relative of B. mori, Masc determines sex in several representatives of distant lepidopteran lineages. We studied the molecular mechanisms of sex determination in the Mediterranean flour moth, Ephestia kuehniella (Pyralidae). We identified an E. kuehniella Masc ortholog, EkMasc, and its paralog resulting from a recent duplication, EkMascB. Both genes are located on the Z chromosome and encode a similar Masc protein that contains two conserved domains but has lost the conserved double zinc finger domain. We developed PCR-based genetic sexing and demonstrated a peak in the expression of EkMasc and EkMascB genes only in early male embryos. Simultaneous knock-down experiments of both EkMasc and EkMascB using RNAi during early embryogenesis led to a shift from male- to female-specific splicing of the E. kuehniella doublesex gene (Ekdsx), their downstream effector, in ZZ embryos and resulted in a strong female-biased sex-ratio. Our results thus confirmed the conserved role of EkMasc and/or EkMascB in masculinization. We suggest that the C-terminal proline-rich domain, we have identified in all functionally confirmed Masc proteins, in conjunction with the masculinizing domain, is important for transcriptional regulation of sex determination in Lepidoptera. The function of the Masc double zinc finger domain is still unknown, but appears to have been lost in E. kuehniella.

The sex-determining cascade in the silkworm, Bombyx mori, differs greatly from those of other insects. In B. mori, female development is initiated by Fem piRNA expressed from the W chromosome during early embryogenesis. Fem piRNA silences Masculinizer (Masc) thereby blocking the male pathway resulting in female development. It is currently unknown whether this cascade is conserved across Lepidoptera. In the Mediterranean flour moth, Ephestia kuehniella, we identified an ortholog of Masc and discovered its functional duplication on the Z chromosome, which has not yet been found in any other lepidopteran species. We provide two lines of evidence that the EkMasc and/or EkMascB genes play an essential role in masculinization: (i) they show a peak of expression during early embryogenesis in ZZ but not in WZ embryos and (ii) their simultaneous silencing by RNAi results in female-specific splicing of the E. kuehniella doublesex gene (Ekdsx) in ZZ embryos and in a female-biased sex ratio. Our results suggest a conserved role of the duplicated Masc gene in sex determination of E. kuehniella.

The direct regulation of Aalbdsx on AalVgR is indispensable for ovarian development in Aedes albopictus

BACKGROUND

Aedes albopictus is an important vector with an extensive worldwide distribution. Only female mosquitoes play a significant role in the transmission of pathogens. Doublesex (dsx) is a central nexus gene in the insect somatic sex determination hierarchy.

RESULTS

In this study, we characterized the full-length sex-specific splicing forms of the Ae. albopictus dsx (Aalbdsx) gene. Then, we identified 15 direct target genes of DSX in adult females using digital gene expression combined with quantitative real-time polymerase chain reaction (qPCR) by performing a chromatin immunoprecipitation (ChIP) assay with specific DSX antibodies. Knockdown of Aalbdsx suppressed ovarian development and decreased the transcript levels of the Aalbdsx target vitellogenin receptor (VgR) gene, whereas vitellogenin (Vg) expression showed an increase in the fat body. Genes in the major Vg regulatory pathway were also up-regulated.

CONCLUSION

Our results suggest that both Vg and VgR are direct target genes of Aalbdsx and that direct regulation of Aalbdsx on VgR is indispensable for ovarian development in Ae. albopictus, which not only provides a reference for the further elucidation of the evolutionarily conserved role of dsx in Ae. albopictus sexual differentiation but also reveals potential molecular targets for application to the development of sterile male mosquitoes to be released for vector control. © 2020 Society of Chemical Industry.

The Sex Determination Cascade in the Silkworm

In insects, sex determination pathways involve three levels of master regulators: primary signals, which determine the sex; executors, which control sex-specific differentiation of tissues and organs; and transducers, which link the primary signals to the executors. The primary signals differ widely among insect species. In Diptera alone, several unrelated primary sex determiners have been identified. However, the doublesex (dsx) gene is highly conserved as the executor component across multiple insect orders. The transducer level shows an intermediate level of conservation. In many, but not all examined insects, a key transducer role is performed by transformer (tra), which controls sex-specific splicing of dsx. In Lepidoptera, studies of sex determination have focused on the lepidopteran model species Bombyx mori (the silkworm). In B. mori, the primary signal of sex determination cascade starts from Fem, a female-specific PIWI-interacting RNA, and its targeting gene Masc, which is apparently specific to and conserved among Lepidoptera. Tra has not been found in Lepidoptera. Instead, the B. mori PSI protein binds directly to dsx pre-mRNA and regulates its alternative splicing to produce male- and female-specific transcripts. Despite this basic understanding of the molecular mechanisms underlying sex determination, the links among the primary signals, transducers and executors remain largely unknown in Lepidoptera. In this review, we focus on the latest findings regarding the functions and working mechanisms of genes involved in feminization and masculinization in Lepidoptera and discuss directions for future research of sex determination in the silkworm.

Transgenic and knockout analyses of Masculinizer and doublesex illuminated the unique functions of doublesex in germ cell sexual development of the silkworm, Bombyx mori

Background

Masculinizer (Masc) plays a pivotal role in male sex determination in the silkworm, Bombyx mori. Masc is required for male-specific splicing of B. mori doublesex (Bmdsx) transcripts. The male isoform of Bmdsx (BmdsxM) induces male differentiation in somatic cells, while females express the female isoform of Bmdsx (BmdsxF), which promotes female differentiation in somatic cells. Our previous findings suggest that Masc could direct the differentiation of genetically female (ZW) germ cells into sperms. However, it remains unclear whether Masc directly induces spermatogenesis or if it promotes male differentiation in germ cells indirectly by inducing the expression of BmdsxM.

Results

In this study, we performed genetic analyses using the transgenic line that expressed Masc, as well as various Bmdsx knockout lines. We found that Masc-expressing females with a homozygous mutation in BmdsxM showed normal development in ovaries. The formation of testis-like tissues was abolished in these females. On the other hand, Masc-expressing females carrying a homozygous mutation in BmdsxF exhibited almost complete male-specific development in gonads and germ cells. These results suggest that BmdsxM has an ability to induce male development in germ cells as well as internal genital organs, while BmdsxF inhibits BmdsxM activity and represses male differentiation. To investigate whether MASC directly controls male-specific splicing of Bmdsx and identify RNAs that form complexes with MASC in testes, we performed RNA immunoprecipitation (RIP) using an anti-MASC antibody. We found that MASC formed a complex with AS1 lncRNA, which is a testis-specific factor involved in the male-specific splicing of Bmdsx pre-mRNA.

Conclusions

Taken together, our findings suggest that Masc induces male differentiation in germ cells by enhancing the production of BmdsxM. Physical interaction between MASC and AS1 lncRNA may be important for the BmdsxM expression in the testis. Unlike in the Drosophila dsx, BmdsxM was able to induce spermatogenesis in genetically female (ZW) germ cells. To the best of our knowledge, this is the first report that the role of dsx in germ cell sexual development is different between insect species.

Mutation of doublesex in Hyphantria cunea results in sex-specific sterility

BACKGROUND

The gene doublesex (dsx) plays pivotal roles in sex determination and controls sexually dimorphic development in certain insects. Importantly, it also displays a potential candidate target for pest management due to its sex-specific splicing. Therefore, we used CRISPR/Cas9-mediated gene disruption to investigate the function of dsx in Hyphantria cunea, an invasive forest pest.

RESULT

In the present study, we identified the dsx gene from H. cunea which showed a sex-biased expression pattern that was different from other lepidopteran insects. Referring to sex-specific functional analyses in Bombyx mori, we performed a site-specific knockout of the Hcdsx gene by using a CRISPR/Cas9 system, which induced severe abnormalities in external genitalia and some incomplete sex reversal phenotypes, which in turn led to reduced sex-specific fecundity. An alternative splicing pattern of Hcdsx was altered by CRISPR/Cas9-induced mutation, and alterations in splicing affected expression of downstream genes encoding pheromone binding protein 1, vg1 and vg2 (encoding vitellogenin), which contributed to the sex-specific sterility phenotypes in the Hcdsx mutants.

CONCLUSION

The Hcdsx gene plays important roles in sexual differentiation in H. cunea. Disruption of Hcdsx induced sex-specific sterility, demonstrating a potential application in control of this pest. © 2019 Society of Chemical Industry.

Spliceosomal Protein Gene BmSPX Regulates Reproductive Organ Development in Bombyx mori

Sex determination and differentiation are nearly universal to all eukaryotic organisms, encompassing diverse systems and mechanisms. Here, we identified a spliceosomal protein gene BmSPX involved in sex determination of the lepidopeteran insect, Bombyx mori. In a transgenic silkworm line that overexpressed the BmSPX gene, transgenic silkworm males exhibited differences in their external genitalia compared to wild-type males, but normal internal genitalia. Additionally, transgenic silkworm females exhibited a developmental disorder of the reproductive organs. Upregulation of BmSPX significantly increased the expression levels of sex-determining genes (BmMasc and BmIMP) and reduced the female-type splice isoform of Bmdsx, which is a key switch gene downstream of the sex-determination pathway. Additionally, co-immunoprecipitation assays confirmed an interaction between the BmSPX protein and BmPSI, an upstream regulatory factor of Bmdsx. Quantitative real-time PCR showed that BmSPX over-expression upregulated the expression of the Hox gene abdominal-B (Adb-B), which is required for specification of the posterior abdomen, external genitalia, and gonads of insects, as well as the genes in the Receptor Tyrosine Kinase (RTK) signaling pathway. In conclusion, our study suggested the involvement of BmSPX, identified as a novel regulatory factor, in the sex-determination pathway and regulation of reproductive organ development in silkworms.

The POU Transcription Factor POU-M2 Regulates Vitellogenin Receptor Gene Expression in the Silkworm, Bombyx mori

Vitellogenin receptors (VgRs) play critical roles in egg formation by transporting vitellogenin (Vg) into oocytes in insects. Although the function of VgR in insects is well studied, the transcriptional regulation of this gene is still unclear. Here, we cloned the promoter of the VgR gene from Bombyx mori (BmVgR), and predicted many POU cis-response elements (CREs) in its promoter. Electrophoretic mobility shift and chromatin immunoprecipitation assays showed that the POU transcription factor POU-M2 bound directly to the CREs of the promoter. Overexpression of POU-M2 in an ovarian cell line (BmNs) enhanced BmVgR transcription and promoter activity detected by quantitative reverse transcription PCR and luciferase reporter assays. Analyses of expression patterns indicated that POU-M2 was expressed in ovary at day two of wandering stage initially, followed by BmVgR. RNA interference of POU-M2 significantly reduced the transcription of BmVgR in ovary and egg-laying rate. Our results suggest a novel function for the POU factor in silkworm oogenesis by its involvement in BmVgR regulation and expands the understanding of POU factors in insect VgR expression.

An antisense lncRNA functions in alternative splicing of Bmdsx in the silkworm, Bombyx mori

Long non-coding RNAs (lncRNAs) are recently thought to play important roles in some physiological processes. In this study, we identified a lncRNA (named as Bmdsx-AS1) which is the antisense transcript and locates on the position of the crucial sex-determining gene Bmdsx in the silkworm. Quantitative real-time PCR and Fluorescence in situ hybridization demonstrated that Bmdsx-AS1 was highly expressed in the silkworm testis. After knock-down of Bmdsx-AS1, the splicing pattern of Bmdsx pre-mRNA was altered in male silkworm. Transgenic overexpression of Bmdsx-AS1 indicated male-specific splicing form of Bmdsx arose in female silkworm. Furthermore, by using RNA-protein pull down, LC-MS/MS and EMSA, we found a splicing factor hnRNPH binding specifically to Bmdsx-AS1. Co-Immunoprecipitation suggested that hnRNPH interacted with BmPSI, one of the upstream regulating factors of Bmdsx. Thus, our results suggested that the antisense lncRNA Bmdsx-AS1 was involved in alternative splicing of Bmdsx in the silkworm.

Disruption of sex-specific doublesex exons results in male- and female-specific defects in the black cutworm, Agrotis ipsilon

BACKGROUND

Doublesex (dsx), the downstream gene in the insect sex-determination pathway, is a key regulator of sexually dimorphic development and behavior across a variety of insects. Manipulating expression of dsx could be useful in the genetic control of insects. However, information on the sex-specific function of dsx in non-model insects is lacking.

RESULTS

In this work, we isolated a dsx homolog, which is alternatively spliced into six female-specific and one male-specific isoforms, from an important agricultural pest, the black cutworm, Agrotis ipsilon. Studies on the expression of sex-specific Aidsx mRNA during embryonic development showed that the sixth hour post oviposition is the key stage for sex determination in A. ipsilon. Functional analysis of Aidsx was conducted using a CRISPR/Cas9 system targeting female- and male-specific Aidsx exons. Disruptions of sex-specific Aidsx exons resulted in sex-specific, sexually dimorphic defects in external genitals, gonads and antennae, and expression of sex-specific genes as well as production of offspring in both sexes.

CONCLUSION

Our results not only demonstrate that dsx is a key player determining A. ipsilon sexually dimorphic traits, but also provide a potential method for the genetic control of this pest. © 2018 Society of Chemical Industry.

P-element Somatic Inhibitor Protein Binding a Target Sequence in dsx Pre-mRNA Conserved in Bombyx mori and Spodoptera litura

Bombyx mori doublesex (Bmdsx) functions as a double-switch gene in the final step of the sex-determination cascade in the silkworm Bombyx mori. The P-element somatic inhibitor (PSI) protein in B. mori interacts with Bmdsx pre-mRNA in CE1 as an exonic splicing silencer to promote male-specific splicing of Bmdsx. However, the character of the interaction between BmPSI and Bmdsx pre-mRNA remains unclear. Electrophoretic mobility shift assay (EMSA) results showed that the four KH_1 motifs in BmPSI are all essential for the binding, especially the former two KH_1 motifs. Three active sites (I116, L127, and IGGI) in the KH_1 motif were found to be necessary for the binding through EMSA, circular dichroism (CD) spectroscopy, and isothermal titration calorimetry (ITC). The PSI homologous protein in S. litura (SlPSI) was purified and the binding of SlPSI and CE1 was verified. Compared with BmPSI, the mutant SlPSI proteins of I116 and IGGI lost their ability to bind to CE1. In conclusion, the binding of PSI and dsx pre-mRNA are generally conserved in both B. mori and S. litura. These findings provide clues for sex determination in Lepidoptera.

Alternative splicing regulation of doublesex gene by RNA-binding proteins in the silkworm Bombyx mori

Doublesex is highly conserved and sex-specifically spliced in insect sex-determination pathways, and its alternative splicing (AS) is regulated by Transformer, an exonic splicing activator, in the model system of Drosophila melanogaster. However, due to the lack of a transformer gene, AS regulation of doublesex remains unclear in Lepidoptera, which contain the economically important silkworm Bombyx mori and thousands of agricultural pests. Here, we use yeast three-hybrid system to screen for RNA-binding proteins that recognize sex-specific exons 3 and 4 of silkworm doublesex (Bm-dsx); this approach identified BxRBP1/Lark binding to the exon 3, and BxRBP2/TBPH and BxRBP3/Aret binding to the exon 4. Investigation of tissues shows that BxRBP1 and BxRBP2 have no sex specificity, but BxRBP3 has - three of its four isoforms are expressed with a sex-bias. Using novel sex-specific silkworm cell lines, we find that BxRBP1 and BxRBP3 directly interact with each other, and cooperatively function as splicing repressors. Over-expression of BxRBP1 and BxRBP3 isoforms efficiently inhibits splicing of the exons 3 and 4 in the female-specific cells and generates the male-specific isoform of Bm-dsx. We also demonstrate that the sex-determination upstream gene Masc regulates alternatively transcribed BxRBP3 isoforms. Thus, we identify a new regulatory mechanism of doublesex AS in the silkworm, revealing an evolutionary divergence in insect sex-determination.

Identification and characterization of doublesex in Bemisia tabaci

Bemisia tabaci (Gennadius) is an important agricultural pest with a worldwide distribution. Although B. tabaci is known to have a unique haplodiploid reproductive strategy, its sex determination mechanism is largely unknown. In this study, we cloned the full-length sequence of B. tabaci doublesex (Btdsx) and found that Btdsx has 28 splicing isoforms. We found two new splicing isoforms of transformer 2 (Bttra2), which encode two proteins. We also confirmed that both genes lack sex-specific splicing isoforms. Real-time quantitative PCR analysis showed that the expression of Btdsx and Bttra2 is higher in males than in females. RNA interference of Bttra2 affected the expression of Btdsx and vice versa. Furthermore, silencing of Bttra2 or Btdsx caused malformation of the male genitalia (anal style). It did not affect the female phenotype, but reduced the expression of vitellogenin gene in females. These results indicate that Btdsx is associated with sex determination in B. tabaci and that Btdsx and Bttra2 affect each other and are important for male genitalia formation. In addition to increasing our understanding of the roles of dsx and tra2 in the sex determination of B. tabaci, the results will be useful for studies of sex determination in other haplodiploid species.

Monosex in Aquaculture

Monosex refers to the culture of either all-male or all-female populations, a sought after approach in aquaculture. This chapter reviews the advantages of monosex population culture and details the mechanisms to achieve it based on different modes of sex determination and sexual differentiation. A recent case study for an aquaculture biotechnology based on sexual manipulation in crustaceans serves in this chapter to identify the key elements for a successful application. This application which makes use of RNA interference with a key regulating hormone opens the pathway toward environmentally friendly applications in fish and additional aquacultured species. This chapter portrays the state of the art in sexual manipulations in aquacultured species, starting with vertebrate species, followed by the case study of the crustacean species and discussion on how the techniques used in this study are applicable for other species.

Single molecule RNA sequencing uncovers trans-splicing and improves annotations in Anopheles stephensi

Single molecule real-time (SMRT) sequencing has recently been used to obtain full-length cDNA sequences that improve genome annotation and reveal RNA isoforms. Here, we used one such method called isoform sequencing from Pacific Biosciences (PacBio) to sequence a cDNA library from the Asian malaria mosquito Anopheles stephensi. More than 600 000 full-length cDNAs, referred to as reads of insert, were identified. Owing to the inherently high error rate of PacBio sequencing, we tested different approaches for error correction. We found that error correction using Illumina RNA sequencing (RNA-seq) generated more data than using the default SMRT pipeline. The full-length error-corrected PacBio reads greatly improved the gene annotation of Anopheles stephensi: 4867 gene models were updated and 1785 alternatively spliced isoforms were added to the annotation. In addition, six trans-splicing events, where exons from different primary transcripts were joined together, were identified in An. stephensi. All six trans-splicing events appear to be conserved in Culicidae, as they are also found in Anopheles gambiae and Aedes aegypti. The proteins encoded by trans-splicing events are also highly conserved and the orthologues of these proteins are cis-spliced in outgroup species, indicating that trans-splicing may arise as a mechanism to rescue genes that broke up during evolution.

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.

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.

Identification and functional characterization of the sex-determining gene doublesex in the sawfly, Athalia rosae (Hymenoptera: Tenthredinidae)

Sexual fate of the sawfly, Athalia rosae (Hymenoptera: Tenthredinidae) is determined by the complementary sex determination (CSD) mechanism as is the case in honeybees. However, to date, genes involved in sex determination have not been identified in this species. In this study, we attempted to identify orthologs of complementary sex-determiner (csd), feminizer (fem), and doublesex (dsx) from the A. rosae genome, all of which are crucial components of the sex determination cascade in the honeybee. As a result, we identified a sawfly ortholog of dsx (designated as Ardsx). Rapid amplification of cDNA ends (RACE) using total RNA extracted from male and female larvae identified three male-specific variants and three female-specific variants. Comparison between the full-length Ardsx cDNAs and the genomic sequence revealed that exon 5 was differentially spliced between the male- and female-specific variants. RT-PCR analysis demonstrated that Ardsx pre-mRNA was spliced alternatively in a sex-dependent manner at almost all the developmental stages. RNAi-mediated knockdown of Ardsx in males caused severe defects in the reproductive organs and, notably, induced development of the ovipository apparatus containing the dorsal pair of blades and the sheath. These males also showed abnormalities in testes and seminal vesicles and lacked mature sperm. The present study provides the first direct evidence that dsx is essential for sexual development in hymenopteran species.

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.

The C-terminus of DSX(F5) protein acts as a novel regulatory domain in Bombyx mori

The doublesex gene regulates the somatic sexual development of Bombyx mori by alternatively splicing into sex-specific splice forms. In our previous study, the splice form Bmdsx (F7) , which encodes the BmDSX(F5) protein, was found to be expressed in a female-specific manner and to contain a novel C-terminus. In this study, we aimed to investigate the role of this C-terminus. Two transgenic lines, L1 and L2, were constructed to ectopically express Bmdsx (F7) in males. Phenotype and W chromosome-specific polymerase chain reaction (PCR) analysis showed that developmental abnormalities and sex reversal did not occur. Moreover, the sex ratio was also normal. Quantitative PCR revealed that the expression levels of SP1 and Vg were upregulated in the fat body of transgenic males. Additionally, the expression level of PBP was downregulated in the antenna of transgenic males. The results suggested that the C-terminus of BmDSX(F5) functioned as a regulatory domain during regulation of downstream target gene expression and that BmDSX(F5) participated in the sexual development of somatic cells together with other DSX proteins in B. mori.

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.

An Unusual Role for doublesex in Sex Determination in the Dipteran Sciara

The gene doublesex, which is placed at the bottom of the sex-determination gene cascade, plays the ultimate discriminatory role for sex determination in insects. In all insects where this gene has been characterized, the dsx premessenger RNA (pre-mRNA) follows a sex-specific splicing pattern, producing male- and female-specific mRNAs encoding the male-DSXM and female-DSXF proteins, which determine male and female development, respectively. This article reports the isolation and characterization of the gene doublesex of dipteran Sciara insects. The Sciara doublesex gene is constitutively transcribed during development and adult life of males and females. Sciara had no sex-specific doublesex mRNAs but the same transcripts, produced by alternative splicing of its primary transcript, were present in both sexes, although their relative abundance is sex specific. However, only the female DSXF protein, but not the male DSXM protein, was produced at similar amounts in both sexes. An analysis of the expression of female and male Sciara DSX proteins in Drosophila showed that these proteins conserved female and male function, respectively, on the control of Drosophila yolk-protein genes. The molecular evolution of gene doublesex of all insects where this gene has been characterized revealed that Sciara doublesex displays a considerable degree of divergence in its molecular organization and its splicing pattern with respect to the rest of dipterans as suggested by its basal position within the doublesex phylogeny. It is suggested that the doublesex gene is involved in Sciara sex determination although it appears not to play the discriminatory role performed in other insects.

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.

Effects of RNAi-mediated knockdown of histone methyltransferases on the sex-specific mRNA expression of Imp in the silkworm Bombyx mori

Sexual differentiation in Bombyx mori is controlled by sex-specific splicing of Bmdsx, which results in the omission of exons 3 and 4 in a male-specific manner. In B. mori, insulin-like growth factor II mRNA-binding protein (Imp) is a male-specific factor involved in male-specific splicing of Bmdsx. Male-specific Imp mRNA results from the male-specific inclusion of exon 8. To verify the link between histone methylation and alternative RNA processing in Imp, we examined the effects of RNAi-mediated knockdown of several histone methyltransferases on the sex-specific mRNA expression of Imp. As a result, male-specific expression of Imp mRNA was completely abolished when expression of the H3K79 methyltransferase DOT1L was repressed to <10% of that in control males. Chromatin immunoprecipitation-quantitative PCR analysis revealed a higher distribution of H3K79me2 in normal males than in normal females across Imp. RNA polymerase II (RNAP II) processivity assays indicated that RNAi knockdown of DOT1L in males caused a twofold decrease in RNAP II processivity compared to that in control males, with almost equivalent levels to those observed in normal females. Inhibition of RNAP II-mediated elongation in male cells repressed the male-specific splicing of Imp. Our data suggest the possibility that H3K79me2 accumulation along Imp is associated with the male-specific alternative processing of Imp mRNA that results from increased RNAP II processivity.

Conserved RNA cis-elements regulate alternative splicing of Lepidopteran doublesex

Doublesex (dsx) is a downstream key regulator in insect sex determination pathway. In Drosophila, alternative splicing of Dm-dsx gene is sex-specifically regulated by transformer (tra), in which the functional TRA promotes female-specific Dm-dsx. However, the sex determination pathway in Lepidoptera is not well understood; here we focused on alternative splicing of doublesex (dsx) in two agricultural pests, Asian corn borer (Ostrinia furnacalis) and cotton bollworm (Helicoverpa armigera), as well as the silkworm (Bombyx mori). More than a dozen new alternative splicing isoforms of dsx were found in the Lepidopteran females, which exist in all tested developmental stages and differentiated tissues. Alignment of mRNA and protein sequences of doublesex revealed high conservation of this gene in Lepidoptera. Strength analysis of splice sites revealed a weak 5' splice site at intron 3 in Lepidopteran dsx, which was experimentally confirmed. Furthermore, we identified highly conserved RNA sequences in the Lepidopteran dsx, including RNA elements I (14 nt), II (11 nt), III (26 nt), IV (17 nt), 3E-1 (8 nt) and 3E-2 (8 nt). The RNA elements III and IV were previously found in exon 4 of B. mori dsx and bound with Bm-PSI, which suppressed the inclusion of exons 3 & 4 into the male-specific Bm-dsx. Then we identified and analyzed the homologous genes of Bm-psi in the two Lepidopteran pests, which expressed at similar levels and exhibited a unique isoform in the males and females from each Lepidoptera. Importantly, mutagenesis of Bm-dsx mini-genes and their expression in BmN cell line demonstrated that three RNA elements are involved in the female-specific alternative splicing of Bm-dsx. Mutations in the RNA cis-elements 3E-1 and 3E-2 resulted in decreased inclusion of exon 3 into the female-specific dsx mRNA, suggesting that these two elements would be exonic splicing enhancers that facilitate the recognition of the weak 5' splice site at intron 3 of Lepidopteran dsx. We propose that the 5' splice sites at intron 3 are weak, resulting in multiple alternative splicing events in intron 3 of female Lepidoptera dsx. Activation of the 5' splice site requires regulatory cis-elements in exons 3 for female-specific splicing of Lepidoptera dsx.

Phylogenetic distribution and evolutionary dynamics of the sex determination genes doublesex and transformer in insects

Sex determination in insects is characterized by a gene cascade that is conserved at the bottom but contains diverse primary signals at the top. The bottom master switch gene doublesex is found in all insects. Its upstream regulator transformer is present in the orders Hymenoptera, Coleoptera and Diptera, but has thus far not been found in Lepidoptera and in the basal lineages of Diptera. transformer is presumed to be ancestral to the holometabolous insects based on its shared domains and conserved features of autoregulation and sex-specific splicing. We interpret that its absence in basal lineages of Diptera and its order-specific conserved domains indicate multiple independent losses or recruitments into the sex determination cascade. Duplications of transformer are found in derived families within the Hymenoptera, characterized by their complementary sex determination mechanism. As duplications are not found in any other insect order, they appear linked to the haplodiploid reproduction of the Hymenoptera. Further phylogenetic analyses combined with functional studies are needed to understand the evolutionary history of the transformer gene among insects.

Manipulation of arthropod sex determination by endosymbionts: diversity and molecular mechanisms

Arthropods exhibit a large variety of sex determination systems both at the chromosomal and molecular level. Male heterogamety, female heterogamety, and haplodiploidy occur frequently, but partially different genes are involved. Endosymbionts, such as Wolbachia, Cardinium,Rickettsia, and Spiroplasma, can manipulate host reproduction and sex determination. Four major reproductive manipulation types are distinguished: cytoplasmic incompatibility, thelytokous parthenogenesis, male killing, and feminization. In this review, the effects of these manipulation types and how they interfere with arthropod sex determination in terms of host developmental timing, alteration of sex determination, and modification of sexual differentiation pathways are summarized. Transitions between different manipulation types occur frequently which suggests that they are based on similar molecular processes. It is also discussed how mechanisms of reproductive manipulation and host sex determination can be informative on each other, with a special focus on haplodiploidy. Future directions on how the study of endosymbiotic manipulation of host reproduction can be key to further studies of arthropod sex determination are shown.

Identification of the key stages for sex determination in the silkworm, Bombyx mori

In general, the master switch gene for sex determination is expressed for a limited period during the early embryonic stage. To increase our understanding of the sex determination mechanism in Bombyx mori, it is important to understand when sex determination takes place. To examine the key stages for sex determination in this insect, we focused on the expression patterns of Bmdsx (a double-switch gene in the sex determination cascade of B. mori) and BmIMP (a gene expressed specifically in males involved in male-specific splicing of Bmdsx). Reverse transcription PCR (RT-PCR) analysis revealed that male-type Bmdsx expression was observed in females at 27 and 29 h after oviposition (hao), and finally disappeared at 32 hao. Moreover, BmIMP mRNA was also expressed in these females, and its expression level was comparable to that of the male-type Bmdsx mRNA. These results demonstrated that female embryos before 32 hao can show male-type expression of Bmdsx and BmIMP, suggesting that sex determination occurs between 29 and 32 hao, which correspond to the developmental stages from the head lobe differentiation to spoon-shaped embryo stages. This also suggests that the master switch gene for sex determination of B. mori is expressed in females during this period and represses the male-specific mode of expression in sex-determining genes.

Sex determination in insects: variations on a common theme

Recent studies in a representative selection of holometabolous insects suggest that, despite diversity at the instructive level, the signal-relaying part of the sex-determining pathway is remarkably well conserved. In principle, it is composed of the transformer gene (tra), which acts as a common binary switch that transduces the selected sexual fate, female when ON, male when OFF, to the downstream effector doublesex(dsx) that controls overt sexual differentiation. An interesting recurrent feature is that tra is switched ON in the early zygote by maternally provisioned tra activity. Different male-determining signals evolved, which prevent maternal activation of zygotic tra to allow for male development. In some species, where lack of maternal activation leaves tra in the OFF state, novel female-determining signals were deployed to activate zygotic tra. It appears that both the instructive end of the pathway upstream of tra as well as the executive end downstream of dsx are primary targets of evolutionary divergence, while the transduction part seems less prone to changes. We propose that this is a feature shared with many other signaling cascades that regulate developmental fates.

Male-specific splicing of the silkworm Imp gene is maintained by an autoregulatory mechanism

Sexual differentiation in the silkworm Bombyx mori is controlled by sex-specific splicing of Bmdsx, in which exons 3 and 4 are skipped in males. B. mori insulin-like growth factor II mRNA-binding protein (Imp) is a factor involved in the male-specific splicing of Bmdsx. In this study, we found that the male-specific Imp mRNA is formed as a result of the inclusion of exon 8 and the promoter-distal poly(A) site choice, whereas non-sex-specific polyadenylation occurs at the promoter-proximal poly(A) site downstream of exon 7. Recent studies revealed that Drosophila Sxl, tra in several dipteran and hymenopteran insects, and fem in Apis mellifera, play a central role in sex determination and maintain their productive mode of expression via an autoregulatory function. To determine whether Imp protein is required for the maintenance of the male-specific splicing of its own pre-mRNA, we knocked down endogenous Imp in male cells and assessed the male-specific splicing of an exogenous Imp minigene. Knockdown of endogenous Imp inhibited the male-specific splicing of the Imp minigene transcript. In contrast, overexpression of Imp in female cells induced the male-specific splicing of the Imp minigene transcript. Moreover, deletion of adenine-rich (A-rich) sequences located downstream of the proximal poly(A) site repressed the male-specific splicing of the Imp minigene transcript. Finally, gel shift analysis demonstrated that Imp binds to the A-rich sequences. These data suggest that Imp binds to the A-rich sequences in its own pre-mRNA to induce the male-specific splicing of its pre-mRNA.

New insights into the genomic organization and splicing of the doublesex gene, a terminal regulator of sexual differentiation in the silkworm Bombyx mori

Sex-determination mechanisms differ among organisms. The primary mechanism is diverse, whereas the terminal regulator is relatively-conserved. We analyzed the transcripts of the Bombyx mori doublesex gene (Bmdsx), and reported novel results concerning the genomic organization and expression of Bmdsx. Bmdsx consists of nine exons and eight introns, of which two exons are novel and have not been reported previously. Bmdsx transcripts are spliced to generate seventeen alternatively-spliced forms and eleven putative trans-spliced variants. Thirteen of the alternatively-spliced forms and five of the putative trans-spliced forms are reported here for the first time. Sequence analysis predicts that ten female-specific, six male-specific splice forms and one splice form found in males and females will result in four female-specific, two male-specific Dsx proteins and one Dsx protein common to males and females. The Dsx proteins are expected to be functional and regulate downstream target genes. Some of the predicted Dsx proteins are described here for the first time. Therefore the expression of the dsx gene in B. mori results in a variety of cis- and trans-spliced transcripts and multiple Dsx proteins. These findings show that in B. mori there is a complicated pattern of dsx splicing, and that the regulation of splicing and sex-specific functions of lepidopteran dsx have evolved complexity.

Transgene-based, female-specific lethality system for genetic sexing of the silkworm, Bombyx mori

Transgene-based genetic sexing methods are being developed for insects of agricultural and public health importance. Male-only rearing has long been sought in sericulture because males show superior economic characteristics, such as better fitness, lower food consumption, and higher silk yield. Here we report the establishment of a transgene-based genetic sexing system for the silkworm, Bombyx mori. We developed a construct in which a positive feedback loop regulated by sex-specific alternative splicing leads to high-level expression of the tetracycline-repressible transactivator in females only. Transgenic animals show female-specific lethality during embryonic and early larval stages, leading to male-only cocoons. This transgene-based female-specific lethal system not only has wide application in sericulture, but also has great potential in lepidopteran pest control.

Engineered female-specific lethality for control of pest Lepidoptera

The sterile insect technique (SIT) is a pest control strategy involving the mass release of radiation-sterilized insects, which reduce the target population through nonviable matings. In Lepidoptera, SIT could be more broadly applicable if the deleterious effects of sterilization by irradiation could be avoided. Moreover, male-only release can improve the efficacy of SIT. Adequate methods of male-only production in Lepidoptera are currently lacking, in contrast to some Diptera. We describe a synthetic genetic system that allows male-only moth production for SIT and also replaces radiation sterilization with inherited female-specific lethality. We sequenced and characterized the doublesex (dsx) gene from the pink bollworm (Pectinophora gossypiella). Sex-alternate splicing from dsx was used to develop a conditional lethal genetic sexing system in two pest moths: the diamondback moth (Plutella xylostella) and pink bollworm. This system shows promise for enhancing existing pink bollworm SIT, as well as broadening SIT-type control to diamondback moth and other Lepidoptera.

Novel female-specific trans-spliced and alternative splice forms of dsx in the silkworm Bombyx mori

The Bombyx mori doublesex gene (Bmdsx) plays an important role in somatic sexual development. Its pre-mRNA splices in a sex-specific manner to generate two female-specific and one male-specific splice forms. The present study investigated six novel dsx variants generated by trans-splicing between female dsx transcripts and two additional novel genes, dsr1 and dsr2. Expression analysis indicated that Bmdsx-dsr1 represented splicing noise, whereas dsr2, which trans-spliced with dsx to generate five variants, regulated the expression of the female-specific B. mori dsx transcript Bmdsx(F)s. We unexpectedly found a novel exon 2n insertion during Bmdsx transcription, which did not influence the validity of the novel protein, BmDSX(F3). Ectopic expression of BmDSX(F3) repressed the pheromone-binding protein gene and the testis-specific gene A2 in males, and activated of the storage protein 1 gene. Our findings suggest that trans-splicing is a novel regulatory function of Bmdsx, which participates in female sexual development by regulating the expression of three BmDSX(F) proteins.