Molecular analysis and developmental expression of the Sex-lethal gene of Sciara ocellaris (Diptera order, Nematocera suborder)

Isolation of the sex-determining gene Sex-lethal (Sxl) in Penaeus (Litopenaeus) vannamei (Boone, 1931) and characterization of its embryogenic, gametogenic, and tissue-specific expression

The Pacific white shrimp Penaeus vannamei is the most cultured shrimp species around the world. Because females grow larger than males, the culture of 'only females' is of great interest, but knowledge on sex determination and differentiation is required for producing only females. In an effort to obtain information associated with reproduction in P. vannamei, transcriptomic data from female gonads was generated, and partial sequences of a transcript were identified as Sex-lethal (Sxl). Its characterization indicated that, differently from other penaeids in which this gene has been isolated, there are six isoforms of the Sxl transcript in P. vannamei (PvanSxl 1-6). These isoforms result from alternative splicing at three splice sites (SS1, SS2, SS3). The first splice-site is unique to P. vannamei, as it has not been reported for other Arthropod species; the second splice-site (SS2) is common among crustaceans, and the third splice-site (SS3) is also unique to P. vannamei and when spliced-out, it is always together with SS2. All isoforms are expressed during embryogenesis as well as gametogenesis of both genders. The two shorter isoforms, PvanSxl-5 and PvanSxl-6, which result from the splicing of SS2 and SS3, were found mostly expressed in adult testis, but PvanSxl-6 was also expressed in oocytes during gametogenesis. During oogenesis, the second largest isoform, PvanSxl-2, which splices-out only SS1, and PvanSxl-4 that splices-out SS1 and SS2 were highly expressed. These two isoforms were also highly expressed during embryonic development. In situ hybridization allowed pinpointing more specifically the cells where the PvanSxl transcripts were expressed. During embryogenesis, hybridization was observed from the one-cell stage embryo to late gastrula. In the female gonad in previtellogenesis, hybridization occurred in the nucleus of oocytes, whereas in secondary vitellogenesis the transcript also hybridized cytoplasmic granules and cortical crypts. Finally, in situ hybridization corroborated the expression of PvanSxl also in the male gonad during spermatogenesis, mostly occurring in the cytoplasm from spermatogonia and spermatocytes.

Beyond DNA puffs: What can we learn from studying sciarids?

Members of the Sciaridae family attracted the interest of researchers because of the demonstration that the DNA puff regions, which are formed in the salivary gland polytene chromosomes at the end of the fourth larval instar, constitute sites of developmentally regulated gene amplification. Besides contributing to a deeper understanding of the process of gene amplification, the study of sciarids has also provided important insights on other biological processes such as sex determination, programmed cell death, insect immunity, telomere maintenance, and nucleolar organizing regions (NOR) formation. Open questions in sciarids include among others, early development, the role of noncoding RNAs in gene amplification and the relationship between gene amplification and transcription in DNA puff forming regions. These and other questions can now be pursued with next generation sequencing techniques and experiments using RNAi experiments, since this latter technique has been shown to be feasible in sciarids. These new perspectives in the field of sciarid biology open the opportunity to consolidate sciarid species as important emerging models. genesis 54:361-378, 2016. © 2016 Wiley Periodicals, Inc.

Identification and functional analysis of a Masculinizer orthologue in Trilocha varians (Lepidoptera: Bombycidae)

We recently showed that the Masculinizer gene (Masc) plays a primary role in sex determination in the lepidopteran model insect Bombyx mori. However, it remains unknown whether this Masc protein-dependent sex determination system is conserved amongst lepidopteran insects or within the family Bombycidae. Here we cloned and characterized a Masc homologue (TvMasc) in Trilocha varians (Lepidoptera: Bombycidae), a species closely related to B. mori. To elucidate the role of TvMasc in the sex determination cascade of T. varians, TvMasc expression was knocked down in early embryos by the injection of small interfering RNAs (siRNAs) that targeted TvMasc mRNAs. Both female- and male-type splice variants of Tvdsx, a doublesex (dsx) homologue in T. varians were observed in control siRNA-injected embryos. By contrast, only female-type splice variants were observed in TvMasc siRNA-injected embryos. These results indicate that the TvMasc protein directly or indirectly regulates the splicing patterns of Tvdsx. Furthermore, we found that male-type splice variants of B. mori dsx (Bmdsx) were produced in TvMasc-overexpressing BmN4 cells. The mRNA level of B. mori Imp, a gene whose product induces male-specific Bmdsx splicing also increased. These results suggest that Masc genes play similar roles in the sex-determination cascade in Bombycidae.

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.

The comparative study of five sex-determining proteins across insects unveils high rates of evolution at basal components of the sex determination cascade

In insects, the sex determination cascade is composed of genes that interact with each other in a strict hierarchical manner, constituting a coadapted gene complex built in reverse order from bottom to top. Accordingly, ancient elements at the bottom are expected to remain conserved ensuring the correct functionality of the cascade. In the present work, we have studied the levels of variation displayed by five key components of the sex determination cascade across 59 insect species, including Sex-lethal, transformer, transformer-2, fruitless, doublesex, and sister-of-Sex-lethal (a paralog of Sxl encompassing sex-independent functions). Surprisingly, our results reveal that basal components of the cascade (doublesex, fruitless) seem to evolve more rapidly than previously suspected. Indeed, in the case of Drosophila, these proteins evolve more rapidly than the master regulator Sex-lethal. These results agree with the notion suggesting that genes involved in early aspects of development will be more constrained due to the large deleterious pleiotropic effects of mutations, resulting in increased levels of purifying selection at top positions of the cascade. The analyses of the selective episodes involved in the recruitment of Sxl into sex-determining functions further support this idea, suggesting the presence of bursts of adaptive selection in the common ancestor of drosophilids, followed by the onset of purifying selection preserving the master regulatory role of this protein on top of the Drosophila sex determination cascade. Altogether, these results underscore the importance of the position of sex determining genes in the cascade, constituting a major constraint shaping the molecular evolution of the insect sex determination pathway.

Practical applications of insects' sexual development for pest control

Elucidation of the sex differentiation pathway in insects offers an opportunity to understand key aspects of evolutionary developmental biology. In addition, it provides the understanding necessary to manipulate insects in order to develop new synthetic genetics-based tools for the control of pest insects. Considerable progress has been made in this, especially in improvements to the sterile insect technique (SIT). Large scale sex separation is considered highly desirable or essential for most SIT targets. This separation can be provided by genetic methods based on sex-specific gene expression. Investigation of sex determination by many groups has provided molecular components and methods for this. Though the primary sex determination signal varies considerably, key regulatory genes and mechanisms remain surprisingly similar. In most cases studied so far, a primary signal is transmitted to a basal gene at the bottom of the hierarchy (dsx) through an alternative splicing cascade; dsx is itself differentially spliced in males and females. A sex-specific alternative splicing system therefore offers an attractive route to achieve female-specific expression. Experience has shown that alternative splicing modules can be developed with cross-species function; modularity and standardisation and re-use of parts are key principles of synthetic biology. Both female-killing and sex reversal (XX females to phenotypic males) can in principle also be used as efficient alternatives to sterilisation in SIT-like methods. Sexual maturity is yet another area where understanding of sexual development may be applied to insect control programmes. Further detailed understanding of this crucial aspect of insect biology will undoubtedly continue to underpin innovative practical applications.

Sex-determining mechanisms in insects based on imprinting and elimination of chromosomes

As a rule, the sex of an individual is fixed at fertilization, and the chromosomal constitution of the zygote is a direct consequence of the chromosomal constitution of the gametes. However, there are cases in which the chromosomal differences determining sex are brought about by elimination or inactivation of chromosomes in the embryo. In Sciaridae insects, all zygotes start with the XXX constitution; the loss of either 1 or 2 X chromosomes determines whether the zygote becomes XX (female) or X0 (male). In Cecydomyiidae and Collembola insects, all zygotes start with the XXXX constitution. If the embryo does not eliminate any X chromosome, this remains XXXX and develops as female, whereas if 2 X chromosomes are eliminated, the embryo becomes XX0 and develops as a male. In the coccids (scale insects), the chromosomal differences between the sexes result from either the elimination or the heterochromatinization (inactivation) of half of the chromosomes giving rise to haploid males and diploid females. The chromosomes that are eliminated or inactivated are those inherited from the father. Therefore, in the formation of the sex-determining chromosomal signal in those insects, a marking ('imprinting') process must occur in one of the parents, which determines that the chromosomes to be eliminated or inactivated are of paternal origin. In this article, the sex determination mechanism of these insects and the associated imprinting process are reviewed.

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.

Biochemical and functional analysis of Drosophila-sciara chimeric sex-lethal proteins

BACKGROUND

The Drosophila SXL protein controls sex determination and dosage compensation. It is a sex-specific factor controlling splicing of its own Sxl pre-mRNA (auto-regulation), tra pre-mRNA (sex determination) and msl-2 pre-mRNA plus translation of msl-2 mRNA (dosage compensation). Outside the drosophilids, the same SXL protein has been found in both sexes so that, in the non-drosophilids, SXL does not appear to play the key discriminating role in sex determination and dosage compensation that it plays in Drosophila. Comparison of SXL proteins revealed that its spatial organisation is conserved, with the RNA-binding domains being highly conserved, whereas the N- and C-terminal domains showing significant variation. This manuscript focuses on the evolution of the SXL protein itself and not on regulation of its expression.

METHODOLOGY

Drosophila-Sciara chimeric SXL proteins were produced. Sciara SXL represents the non-sex-specific function of ancient SXL in the non-drosophilids from which presumably Drosophila SXL evolved. Two questions were addressed. Did the Drosophila SXL protein have affected their functions when their N- and C-terminal domains were replaced by the corresponding ones of Sciara? Did the Sciara SXL protein acquire Drosophila sex-specific functions when the Drosophila N- and C-terminal domains replaced those of Sciara? The chimeric SXL proteins were analysed in vitro to study their binding affinity and cooperative properties, and in vivo to analyse their effect on sex determination and dosage compensation by producing Drosophila flies that were transgenic for the chimeric SXL proteins.

CONCLUSIONS

The sex-specific properties of extant Drosophila SXL protein depend on its global structure rather than on a specific domain. This implies that the modifications, mainly in the N- and C-terminal domains, that occurred in the SXL protein during its evolution within the drosophilid lineage represent co-evolutionary changes that determine the appropriate folding of SXL to carry out its sex-specific functions.

The gene transformer-2 of Sciara (Diptera, Nematocera) and its effect on Drosophila sexual development

BACKGROUND

The gene transformer-2, which is involved in sex determination, has been studied in Drosophila, Musca, Ceratitis, Anastrepha and Lucilia. All these members of Diptera belong to the suborder Brachycera. In this work, it is reported the isolation and characterisation of genes transformer-2 of the dipterans Sciara ocellaris and Bradysia coprophila (formerly Sciara coprophila), which belong to the much less extensively analysed Sciaridae Family of the Suborder Nematocera, which is paraphyletic with respect to Suborder Brachycera.

RESULTS

The transformer-2 genes of the studied Sciara species were found to be transcribed in both sexes during development and adult life, in both the soma and germ lines. They produced a single primary transcript, which follows the same alternative splicing in both sexes, giving rise to different mRNAs isoforms. In S. ocellaris the most abundant mRNA isoform encoded a full-length protein of 251 amino acids, while that of B. coprophila encoded a protein of 246 amino acids. Both showed the features of the SR protein family. The less significant mRNA isoforms of both species encoded truncated, presumably non-functional Transformer-2 proteins. The comparison of the functional Sciara Transformer-2 proteins among themselves and those of other insects revealed the greatest degree of conservation in the RRM domain and linker region. In contrast, the RS1 and RS2 domains showed extensive variation with respect to their number of amino acids and their arginine-serine (RS) dipeptide content. The expression of S. ocellaris Transformer-2 protein in Drosophila XX pseudomales lacking the endogenous transformer-2 function caused their partial feminisation.

CONCLUSIONS

The transformer-2 genes of both Sciaridae species encode a single protein in both sexes that shares the characteristics of the Transformer-2 proteins of other insects. These proteins showed conserved sex-determination function in Drosophila; i.e., they were able to form a complex with the endogenous Drosophila Transformer protein that controls the female-specific splicing of the Drosophila doublesex pre-mRNA. However, it appears that the complex formed between the Drosophila Transformer protein and the Sciara Transformer-2 protein is less effective at inducing the female-specific splicing of the endogenous Drosophila doublesex pre-mRNA than the DrosophilaTransformer-Transformer2 complex. This suggests the existence of species-specific co-evolution of the Transformer and Transformer-2 proteins.

The transformer gene of Ceratitis capitata: a paradigm for a conserved epigenetic master regulator of sex determination in insects

The transformer gene in Ceratitis capitata (Cctra(ep)) is the founding member of a family of related SR genes that appear to act as the master epigenetic switch in sex determination in insects. A functional protein seems to be produced only in individuals with a female XX karyotype where it is required to maintain the productive mode of expression through a positive feedback loop and to direct female development by instructing the downstream target genes accordingly. When zygotic activation of this loop is prevented, male development follows. Recently, tra(ep) orthologues were isolated in more distantly related dipteran species including Musca domestica, Glossina morsitans and Lucilia cuprina and in the Hymenopterans Apis mellifera and Nasonia vitripennis. All of these tra(ep) orthologues seem to act as binary switches that govern all aspects of sexual development. Transient silencing leads to complete masculinization of individuals with a female karyotype. Reciprocally, in some systems it has been shown that transient expression of the functional TRA product is sufficient to transactivate the endogenous gene and implement female development in individuals with a male karyotype. Hence, a mechanism based on tra(ep) epigenetic autoregulation seems to represent a common and presumably ancestral single principle of sex determination in Insecta. The results of these studies will not only be important for understanding divergent evolution of basic developmental processes but also for designing new strategies to improve genetic sexing in different insect species of economical or medical importance.

Gene regulation by chromatin structure: paradigms established in Drosophila melanogaster

Studies in Drosophila melanogaster have revealed paradigms for regulating gene expression through chromatin structure, including mechanisms of gene activation and silencing. Regulation occurs at the level of individual genes, chromosomal domains, and entire chromosomes. The chromatin state is dynamic, allowing for changes in gene expression in response to cellular signals and/or environmental cues. Changes in chromatin result from the action of ATP-dependent chromatin-remodeling complexes, reversible epigenetic histone modifications, and the incorporation of histone variants. Many of the chromatin-based transcriptional regulatory mechanisms discovered in D. melanogaster are evolutionarily conserved and therefore serve as a foundation for studies in other organisms.

Phylogeny of the sex-determining gene Sex-lethal in insects

The Sex-lethal (SXL) protein belongs to the family of RNA-binding proteins and is involved in the regulation of pre-mRNA splicing. SXL has undergone an obvious change of function during the evolution of the insect clade. The gene has acquired a pivotal role in the sex-determining pathway of Drosophila, although it does not act as a sex determiner in non-drosophilids. We collected SXL sequences of insect species ranging from the pea aphid (Acyrtho siphom pisum) to Drosophila melanogaster by searching published articles, sequencing cDNAs, and exploiting homology searches in public EST and whole-genome databases. The SXL protein has moderately conserved N- and C-terminal regions and a well-conserved central region including 2 RNA recognition motifs. Our phylogenetic analysis shows that a single orthologue of the Drosophila Sex-lethal (Sxl) gene is present in the genomes of the malaria mosquito Anopheles gambiae, the honeybee Apis mellifera, the silkworm Bombyx mori, and the red flour beetle Tribolium castaneum. The D. melanogaster, D. erecta, and D. pseudoobscura genomes, however, contain 2 paralogous genes, Sxl and CG3056, which are orthologous to the Anopheles, Apis, Bombyx, and Tribolium Sxl. Hence, a duplication in the fly clade generated Sxl and CG3056. Our hypothesis maintains that one of the genes, Sxl, adopted the new function of sex determiner in Drosophila, whereas the other, CG3056, continued to serve some or all of the yet-unknown ancestral functions.

The gene Sex-lethal of the Sciaridae family (order Diptera, suborder Nematocera) and its phylogeny in dipteran insects

This article reports the cloning and characterization of the gene homologous to Sex-lethal (Sxl) of Drosophila melanogaster from Sciara coprophila, Rhynchosciara americana, and Trichosia pubescens. This gene plays the key role in controlling sex determination and dosage compensation in D. melanogaster. The Sxl gene of the three species studied produces a single transcript encoding a single protein in both males and females. Comparison of the Sxl proteins of these Nematocera insects with those of the Brachycera showed their two RNA-binding domains (RBD) to be highly conserved, whereas significant variation was observed in both the N- and C-terminal domains. The great majority of nucleotide changes in the RBDs were synonymous, indicating that purifying selection is acting on them. In both sexes of the three Nematocera insects, the Sxl protein colocalized with transcription-active regions dependent on RNA polymerase II but not on RNA polymerase I. Together, these results indicate that Sxl does not appear to play a discriminatory role in the control of sex determination and dosage compensation in nematocerans. Thus, in the phylogenetic lineage that gave rise to the drosophilids, evolution coopted for the Sxl gene, modified it, and converted it into the key gene controlling sex determination and dosage compensation. At the same time, however, certain properties of the recruited ancestral Sxl gene were beneficial, and these are maintained in the evolved Sxl gene, allowing it to exert its sex-determining and dose compensation functions in Drosophila.

Isolation and characterization of the Bactrocera oleae genes orthologous to the sex determining Sex-lethal and doublesex genes of Drosophila melanogaster

Here we report the isolation and characterization of the olive fruit fly Bactrocera oleae genes orthologous to the Drosophila melanogaster sex-determining genes Sex-lethal (Sxl) and doublesex (dsx). Fragments of the Sxl and dsx orthologous were isolated with RT-PCR. Genomic and cDNA clones were then obtained by screening a genomic library and separate male and female cDNA adult libraries using the RT-PCR products as probes in both cases. B. oleae Sxl gene (BoSxl) expresses the same pattern of transcripts which encode for a single common polypeptide in both male and female flies. The gene shares a high degree of similarity in sequence and expression to its Ceratitis capitata orthologous and does not appear to play a key regulatory role in the sex-determining cascade. B. oleae dsx gene (Bodsx) expands in a chromosomal region of more than 50 kb, with 6 exons-5 introns, producing different sex-specific mRNAs, according to the Drosophila model. The cDNA sequences are almost identical to the gene orthologous of Bactrocera tryoni. Four repeat elements identical to the D. melanogaster TRA/TRA-2 binding sites have been found in the untranslated region of the female-specific exon 4, predicting a common regulatory splicing mechanism in all studied species of Diptera.