Genomic organization and splicing evolution of the doublesex gene, a Drosophila regulator of sexual differentiation, in the dengue and yellow fever mosquito Aedes aegypti

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

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

The developmental transcriptome of the mosquito Aedes aegypti, an invasive species and major arbovirus vector

Mosquitoes are vectors of a number of important human and animal diseases. The development of novel vector control strategies requires a thorough understanding of mosquito biology. To facilitate this, we used RNA-seq to identify novel genes and provide the first high-resolution view of the transcriptome throughout development and in response to blood feeding in a mosquito vector of human disease, Aedes aegypti, the primary vector for Dengue and yellow fever. We characterized mRNA expression at 34 distinct time points throughout Aedes development, including adult somatic and germline tissues, by using polyA+ RNA-seq. We identify a total of 14,238 novel new transcribed regions corresponding to 12,597 new loci, as well as many novel transcript isoforms of previously annotated genes. Altogether these results increase the annotated fraction of the transcribed genome into long polyA+ RNAs by more than twofold. We also identified a number of patterns of shared gene expression, as well as genes and/or exons expressed sex-specifically or sex-differentially. Expression profiles of small RNAs in ovaries, early embryos, testes, and adult male and female somatic tissues also were determined, resulting in the identification of 38 new Aedes-specific miRNAs, and ~291,000 small RNA new transcribed regions, many of which are likely to be endogenous small-interfering RNAs and Piwi-interacting RNAs. Genes of potential interest for transgene-based vector control strategies also are highlighted. Our data have been incorporated into a user-friendly genome browser located at www.Aedes.caltech.edu, with relevant links to Vectorbase (www.vectorbase.org)

Genetic control of courtship behavior in the housefly: evidence for a conserved bifurcation of the sex-determining pathway

In Drosophila melanogaster, genes of the sex-determination hierarchy orchestrate the development and differentiation of sex-specific tissues, establishing sex-specific physiology and neural circuitry. One of these sex-determination genes, fruitless (fru), plays a key role in the formation of neural circuits underlying Drosophila male courtship behavior. Conservation of fru gene structure and sex-specific expression has been found in several insect orders, though it is still to be determined whether a male courtship role for the gene is employed in these species due to the lack of mutants and homologous experimental evidence. We have isolated the fru ortholog (Md-fru) from the common housefly, Musca domestica, and show the gene's conserved genomic structure. We demonstrate that male-specific Md-fru transcripts arise by conserved mechanisms of sex-specific splicing. Here we show that Md-fru, is similarly involved in controlling male courtship behavior. A male courtship behavioral function for Md-fru was revealed by the behavioral and neuroanatomical analyses of a hypomorphic allele, Md-tra(man) , which specifically disrupted the expression of Md-fru in males, leading to severely impaired male courtship behavior. In line with a role in nervous system development, we found that expression of Md-fru was confined to neural tissues in the brain, most prominently in optic neuropil and in peripheral sensory organs. We propose that, like in Drosophila, overt sexual differentiation of the housefly depends on a sex-determining pathway that bifurcates downstream of the Md-tra gene to coordinate dimorphic development of non-neuronal tissues mediated by Md-dsx with that of neuronal tissues largely mediated by Md-fru.

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.

Targeting gene expression to the female larval fat body of transgenic Aedes aegypti mosquitoes

As the fat body is a critical tissue for mosquito development, metamorphosis, immune and reproductive system function, the characterization of regulatory modules targeting gene expression to the female mosquito fat body at distinct life stages is much needed for multiple, varied strategies for controlling vector-borne diseases such as dengue and malaria. The hexameric storage protein, Hexamerin-1.2, of the mosquito Aedes atropalpus is female-specific and uniquely expressed in the fat body of fourth instar larvae and young adults. We have identified in the Hex-1.2 gene, a short regulatory module that directs female-, tissue-, and stage-specific lacZ reporter gene expression using a heterologous promoter in transgenic lines of the dengue vector Aedes aegypti. Male transgenic larvae and pupae of one line expressed no Escherichia coli β-galactosidase or transgene product; in two other lines reporter gene activity was highly female-biased. All transgenic lines expressed the reporter only in the fat body; however, lacZ mRNA levels were no different in males and females at any stage examined, suggesting that the gene regulatory module drives female-specific expression by post-transcriptional regulation in the heterologous mosquito. This regulatory element from the Hex-1.2 gene thus provides a new molecular tool for transgenic mosquito control as well as functional genetic analysis in aedine mosquitoes.

Doublesex target genes in the red flour beetle, Tribolium castaneum

Sex determination cascade in insects terminates with the production of sex-specific protein, Doublesex (Dsx). We identified the dsx homolog (Tcdsx) in Tribolium castaneum. The pre-mRNA of Tcdsx is sex-specifically spliced into three female (Tcdsxf1, Tcdsxf2 and Tcdsxf3) and one male-specific (Tcdsxm) isoforms. Cis-regulatory elements potentially involved in sex-specific splicing of the Tcdsx pre-mRNA were identified in the female-specific exon and the adjoining intronic sequences. All the three female-specific TcDsx proteins share common OD1 and OD2 domains and differ in their C-terminal sequences. Knockdown of Tcdsx resulted in a reduction in the oocyte development, egg production and hatching of eggs laid. Several genes, including those coding for Vitellogenins and Vitellogenin receptors were identified as targets of TcDsx. RNAi experiments showed an isoform-specific targeting of identified target genes by TcDsx as knockdown in the expression of Tcdsx isoforms individually or in combinations resulted in differential effects on the expression of target genes.

Sex determination in beetles: production of all male progeny by parental RNAi knockdown of transformer

Sex in insects is determined by a cascade of regulators ultimately controlling sex-specific splicing of a transcription factor, Doublesex (Dsx). We recently identified homolog of dsx in the red flour beetle, Tribolium castaneum (Tcdsx). Here, we report on the identification and characterization of a regulator of Tcdsx splicing in T. castaneum. Two male-specific and one female-specific isoforms of T. castaneum transformer (Tctra) were identified. RNA interference-aided knockdown of Tctra in pupa or adults caused a change in sex from females to males by diverting the splicing of Tcdsx pre-mRNA to male-specific isoform. All the pupa and adults developed from Tctra dsRNA injected final instar larvae showed male-specific sexually dimorphic structures. Tctra parental RNAi caused an elimination of females from the progeny resulting in production of all male progeny. Transformer parental RNAi could be used to produce all male population for use in pest control though sterile male release methods.

Dmrt genes in the development and evolution of sexual dimorphism

Most animals are sexually dimorphic, but different taxa have different sex-specific traits. Despite major differences in the genetic control of sexual development among animal lineages, the doublesex/mab-3 related (Dmrt) family of transcription factors has been shown to be involved in sex-specific differentiation in all animals that have been studied. In recent years the functions of Dmrt genes have been characterized in many animal groups, opening the way to a broad comparative perspective. This review focuses on the similarities and differences in the functions of Dmrt genes across the animal kingdom. I highlight a number of common themes in the sexual development of different taxa, discuss how Dmrt genes have acquired new roles during animal evolution, and show how they have contributed to the origin of novel sex-specific traits.

Molecular cloning and preliminary function study of iron responsive element binding protein 1 gene from cypermethrin-resistant Culex pipiens pallens

Background

Insecticide resistance jeopardizes the control of mosquito populations and mosquito-borne disease control, which creates a major public health concern. Two-dimensional electrophoresis identified one protein segment with high sequence homology to part of Aedes aegypti iron-responsive element binding protein (IRE-BP).

Method

RT-PCR and RACE (rapid amplification of cDNA end) were used to clone a cDNA encoding full length IRE-BP 1. Real-time quantitative RT-PCR was used to evaluate the transcriptional level changes in the Cr-IRE strain Aedes aegypti compared to the susceptible strain of Cx. pipiens pallens. The expression profile of the gene was established in the mosquito life cycle. Methyl tritiated thymidine (³H-TdR) was used to observe the cypermethrin resistance changes in C6/36 cells containing the stably transfected IRE-BP 1 gene of Cx. pipiens pallens.

Results

The complete sequence of iron responsive element binding protein 1 (IRE-BP 1) has been cloned from the cypermethrin-resistant strain of Culex pipiens pallens (Cr-IRE strain). Quantitative RT-PCR analysis indicated that the IRE-BP 1 transcription level was 6.7 times higher in the Cr-IRE strain than in the susceptible strain of 4th instar larvae. The IRE-BP 1 expression was also found to be consistently higher throughout the life cycle of the Cr-IRE strain. A protein of predicted size 109.4 kDa has been detected by Western blotting in IRE-BP 1-transfected mosquito C6/36 cells. These IRE-BP 1-transfected cells also showed enhanced cypermethrin resistance compared to null-transfected or plasmid vector-transfected cells as determined by ³H-TdR incorporation.

Conclusion

IRE-BP 1 is expressed at higher levels in the Cr-IRE strain, and may confer some insecticide resistance in Cx. pipiens pallens.

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.