The gene doublesex of the fruit fly Anastrepha obliqua (Diptera, Tephritidae)

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.

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.

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.

Transcriptome analysis of Anastrepha fraterculus sp. 1 males, females, and embryos: insights into development, courtship, and reproduction

BACKGROUND

Anastrepha fraterculus sp. 1 is considered a quarantine pest in several American countries. Since chemical control applied in an integrated pest management program is the only strategy utilized against this pest, the development of pesticide-free methods, such as the Sterile Insect Technique, is being considered. The search for genes involved in sex-determination and differentiation, and in metabolic pathways associated with communication and mating behaviour, contributes with key information to the development of genetic control strategies. The aims of this work were to perform a comprehensive analysis of A. fraterculus sp. 1 transcriptome and to obtain an initial evaluation of genes associated with main metabolic pathways by the expression analysis of specific transcripts identified in embryos and adults.

RESULTS

Sexually mature adults of both sexes and 72 h embryos were considered for transcriptome analysis. The de novo transcriptome assembly was fairly complete (62.9% complete BUSCO orthologs detected) with a total of 86,925 transcripts assembled and 28,756 GO annotated sequences. Paired-comparisons between libraries showed 319 transcripts differently expressed between embryos and females, 1242 between embryos and males, and 464 between sexes. Using this information and genes searches based on published studies from other tephritid species, we evaluated a set of transcripts involved in development, courtship and metabolic pathways. The qPCR analysis evidenced that the early genes serendipity alpha and transformer-2 displayed similar expression levels in the analyzed stages, while heat shock protein 27 is over-expressed in embryos and females in comparison to males. The expression of genes associated with courtship (takeout-like, odorant-binding protein 50a1) differed between males and females, independently of their reproductive status (virgin vs mated individuals). Genes associated with metabolic pathways (maltase 2-like, androgen-induced gene 1) showed differential expression between embryos and adults. Furthermore, 14,262 microsatellite motifs were identified, with 11,208 transcripts containing at least one simple sequence repeat, including 48% of di/trinucleotide motifs.

CONCLUSION

Our results significantly expand the available gene space of A. fraterculus sp. 1, contributing with a fairly complete transcript database of embryos and adults. The expression analysis of the selected candidate genes, along with a set of microsatellite markers, provides a valuable resource for further genetic characterization of A. fraterculus sp. 1 and supports the development of specific genetic control strategies.

The transformer-2 and fruitless characterisation with developmental expression profiles of sex-determining genes in Bactrocera dorsalis and B. correcta

Sex determination in tephritid fruit flies involves a signaling cascade of alternatively spliced genes. The Transformer (TRA) and Transformer-2 (TRA-2) complex establishes an autoregulatory loop switching sex-specific splicing of tra pre-mRNA in females. The TRA/TRA-2 complex also regulates the sex-specific splicing of downstream effector genes, doublesex (dsx) and fruitless (fru). In Ceratitis capitata, a Maleness-on the-Y (MoY) gene modulates sex-specifically spliced Cctra pre-mRNA and results in the breakdown of the Cctra autoregulatory loop in males. In this study, the tra-2 and fru genes were characterised in two key pests, Bactrocera dorsalis and B. correcta. The tra-2 genes showed high degrees of conservation among tephritids. The complex gene organisation for each of Bdfru and Bcfru were identified. There are sex-specific and non sex-specific transcripts generated by alternative promoters as found in Drosophila melanogaster and other insects. RNAi knockdown of Bdtra transcripts showed that BdTRA controls the sex-specific splicing of Bddsx and Bdfru pre-mRNAs. Developmental expression analysis shows that multiple splice variants of Bdtra and Bctra RNAs are present before and during cellular blastoderm formation and that the mature sex-specific variants become fixed later in embryogenesis. Furthermore, the Bddsx^M splice variants are found in early embryos at the beginning of gastulation, but Bdfru^M does not appear until the larval stage. We proposed that the zygotic tra loop is initiated in both female and male embryos before becoming automatised or abolished by MoY, respectively.

Hemimetabolous insects elucidate the origin of sexual development via alternative splicing

Insects are the only known animals in which sexual differentiation is controlled by sex-specific splicing. The doublesex transcription factor produces distinct male and female isoforms, which are both essential for sex-specific development. dsx splicing depends on transformer, which is also alternatively spliced such that functional Tra is only present in females. This pathway has evolved from an ancestral mechanism where dsx was independent of tra and expressed and required only in males. To reconstruct this transition, we examined three basal, hemimetabolous insect orders: Hemiptera, Phthiraptera, and Blattodea. We show that tra and dsx have distinct functions in these insects, reflecting different stages in the changeover from a transcription-based to a splicing-based mode of sexual differentiation. We propose that the canonical insect tra-dsx pathway evolved via merger between expanding dsx function (from males to both sexes) and narrowing tra function (from a general splicing factor to dedicated regulator of dsx).

Identification and functional analysis of the doublesex gene in the sexual development of a hemimetabolous insect, the brown planthopper

In the sex determination cascade, the genes dsx (doublesex) in insects, mab-3 (male abnormal 3) in nematodes, and Dmrt1 (dsx/mab-3 related transcription factor-1) in vertebrates act as the base molecular switches and play important roles. Moreover, these genes share the same conserved feature domain-DNA-binding oligomerization domain (OD1), and female-specific dsx also has a conserved oligomerization domain 2 (OD2). Although sex determination and the functions of dsx in several holometabolous insects have been well documented, sex determination and the function of dsx in hemimetabolous insects remain a mystery. In this study, four dsx homologs were unexpectedly found in the Nilaparvata lugens (brown planthopper, BPH, order Hemiptera), which also showed a different evolutionary status. We found that only one of the four homologs, Nldsx, which has three alternative splicing variants (female-specific Nldsx^F, male-specific Nldsx^M, non-sex-specific Nldsx^C), was required in the sexual development of N. lugens. Compared with that of holometabolous species, the dsx of N. lugens contains a less conserved OD1, while the OD2 domain of BPH was not identifiable because the common region is poorly conserved, and the female-specific region is short. RNAi-mediated knockdown of Nldsx in female BPH resulted in a larger body size with a normal abdomen and reproductive system, while no changes in fertility were noted. However, adult males with RNA interference knockdown of Nldsx^M in nymphs became pseudofemales, were infertile, had abnormal copulatory organs, and had impassable deferent ducts with hyperplastic walls; additionally, the pseudofemales could not produce the normal courtship signals. Our results suggest that dsx plays a critical role in male BPH somatic development and mating behavior. This is the first study to show that dsx is essential for sexual development in a hemipteran species.

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.

Introduction of the RTA-Bddsx gene induces female-specific lethal effects in transformed Bactrocera dorsalis (Hendel)

BACKGROUND

The oriental fruit fly, Bactrocera dorsalis (Hendel), can reduce fruit production and quality and is considered to be a major insect pest in many Asian countries. A system combining the toxicity of ricin and the alternative RNA splicing properties of doublesex (RTA-Bddsx) has been proposed that results in differential sexual processing in vitro. A transgenic approach was used in this study to confirm the existence of female-specific lethal effects in vivo.

RESULTS

The piggyBac-based vector PB-Acp-CF21-26, which carries the actin 5C promoter and RTA-Bddsx, was used to establish transgenic lines. Five surviving male flies (F1) demonstrated the presence of selection marker Ds-Red((+)) throughout their entire bodies following single-pair mating with wild-type females, indicating germline transmission. A high percentage of males (59.6-100%) were observed in transformed F3 offspring, and this skewed sex ratio indicated that the female-lethal effects of the RTA-Bddsx system were heritable and functioned well in B. dorsalis. Some transformed female flies were observed, and these unexpected results were attributed to the loss of the intact transgene after genomic PCR analyses.

CONCLUSION

This transgenic study provides direct evidence for the female-specific lethal effects of RTA-Bddsx in B. dorsalis and offers a novel and promising approach for the control of B. dorsalis in the future. © 2015 Society of Chemical Industry.

Establishment of an RTA-Bddsx hybrid system for female-specific splicing that can affect the sex ratio of Bactrocera dorsalis (Hendel) after embryonic injection

BACKGROUND

The oriental fruit fly, Bactrocera dorsalis (Hendel), a very destructive insect pest in many areas of Asia, including Taiwan, can cause significant damage by ovipositing in and larval feeding on many kinds of fruit. A female lethal system, combining the splicing property of doublesex (dsx) with the toxicity of ricin A chain (RTA), has been developed. In this system, a modified RTA is separated by Bddsx intron 3; the expressed RNA can only be spliced in females, with toxic effects, whereas the immature RTA in males is harmless.

RESULTS

Two RTA-Bddsx constructs, clone BE 24-7 and clone CF 26-21, containing Bddsx intron 3 and its flanking exonic sequences, with four nucleotides at the 5'-end and five nucleotides at the 3'-end, correctly spliced in a sex-specific manner. Wild-type and modified RTAs expressed in an Escherichia coli system retained their ability to suppress protein synthesis: 90.4% for Ricin-WT, 71.3% for Ricin-LERQ and 58.0% for Ricin-FEGQ. Embryonic injection of Acp-CF26-21, the RTA-Bddsx gene driven by the actin 5C promoter, resulted in a significant increase in male percentage in the eclosed adults.

CONCLUSION

Our results indicate that the RTA-Bddsx hybrid system offers a novel and promising approach for oriental fruit fly control.

The study of the transformer gene from Bactrocera dorsalis and B. correcta with putative core promoter regions

Background

The transformer (tra) is a sex determining switch in different orders of insects, including Diptera, as in the family Tephritidae. The lifelong autoregulatory loop of tra female-specific splicing can be reset by the intervention of male-specific primary signals (M factor). In early development, the functional female and truncated male TRA proteins relay the sexual fates to the alternative splicing of a bisexual switch gene, doublesex (dsx) cascading the sexual differentiation processes. Bactrocera dorsalis (Hendel) and Bactrocera correcta (Bezzi) are among the Bactrocera model worldwide key pests. Area-wide integrated pest management using the male-only Sterile Insect Technique (SIT) relying on genetic sexing systems is effective in control programs. We undertook the molecular characterization and comparative studies of the tra orthologues in the Bactrocera species, including the Salaya1 genetic sexing strain (GSS).

Results

RT-PCR revealed that B. dorsalis tra (Bdtra) and B. correcta tra (Bctra) transcripts contained conservation of both constitutive exons and male-specific exons as in other Bactrocera. However, new Bdtra male-specific exons were retained, diversifying the pattern of the male-specifically spliced transcripts. The coding sequences of tra were highly conserved in Bactrocera (86–95 %) but less so among related genera (61–65 %) within the same Tephritidae family. A conservation of deduced amino acid sequences (18 residues), called the TEP region, was identified to be distinctive among tephritids. The 5’ regulatory sequence containing many structural characteristics of the putative core promoter was discovered in B. correcta. The expression patterns of Bdtra and Bctra were sex-specifically spliced and the signals relayed to the dsx genes in the adult wild-types. However, the coexistence of male- and female-specifically spliced transcripts (980 and 626 bp, respectively) of the B. dorsalis wild-type strain was found in the Salaya1 GSS adult males. The Bdtra RNA interference masculinized the XX karyotype females into pseudomales, but their testes were mostly not well developed.

Conclusions

Bdtra and Bctra have sex-specific splicing, similar to Bactroceras, Ceratitis capitata (Wiedemann), and Anastrephas. A newly identified TEP region is proposed in tephritids. A putative core promoter has been discovered in Bctra.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-016-0342-0) contains supplementary material, which is available to authorized users.

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.

Genetics and biology of Anastrepha fraterculus: research supporting the use of the sterile insect technique (SIT) to control this pest in Argentina

Two species of true fruit flies (taxonomic family Tephritidae) are considered pests of fruit and vegetable production in Argentina: the cosmopolitan Mediterranean fruit fly (Ceratitis capitata Wiedemann) and the new world South American fruit fly (Anastrepha fraterculus Wiedemann). The distribution of these two species in Argentina overlaps north of the capital, Buenos Aires. Regarding the control of these two pests, the varied geographical fruit producing regions in Argentina are in different fly control situations. One part is under a programme using the sterile insect technique (SIT) for the eradication of C. capitata, because A. fraterculus is not present in this area. The application of the SIT to control C. capitata north of the present line with the possibility of A. fraterculus occupying the niche left vacant by C. capitata becomes a cause of much concern. Only initial steps have been taken to investigate the genetics and biology of A. fraterculus. Consequently, only fragmentary information has been recorded in the literature regarding the use of SIT to control this species. For these reasons, the research to develop a SIT protocol to control A. fraterculus is greatly needed. In recent years, research groups have been building a network in Argentina in order to address particular aspects of the development of the SIT for Anastrepha fraterculus. The problems being addressed by these groups include improvement of artificial diets, facilitation of insect mass rearing, radiation doses and conditions for insect sterilisation, basic knowledge supporting the development of males-only strains, reduction of male maturation time to facilitate releases, identification and isolation of chemical communication signals, and a good deal of population genetic studies. This paper is the product of a concerted effort to gather all this knowledge scattered in numerous and often hard-to-access reports and papers and summarize their basic conclusions in a single publication.

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.

Transcriptome analysis of female reproductive tissues of Anastrepha obliqua and molecular evolution of eggshell proteins in the fraterculus group

The investigation of cDNA libraries has been an important tool for the identification of new genes in nonmodel species such as the fruit flies from the Anastrepha fraterculus group. In the present study, we constructed a cDNA library from the female reproductive tissues of Anastrepha obliqua aiming to identify genes with high evolutionary rates. We sequenced 2304 clones obtained from the female reproductive tissues of A. obliqua flies. The expressed sequence tags generated a total of 816 unigenes which were classified into different protein classes. Among these,we identified chorionic and vitelline protein genes as being among the most highly expressed. We used unigene sequences to amplify a set of chorionic and vitelline genes, involved in the formation of the eggshell,in species of the fraterculus group. Four chorionic genes and two vitelline genes showed evidence of positive selection along the Anastrepha and/or Tephritidae lineage. The signal of selection detected for Vm26Aa was possibly generated by a gene duplication event. The rapid evolutionary rates indicate that these genes could serve as important markers in population and evolutionary studies, not only for species of this group, but possibly also for other Diptera.

The orthologue of the fruitfly sex behaviour gene fruitless in the mosquito Aedes aegypti: evolution of genomic organisation and alternative splicing

In Drosophila melanogaster the doublesex (dsx) and fruitless (fru) regulatory genes act at the bottom of the somatic sex determination pathway. Both are regulated via alternative splicing by an upstream female-specific TRA/TRA-2 complex, recognizing a common cis element. dsx controls somatic sexual differentiation of non-neural as well as of neural tissues. fru, on the other hand, expresses male-specific functions only in neural system where it is required to built the neural circuits underlying proper courtship behaviour. In the mosquito Aedes aegypti sex determination is different from Drosophila. The key male determiner M, which is located on one of a pair of homomorphic sex chromosomes, controls sex-specific splicing of the mosquito dsx orthologue. In this study we report the genomic organization and expression of the fru homologue in Ae. aegypti (Aeafru). We found that it is sex-specifically spliced suggesting that it is also under the control of the sex determination pathway. Comparative analyses between the Aeafru and Anopheles gambiae fru (Angfru) genomic loci revealed partial conservation of exon organization and extensive divergence of intron lengths. We find that Aeadsx and Aeafru share novel cis splicing regulatory elements conserved in the alternatively spliced regions. We propose that in Aedes aegypti sex-specific splicing of dsx and fru is most likely under the control of splicing regulatory factors which are different from TRA and TRA-2 found in other dipteran insects and discuss the potential use of fru and dsx for developing new genetic strategies in vector control.

The Am-tra2 gene is an essential regulator of female splice regulation at two levels of the sex determination hierarchy of the honeybee

Heteroallelic and homo- or hemiallelic Complementary sex determiner (Csd) proteins determine sexual fate in the honeybee (Apis mellifera) by controlling the alternative splicing of the downstream gene fem (feminizer). Thus far, we have little understanding of how heteroallelic Csd proteins mediate the splicing of female fem messenger RNAs (mRNAs) or how Fem proteins direct the splicing of honeybee dsx (Am-dsx) pre-mRNAs. Here, we report that Am-tra2, which is an ortholog of Drosophila melanogaster tra2, is an essential component of female splicing of the fem and Am-dsx transcripts in the honeybee. The Am-tra2 transcripts are alternatively (but non-sex-specifically) spliced, and they are translated into six protein isoforms that all share the basic RNA-binding domain/RS (arginine/serine) domain structure. Knockdown studies showed that the Am-tra2 gene is required to splice fem mRNAs into the productive female and nonproductive male forms. We suggest that the Am-Tra2 proteins are essential regulators of fem pre-mRNA splicing that, together with heteroallelic Csd proteins and/or Fem proteins, implement the female pathway. In males, the Am-Tra2 proteins may enhance the switch of fem transcripts into the nonproductive male form when heteroallelic Csd proteins are absent. This dual function of Am-Tra2 proteins possibly enhances and stabilizes the binary decision process of male/female splicing. Our knockdown studies also imply that the Am-Tra2 protein is an essential regulator for Am-dsx female splice regulation, suggesting an ancestral role in holometabolous insects. We also provide evidence that the Am-tra2 gene has an essential function in honeybee embryogenesis that is unrelated to sex determination.

Positive and purifying selection influence the evolution of doublesex in the Anastrepha fraterculus species group

The gene doublesex (dsx) is considered to be under strong selective constraint along its evolutionary history because of its central role in somatic sex differentiation in insects. However, previous studies of dsx used global estimates of evolutionary rates to investigate its molecular evolution, which potentially miss signals of adaptive changes in generally conserved genes. In this work, we investigated the molecular evolution of dsx in the Anastrepha fraterculus species group (Diptera, Tephritidae), and test the hypothesis that this gene evolved solely by purifying selection using divergence-based and population-based methods. In the first approach, we compared sequences from Anastrepha and other Tephritidae with other Muscomorpha species, analyzed variation in nonsynonymous to synonymous rate ratios (dN/dS) in the Tephritidae, and investigated radical and conservative changes in amino acid physicochemical properties. We show a general selective constraint on dsx, but with signs of positive selection mainly in the common region. Such changes were localized in alpha-helices previously reported to be involved in dimer formation in the OD2 domain and near the C-terminal of the OD1 domain. In the population-based approach, we amplified a region of 540 bp that spanned almost all of the region common to both sexes from 32 different sites in Brazil. We investigated patterns of selection using neutrality tests based on the frequency spectrum and locations of synonymous and nonsynonymous mutations in a haplotype network. As in the divergence-based approach, these analyses showed that dsx has evolved under an overall selective constraint, but with some events of positive selection. In contrast to previous studies, our analyses indicate that even though dsx has indeed evolved as a conserved gene, the common region of dsx has also experienced bouts of positive selection, perhaps driven by sexual selection, during its evolution.

Function and evolution of sex determination mechanisms, genes and pathways in insects

Animals have evolved a bewildering diversity of mechanisms to determine the two sexes. Studies of sex determination genes – their history and function – in non-model insects and Drosophila have allowed us to begin to understand the generation of sex determination diversity. One common theme from these studies is that evolved mechanisms produce activities in either males or females to control a shared gene switch that regulates sexual development. Only a few small-scale changes in existing and duplicated genes are sufficient to generate large differences in sex determination systems. This review summarises recent findings in insects, surveys evidence of how and why sex determination mechanisms can change rapidly and suggests fruitful areas of future research.

Display female-specific doublesex RNA interference in early generations of transformed oriental fruit fly, Bactrocera dorsalis (Hendel)

BACKGROUND

The oriental fruit fly, Bactrocera dorsalis (Hendel), is one of the most destructive pests in many Asian countries. An effective strategy to reduce fly density in the field is urgently required. Recently, the doublesex of B. dorsalis (Bddsx(f) ) has been cloned, and RNA interference (RNAi) indicates that it can reduce the offspring in vitro. In this study, a piggyBac-based construct that generates short hairpin RNA (shRNA) against the female-specific region of Bddsx was introduced into the pest to test the RNAi effects on reproductive functions in vivo.

RESULTS

After embryonic injection and backcross, 21 transgenic lines with germline transformation were identified. Genomic DNA analysis showed that the exogenous transgene including short hairpin Bddsx(f) and a DsRed marker had integrated into the genomes of 11 transformed lines. Northern blot analysis indicated the presence of Bddsx(f) short interfering RNA (siRNA) under the control of a U6 promoter in transformed flies. As expected, the specific effects of RNAi led to the delay of egg maturation, and the offspring was significantly reduced. Reverse transcription real-time PCR further demonstrated that in vivo interference not only specifically inhibited the Bddsx(f) transcript but also repressed expression of the downstream yolk protein gene (Bdyp1).

CONCLUSION

The results clearly indicate that RNAi is heritable through the expression of specific siRNA in early generations of transformed oriental fruit fly. These results can broaden the understanding of sex-related developmental mechanisms in the fly, and also offer a possible molecular approach for pest control in the future.

Identification of a male-specific RNA binding protein that regulates sex-specific splicing of Bmdsx by increasing RNA binding activity of BmPSI

Bmdsx is a sex-determining gene in the silkworm and is alternatively spliced in males and females. CE1 is a splicing silencer element responsible for the sex-specific splicing of Bmdsx. To identify sex-specific factors implicated in the sex-specific splicing of Bmdsx, we performed RNA affinity chromatography using CE1 RNA as a ligand. We have identified BmIMP, a Bombyx homolog of IGF-II mRNA binding protein (IMP), as a male-specific factor that specifically binds to CE1. The gene encoding BmIMP is localized on the Z chromosome and is male-specifically expressed in various tissues. Antisense inhibition of BmIMP expression increased female-specific splicing of Bmdsx pre-mRNA. Coimmunoprecipitation and glutathione S-transferase (GST) pulldown analyses demonstrated that BmIMP physically interacts with BmPSI, which has been identified as a factor implicated in the sex-specific splicing of Bmdsx, through the KH domains of BmIMP. The functional consequence of this interaction was examined using RNA mobility shift analysis. BmIMP increased BmPSI-CE1 RNA binding activity by decreasing the rate of BmPSI dissociation from CE1 RNA. Truncation analysis of BmIMP suggested that the KH domains are responsible for enhancing BmPSI-CE1 RNA binding activity. These results suggest that BmIMP may enhance the male-specific splicing of Bmdsx pre-mRNA by increasing RNA binding activity of BmPSI.

Evidence for positive selection in the gene fruitless in Anastrepha fruit flies

Background

Many genes involved in the sex determining cascade have indicated signals of positive selection and rapid evolution across different species. Even though fruitless is an important gene involved mostly in several aspects of male courtship behavior, the few studies so far have explained its high rates of evolution by relaxed selective constraints. This would indicate that a large portion of this gene has evolved neutrally, contrary to what has been observed for other genes in the sex cascade.

Results

Here we test whether the fruitless gene has evolved neutrally or under positive selection in species of Anastrepha (Tephritidae: Diptera) using two different approaches, a long-term evolutionary analysis and a populational genetic data analysis. The first analysis was performed by using sequences of three species of Anastrepha and sequences from several species of Drosophila using the ratio of nonsynonymous to synonymous rates of evolution in PAML, which revealed that the fru region here studied has evolved by positive selection. Using Bayes Empirical Bayes we estimated that 16 sites located in the connecting region of the fruitless gene were evolving under positive selection. We also investigated for signs of this positive selection using populational data from 50 specimens from three species of Anastrepha from different localities in Brazil. The use of standard tests of selection and a new test that compares patterns of differential survival between synonymous and nonsynonymous in evolutionary time also provide evidence of positive selection across species and of a selective sweep for one of the species investigated.

Conclusions

Our data indicate that the high diversification of fru connecting region in Anastrepha flies is due at least in part to positive selection, not merely as a consequence of relaxed selective constraint. These conclusions are based not only on the comparison of distantly related taxa that show long-term divergence time, but also on recently diverged lineages and suggest that episodes of adaptive evolution in fru may be related to sexual selection and/or conflict related to its involvement in male courtship behavior.

The gene transformer-2 of Anastrepha fruit flies (Diptera, Tephritidae) and its evolution in insects

BACKGROUND

In the tephritids Ceratitis, Bactrocera and Anastrepha, the gene transformer provides the memory device for sex determination via its auto-regulation; only in females is functional Tra protein produced. To date, the isolation and characterisation of the gene transformer-2 in the tephritids has only been undertaken in Ceratitis, and it has been shown that its function is required for the female-specific splicing of doublesex and transformer pre-mRNA. It therefore participates in transformer auto-regulatory function. In this work, the characterisation of this gene in eleven tephritid species belonging to the less extensively analysed genus Anastrepha was undertaken in order to throw light on the evolution of transformer-2.

RESULTS

The gene transformer-2 produces a protein of 249 amino acids in both sexes, which shows the features of the SR protein family. No significant partially spliced mRNA isoform specific to the male germ line was detected, unlike in Drosophila. It is transcribed in both sexes during development and in adult life, in both the soma and germ line. The injection of Anastrepha transformer-2 dsRNA into Anastrepha embryos caused a change in the splicing pattern of the endogenous transformer and doublesex pre-mRNA of XX females from the female to the male mode. Consequently, these XX females were transformed into pseudomales. The comparison of the eleven Anastrepha Transformer-2 proteins among themselves, and with the Transformer-2 proteins of other insects, suggests the existence of negative selection acting at the protein level to maintain Transformer-2 structural features.

CONCLUSIONS

These results indicate that transformer-2 is required for sex determination in Anastrepha through its participation in the female-specific splicing of transformer and doublesex pre-mRNAs. It is therefore needed for the auto-regulation of the gene transformer. Thus, the transformer/transfomer-2 > doublesex elements at the bottom of the cascade, and their relationships, probably represent the ancestral state (which still exists in the Tephritidae, Calliphoridae and Muscidae lineages) of the extant cascade found in the Drosophilidae lineage (in which tra is just another component of the sex determination gene cascade regulated by Sex-lethal). In the phylogenetic lineage that gave rise to the drosophilids, evolution co-opted for Sex-lethal, modified it, and converted it into the key gene controlling sex determination.

Maternal control of haplodiploid sex determination in the wasp Nasonia

All insects in the order Hymenoptera have haplodiploid sex determination, in which males emerge from haploid unfertilized eggs and females are diploid. Sex determination in the honeybee Apis mellifera is controlled by the complementary sex determination (csd) locus, but the mechanisms controlling sex determination in other Hymenoptera without csd are unknown. We identified the sex-determination system of the parasitic wasp Nasonia, which has no csd locus. Instead, maternal input of Nasonia vitripennis transformer (Nvtra) messenger RNA, in combination with specific zygotic Nvtra transcription, in which Nvtra autoregulates female-specific splicing, is essential for female development. Our data indicate that males develop as a result of maternal imprinting that prevents zygotic transcription of the maternally derived Nvtra allele in unfertilized eggs. Upon fertilization, zygotic Nvtra transcription is initiated, which autoregulates the female-specific transcript, leading to female development.

BmHrp28 is a RNA-binding protein that binds to the female-specific exon 4 of Bombyx mori dsx pre-mRNA

The Bombyx mori sex determination gene Bmdsx is alternatively spliced in the male and female to produce the male- and female-specific proteins. In an effort to better understand the mechanism of the alternative splicing regulation of Bmdsx, we conducted a gel-shift assay followed by LC-MS/MS analysis to identify the putative proteins bound to the cis-element CE1+6 in the exon 4 of Bmdsx. A protein named as BmHrp28 which is homologous to the Drosophila Hrp48, a member of the hnRNPA/B family, was identified and expressed in Escherichia coli for testing RNA-protein binding in vitro. All of the results showed that BmHrp28 specifically bound to the CE1+6 RNA probe. BmHrp28 has two RNA recognition motifs at the N-terminal and a glycine-rich motif at the C-terminal. It might be one of the factors involved in the male-specific splicing of Bmdsx.

Identification and characterization of the doublesex gene of Nasonia

The doublesex (dsx) gene of the parasitic wasp Nasonia vitripennis is described and characterized. Differential splicing of dsx transcripts has been shown to induce somatic sexual differentiation in Diptera and Lepidoptera, but not yet in other insect orders. Two spliceforms of Nasonia dsx mRNA are differentially expressed in males and females. In addition, in a gynandromorphic line that produces haploids (normally males) with full female phenotypes, these individuals show the female spliceform, providing the first demonstration of a direct association of dsx with somatic sex differentiation in Hymenoptera. Finally, the DNA binding (DM) domain of Nasonia dsx clusters phylogenetically with dsx from other insects, and Nasonia dsx shows microsynteny with dsx of Apis, further supporting identification of the dsx orthologue in Nasonia.

Sexual development in Lucilia cuprina (Diptera, Calliphoridae) is controlled by the transformer gene

Insects use an amazing variety of genetic systems to control sexual development. A Y-linked male determining gene (M) controls sex in the Australian sheep blowfly Lucilia cuprina, an important pest insect. In this study, we isolated the L. cuprina transformer (Lctra) and transformer2 (Lctra2) genes, which are potential targets of M. The LCTRA and LCTRA2 proteins are significantly more similar to homologs from tephritid insects than Drosophila. The Lctra transcript is alternatively spliced such that only females make a full-length protein and the presence of six TRA/TRA2 binding sites in the female first intron suggest that Lctra splicing is autoregulated as in tephritids. LCTRA is essential for female development as RNAi knockdown of Lctra mRNA leads to the development of male genitalia in XX adults. Analysis of Lctra expression during development shows that early and midstage male and female embryos express the female form of Lctra and males express only the male form by the first instar larval stage. Our results suggest that an autoregulatory loop sustains female development and that expression of M inhibits Lctra autoregulation, switching its splicing to the male form. The conservation of tra function and regulation in a Calliphorid insect shows that this sex determination system is not confined to Tephritidae. Isolation of these genes is an important step toward the development of a strain of L. cuprina suitable for a genetic control program.

Effect of the gene doublesex of anastrepha on the somatic sexual development of Drosophila

BACKGROUND

The gene doublesex (dsx) is at the bottom of the sex determination genetic cascade and is transcribed in both sexes, but gives rise to two different proteins, DsxF and DsxM, which impose female and male sexual development respectively via the sex-specific regulation of the so-called sexual cyto-differentiation genes. The present manuscript addressed the question about the functional conservation of the tephritid Anastrepha DsxF and DsxM proteins to direct the sexual development in Drosophila (Drosophilidae).

METHODOLOGY

To express these proteins in Drosophila, the GAL4-UAS system was used. The effect of these proteins was monitored in the sexually dimorphic regions of the fly: the foreleg basitarsus, the 5th, 6th and 7th tergites, and the external terminalia. In addition, we analysed the effect of Anastrepha DsxF and DsxM proteins on the regulation of Drosophila yolk protein genes, which are expressed in the fat body of adult females under the control of dsx.

CONCLUSIONS

The Anastrepha DsxF and DsxM proteins transformed doublesex intersexual Drosophila flies into females and males respectively, though this transformation was incomplete and the extent of their influence varied in the different sexually dimorphic regions of the adult fly. The Anastrepha DsxF and DsxM proteins also behaved as activators and repressors, respectively, of the Drosophila yolk protein genes, as do the DsxF and DsxM proteins of Drosophila itself. Finally, the Anastrepha DsxF and DsxM proteins were found to counteract the functions of Drosophila DsxM and DsxF respectively, reflecting the normal behaviour of the latter proteins towards one another. Collectively, these results indicate that the Anastrepha DsxF and DsxM proteins show conserved female and male sex-determination function respectively in Drosophila, though it appears that they cannot fully substitute the latter's own Dsx proteins. This incomplete function might be partly due to a reduced capacity of the Anastrepha Dsx proteins to completely control the Drosophila sexual cyto-differentiation genes, a consequence of the accumulation of divergence between these species resulting in the formation of different co-adapted complexes between the Dsx proteins and their target genes.

Female-specific doublesex dsRNA interrupts yolk protein gene expression and reproductive ability in oriental fruit fly, Bactrocera dorsalis (Hendel)

A homologue of the doublesex gene (Bddsx) has been cloned from the oriental fruit fly, Bactrocera dorsalis (Hendel). Northern analysis indicates a differential expression of Bddsx in male and female flies, as reported for other dsx genes. A structural conservation of DNA binding domain/oligomerization domain 1 and oligomerization domain 2 suggests that the doublesex protein (BdDSX) of this fruit fly serves as a transcriptional factor for downstream sex-specific gene expression. The putative transformer/transformer-2 protein binding sequence in female-specific transcript suggests that a preserved alternative splicing process found in other flies mediates the synthesis of Bddsx transcript. RNA interference (RNAi) data from adult abdominal dsRNA injection assays indicate that female-specific dsx dsRNA reduces specifically its own transcript, inhibits selectively expression of the yolk protein gene (Bdyp1), and delays ovary development. The number of matured eggs is significant reduced after RNAi treatment, but the sex ratio of offspring is not biased. Moreover, 27% of female progeny with RNAi show deformed ovipositor, but male flies are not affected. Although this is a transient treatment, the specific Bddsx(f) interference offers a promising and novel approach to oriental fruit fly control in the future.

The gene transformer of anastrepha fruit flies (Diptera, tephritidae) and its evolution in insects

In the tephritids Ceratitis capitata and Bactrocera oleae, the gene transformer acts as the memory device for sex determination, via an auto-regulatory function; and functional Tra protein is produced only in females. This paper investigates the evolution of the gene tra, which was characterised in twelve tephritid species belonging to the less extensively analysed genus Anastrepha. Our study provided the following major conclusions. Firstly, the memory device mechanism used by this gene in sex determination in tephritids likely existed in the common ancestor of the Ceratitis, Bactrocera and Anastrepha phylogenetic lineages. This mechanism would represent the ancestral state with respect to the extant cascade seen in the more evolved Drosophila lineage. Secondly, Transformer2-specific binding intronic splicing silencer sites were found in the splicing regulatory region of transformer but not in doublesex pre-mRNAs in these tephritids. Thus, these sites probably provide the discriminating feature for the putative dual splicing activity of the Tra-Tra2 complex in tephritids. It acts as a splicing activator in dsx pre-mRNA splicing (its binding to the female-specific exon promotes the inclusion of this exon into the mature mRNA), and as a splicing inhibitor in tra pre-mRNA splicing (its binding to the male-specific exons prevents the inclusion of these exons into the mature mRNA). Further, a highly conserved region was found in the specific amino-terminal region of the tephritid Tra protein that might be involved in Tra auto-regulatory function and hence in its repressive splicing behaviour. Finally, the Tra proteins conserved the SR dipeptides, which are essential for Tra functionality.

Sex-specific splicing of the honeybee doublesex gene reveals 300 million years of evolution at the bottom of the insect sex-determination pathway

Sex-determination mechanisms vary greatly among taxa. It has been proposed that genetic sex-determination pathways evolve in reverse order from the final step in the pathway to the first step. Consistent with this hypothesis, doublesex (dsx), the most downstream gene in the Drosophila sex-determination cascade that determines most sexual phenotypes also determines sex in other dipterans and the silk moth, while the upstream genes vary among these species. However, it is unknown when dsx was recruited to the sex-determination pathway during insect evolution. Furthermore, sex-specific splicing of dsx, by which dsx determines sex, is different in pattern and mechanism between the moth and the fly, raising an interesting question of how these insects have kept the executor of sex determination while allowing flexibility in the means of execution. To address these questions, here we study the dsx gene of the honeybee Apis mellifera, a member of the most basal lineage of holometabolous insects. We report that honeybee dsx is sex-specifically spliced and that it produces both the fly-type and moth-type splicing forms, indicating that the use of different splicing forms of Dsx in controlling sexual differentiation was present in the common ancestor of holometabolous insects. Our data suggest that in ancestral holometabolous insects the female Dsx form is the default and the male form is generated by suppressing the splicing of the female form. Thus, it is likely that the dsx splicing activator system in flies, where the male form is the default, arose during early dipteran evolution.

The gene doublesex of Anastrepha fruit flies (Diptera, Tephritidae) and its evolution in insects

The doublesex (dsx) gene of several Anastrepha species was isolated and characterised. Its molecular organisation was found to be the same in all the species examined. This gene is composed of four exons: Exons 1 and 2 are common to both sexes, exon 3 is female specific, and exon 4 is male specific. It codes for both the female DsxF and male DsxM proteins, corresponding to the sex-specific splicing product of its primary transcript; male-specific splicing is the default mode. A comparison of the Dsx proteins of different Anastrepha species with those of other insects showed them to be very similar. Molecular evolutionary analysis (both at the nucleotide and amino acid levels) of dsx in different insects revealed a topology in good agreement with their owners' taxonomic relationships. The great majority of the nucleotide changes detected in the dsx gene of the analysed species were significantly synonymous, evidence that strong purifying selection has acted on dsx so that the functional structure of the Dsx proteins is preserved. However, the common region of DsxF and DsxM proteins appeared to be the main target for selection acting upon the long-term evolution of gene dsx.

The transformer gene in Bactrocera oleae: the genetic switch that determines its sex fate

Transformer (tra) is the second gene of a regulatory cascade based on RNA splicing that determines sex in Drosophila melanogaster. Splicing of tra transcripts is regulated by the master gene Sex lethal and tra itself regulates splicing of the transcriptional regulator doublesex (dsx). We present the isolation and characterization of Botra, the olive fruit fly Bactrocera oleae orthologue to the Drosophila gene transformer. As in Drosophila, Botra transcripts are spliced in a sex-specific manner so that only females encode a functional polypeptide of 422 amino acids, whereas males encode presumably nonfunctional peptide(s). The identification of multiple TRA/TRA-2 binding sites within the Botra male-specific exons, suggests an autoregulation mechanism of tra, through TRA/TRA2 activities. The fundamental role of the TRA protein in sex determination of Bactrocera was investigated by RNA interference, where the introduction of Botra dsRNA into embryos resulted in complete transformation of XX flies into fertile males.

Sex determination in the haplodiploid wasp Nasonia vitripennis (Hymenoptera: Chalcidoidea): a critical consideration of models and evidence

Sex determining mechanisms are highly diverse. Like all Hymenoptera, the parasitic wasp Nasonia vitripennis reproduces by haplodiploidy: males are haploid and females are diploid. Sex in Nasonia is not determined by complementary alleles at sex loci. Evidence for several alternative models is considered. Recent studies on a polyploid and a gynandromorphic mutant strain point to a maternal product that is balanced against the number of chromosomal complements in the zygote and a parent-specific (imprinting) effect. Research is now focused on the molecular details of sex determination in Nasonia.