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

Cell-based analysis reveals that sex-determining gene signals in Ostrinia are pivotally changed by male-killing Wolbachia

Wolbachia, a maternally transmitted bacterium, shows male-killing, an adaptive phenotype for cytoplasmic elements, in various arthropod species during the early developmental stages. In lepidopteran insects, lethality of males is accounted for by improper dosage compensation in sex-linked genes owing to Wolbachia-induced feminization. Herein, we established Ostrinia scapulalis cell lines that retained sex specificity per the splicing pattern of the sex-determining gene doublesex (Osdsx). We found that Wolbachia transinfection in male cell lines enhanced the female-specific splice variant of Osdsx (Osdsx^F ) while suppressing the male-specific variant (Osdsx^M ), indicating that Wolbachia affects sex-determining gene signals even in vitro. Comparative transcriptome analysis isolated only two genes that behave differently upon Wolbachia infection. The two genes were respectively homologous to Masculinizer (BmMasc) and zinc finger-2 (Bmznf-2), male-specifically expressed sex-determining genes of the silkworm Bombyx mori that encode CCCH-type zinc finger motif proteins. By using cultured cells and organismal samples, OsMasc and Osznf-2 were found to be sex-determining genes of O. scapulalis that are subjected to sex-specific alternative splicing depending upon the chromosomal sex, developmental stage, and infection status. Overall, our findings expound the cellular autonomy in insect sex determination and the mechanism through which sex is manipulated by intracellular selfish microbes.

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.

A CCCH zinc finger gene regulates doublesex alternative splicing and male development in Bombyx mori

Recent identification of a Piwi-interacting RNA (piRNA)-initiated sex determination cascade in the silkworm, Bombyx mori, provides novel insights into high diversity of insect sex determination pathways. In this system, the W-chromosome-derived Fem piRNA is the primary sex determination signal. A CCCH-type zinc finger gene Masculinizer (Masc), which is targeted by Fem piRNA-PIWI complex in female animals, is indispensable for male-specific splicing of B. mori doublesex (Bmdsx). Although many genes involved in this cascade have been identified, the regulatory mechanisms of silkworm sex determination remain to be elucidated. Here we show that another CCCH-type zinc finger gene, Bmznf-2, is a masculinization factor in B. mori. Bmznf-2 shows testis-abundant expression and loss of Bmznf-2 function via clustered regularly interspaced short palindromic repeats / single-guide RNA-mediated mutagenesis results in feminized differentiation and appearance of the female-specific splicing variants of Bmdsx transcripts in males. In contrast, there is no phenotypic consequence in mutant females. In mutant males, relative messenger RNA expression levels of female-dominant genes such as vitellogenin and sex-specific storage protein 1 are significantly elevated while several male-dominant genes are significantly down-regulated. Furthermore, male mutants show delayed developmental timing, smaller body sizes of larvae and malformation of moth wings. Our data thus reveal that Bmznf-2 plays an indispensable role in silkworm male sexual differentiation.

Identification and Characterization of the Masculinizing Function of the Helicoverpa armigera Masc Gene

The Masculinizer (Masc) gene has been known to control sex development and dosage compensation in lepidopterans. However, it remains unclear whether its ortholog exists and plays the same roles in distantly related lepidopterans such as Helicoverpa armigera. To address this question, we cloned Masc from H. armigera (HaMasc), which contains all essential functional domains of BmMasc, albeit with less than 30% amino acid sequence identity with BmMasc. Genomic PCR and qPCR analyses showed that HaMasc is a Z chromosome-linked gene since its genomic content in males (ZZ) was two times greater than that in females (ZW). RT-PCR and RT-qPCR analyses revealed that HaMasc expression was sex- and stage-biased, with significantly more transcripts in males and eggs than in females and other stages. Transfection of a mixture of three siRNAs of HaMasc into a male embryonic cell line of H. armigera led to the appearance of female-specific mRNA splicing isoforms of H. armigeradoublesex (Hadsx), a downstream target gene of HaMasc in the H. armigera sex determination pathway. The knockdown of HaMasc, starting from the third instar larvae resulted in a shift of Hadsx splicing from male to female isoforms, smaller male pupa and testes, fewer but larger/longer spermatocytes and sperm bundles, delayed pupation and internal fusion of the testes and follicles. These data demonstrate that HaMasc functions as a masculinizing gene in the H. armigera sex-determination cascade.

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.

Novel doublesex duplication associated with sexually dimorphic development of dogface butterfly wings

Sexually dimorphic development is responsible for some of the most remarkable phenotypic variation found in nature. Alternative splicing of the transcription factor gene doublesex (dsx) is a highly conserved developmental switch controlling the expression of sex-specific pathways. Here, we leverage sex-specific differences in butterfly wing color pattern to characterize the genetic basis of sexually dimorphic development. We use RNA-seq, immunolocalization, and motif binding site analysis to test specific predictions about the role of dsx in the development of structurally based ultraviolet (UV) wing patterns in Zerene cesonia (Southern Dogface). Unexpectedly, we discover a novel duplication of dsx that shows a sex-specific burst of expression associated with the sexually dimorphic UV coloration. The derived copy consists of a single exon that encodes a DNA binding but no protein-binding domain and has experienced rapid amino-acid divergence. We propose the novel dsx paralog may suppress UV scale differentiation in females, which is supported by an excess of Dsx-binding sites at cytoskeletal and chitin-related genes with sex-biased expression. These findings illustrate the molecular flexibility of the dsx gene in mediating the differentiation of secondary sexual characteristics.

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.

Two CCCH-type zinc finger domains in the Masc protein are dispensable for masculinization and dosage compensation in Bombyx mori

The Masculinizer (Masc) gene encodes a novel lepidopteran-specific protein that controls both masculinization and dosage compensation in the silkworm Bombyx mori. The Masc protein possesses two CCCH-type zinc finger domains (ZFs), a nuclear localization signal, and an 11-amino-acid region that is highly conserved among lepidopteran insects. Using a cell-based assay system, we revealed that two cysteine residues localized in the conserved region, but not ZFs, are required for masculinization. In addition, nuclear localization of the Masc protein is not associated with masculinizing activity. Because dosage compensation is considered to occur in the nucleus, we inferred that the two ZFs play a role in the establishment of dosage compensation. To investigate this hypothesis at the organism level, we utilized the CRISPR/Cas9 system and established three B. mori strains whose Masc is partially deleted at different regions. The strain lacking the 210 C-terminal amino acids of the Masc protein showed male-specific embryonic lethality due to its low abundance and/or instability. The male embryos of this strain expressed the female-type splice variants of B. mori doublesex and did not express the male-type mRNA of B. mori IGF-II mRNA-binding protein. Furthermore, mRNA levels of Z-linked genes were abnormally enhanced only in male embryos. In contrast, the strain lacking both ZFs grew normally and did not show any defective phenotypes including sexual differentiation and the expression of Z-linked genes, demonstrating that the two CCCH-type ZFs, which are conserved in lepidopteran Masc homologs, are dispensable for masculinization and dosage compensation.

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.