The role of the transformer gene in sex determination and reproduction in the tephritid fruit fly, Bactrocera dorsalis (Hendel)

Function of doublesex and transformer-2 genes and its respond to environment factors in Dendroctonus armandi Tsai et Li (Coleoptera: Curculionidae: Scolytinae)

The doublesex (dsx) and transformer-2 (tra-2) genes play important roles in insect sex determination. However, the sex determination of Dendroctonus armandi in the native and coniferous forests of the Qinling Mountains is still unclear. In this study, we cloned and identified the full-length sequences of two Dadsx isoform (Dadsx1 and Dadsx2) and two isoform Datra-2 (Datra-2A and Datra-2B) in D. armandi. These four isoforms are expressed in larvae, pupae, as well as in males and females, and their expression levels are highest during the pupae stage. The relative expression levels of Dadsx and Datra-2 genes showed significant differences under different environment treatments (including temperature, nutrient, feeding duration, and terpenoid), with male Dadsx1 relative expression levels significantly higher than females and female Dadsx2 relative expression levels significantly higher than males. The silencing of Datra-2A and Datra-2B isoforms both leads to the downregulation of Dadsx1 isoform. Compared to the control group (average mortality rate: 36.7 %; average deformity rate: 0.2 %; emergence rate: 60.9 %; sex ratio: 1.11), when Dadsx1 (average mortality rate: 69.0 %; average deformity rate: 16.5 %; emergence rate: 29.6 %; sex ratio: 0.28), Dadsx2 (average mortality rate: 64.4 %; average deformity rate: 17.4 %; emergence rate: 25.4 %; sex ratio: 2.41), Datra-2A (average mortality rate: 65.1 %; average deformity rate: 16.1 %; emergence rate: 15.4 %; sex ratio: 0.31) and Datra-2B (average mortality rate: 65.4 %; average deformity rate: 17.2 %; emergence rate: 13.8 %; sex ratio: 0.33) isoform s were silenced, it showed a significant increase in mortality and deformity rates, a significant decrease in emergence rate, and a severe sex imbalance. The results indicate that the relative expression levels of Dadsx and Datra-2 genes are influenced by external factors and play a crucial role in maintaining the sex ratio of D. armandi and ensuring its lifecycle development.

Genetic engineering for SIT application: a fruit fly-focused review

Sterile insect technique (SIT) has become a key component of efficient pest control. Fruit fly pests from the Drosophilidae and Tephritidae families pose a substantial and overwhelmingly increasing threat to the agricultural industry, aggravated by climate change and globalization among other contributors. In this review, we discuss the advances in genetic engineering aimed to improve the SIT-mediated fruit fly pest control. This includes SIT enhancement strategies such as novel genetic sexing strain and female lethality approaches. Self-pervasive X-shredding and X-poisoning sex distorters, alongside gene drive varieties are also reviewed. The self-limiting precision-guided SIT, which aims to tackle female removal and male fertility via CRISPR/Cas9, is additionally introduced. By using examples of existing genetic tools in the fruit fly pests of interest, as well as model species, we illustrate that the population control intensity may be modulated depending on strategy selection.

Next-generation genetic sexing strain establishment in the agricultural pest Ceratitis capitata

Tephritid fruit fly pests pose an increasing threat to the agricultural industry due to their global dispersion and a highly invasive nature. Here we showcase the feasibility of an early-detection SEPARATOR sex sorting approach through using the non-model Tephritid pest, Ceratitis capitata. This system relies on female-only fluorescent marker expression, accomplished through the use of a sex-specific intron of the highly-conserved transformer gene from C. capitata and Anastrepha ludens. The herein characterized strains have 100% desired phenotype outcomes, allowing accurate male-female separation during early development. Overall, we describe an antibiotic and temperature-independent sex-sorting system in C. capitata, which, moving forward, may be implemented in other non-model Tephritid pest species. This strategy can facilitate the establishment of genetic sexing systems with endogenous elements exclusively, which, on a wider scale, can improve pest population control strategies like sterile insect technique.

CRISPR/Cas9-induced Mutation of Sex Peptide Receptor Gene Bdspr Affects Ovary, Egg Laying, and Female Fecundity in Bactrocera dorsalis (Hendel) (Diptera: Tephritidae)

The oriental fruit fly, Bactrocera dorsalis (Hendel) (Diptera: Tephritidae), is an invasive and polyphagous pest of horticultural crops, and it can cause huge economic losses in agricultural production. The rapid development of CRISPR/Cas9 gene editing technology has provided new opportunities for the scientific control of agricultural pests. Here, we explore the applicability of the B. dorsalis sex peptide receptor (Bdspr) as a target gene for the CRISPR/Cas9-based sterile insect technique (SIT) in B. dorsalis. We screened two high-efficient single guide RNAs (sgRNAs) for gene editing. The results showed that both mutation efficiency and germline transmission rate were 100% in the surviving G0 females (8/8) from injected embryos, and that 75% of mosaically mutated G0 females (6/8) were sterile. The 50% of heterozygous G1 females (4/8) could not lay eggs; 100% of eggs laid by them could not survive; and 62.5% of individual females (5/8) had abnormal ovaries. These results indicate that Bdspr plays an important role in regulating fertility, egg viability, and ovary development in female B. dorsalis, suggesting that the spr gene can be used for CRISPR/Cas9-based SIT in B. dorsalis.

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.

Early embryonic transcriptomes of Zeugodacus tau provide insight into sex determination and differentiation genes

Zeugodacus tau (Walker) is an invasive pest. The sterile insect technique is an environment-friendly method for pest control. Understanding the mechanism of sex determination will contribute to improving efficiency of this technique. In this study, we identified the transformer (tra) gene in Z. tau. One female-specific and two male-specific isoforms of tra were found in Z. tau, and the male-specific splicing pattern of tra was found to occur 5 h after egg laying. We performed transcriptome sequencing at 1 h (E1), 5 h (E5), and 9 h (E9) after egg laying and obtained high-quality transcriptome libraries of early embryo development. We identified 13 297 and 11 713 differentially expressed genes (DEGs) from E5 versus E1 and E9 versus E1 comparisons, respectively. To explore the potential functions of the DEGs during embryonic development, Gene Ontology, Clusters of Orthologous Groups of proteins, and Kyoto Encyclopedia of Genes and Genomes analyses were performed. Twenty-six genes potentially involved in sex determination or differentiation, including Maleness-on-the-Y (MoY), were identified in Z. tau. To verify the transcriptome results, 15 genes were selected for quantitative real-time PCR validation. The results were consistent with the transcriptome sequencing results. Moreover, U2 small nuclear riboprotein auxiliary factor (U2AF-50), female lethal d (fl(2)d), and virilizer (vir) were highly expressed at E5, indicating that they may be related to the sex-specific splicing of tra. Further functional analysis is needed to confirm this speculation. Our data provide an insight into the mechanism underlying sex determination and differentiation in tephritid species.

Delivery of Double-Stranded RNAs (dsRNAs) Produced by Escherichia coli HT115(DE3) for Nontransgenic RNAi-Based Insect Pest Management

RNA interference (RNAi) is a powerful mechanism that can be exploited not only for physiology research but also for designing insect pest management approaches. Some insects cause harm by vectoring diseases dangerous to humans, livestock, or plants or by damaging crops. For at least a decade now, different insect control strategies that induce RNAi by delivering double stranded RNA (dsRNA) targeting essential genes have been proposed. Here, we focus on nontransgenic RNAi-based approaches that use oral delivery of dsRNA through feeding of inactivated bacteria to produce RNAi in disease vectors and in a crop pest. This potential pest management method could be easily adapted to target different genes or similar organisms.

Introduction of a cold sensitivity-conferring mutation into the RTA-Bddsx hybrid system of Bactrocera dorsalis for establishment of a thermally controllable homozygous line

BACKGROUND

For efficient control of the economically important fruit pest Bactrocera dorsalis, a hybrid system combining ricin toxicity and sex-related alternative splicing of the doublesex gene has been developed. This system exhibits the expected female-specific lethal effect; however, the transgenic females do not survive, making it difficult to raise stable homozygous lines. Since modification of ricin toxin A chain (RTA) through a single-residue change (Gly²¹² > Arg²¹² ) leads to cold-sensitive posttranslational repression of its toxicity, we utilized this unique property to obtain RTA-Bddsx females that survive at low temperature for line maintenance.

RESULTS

In transient expression experiments using embryonic injection, two groups treated with RTAcs-derived DNA (LERQcs and RTAcs) exhibited temperature-dependent effects. The toxicity was higher at 29 °C than at 18 °C. The proportion of males was close to 50% at 18 °C in all the tested groups except LERQcs-treated flies, which exhibited a high proportion of males (over 70%) at 29 °C. The results indicate the cold-sensitive responses of RTA and further suggest a female-specific lethal effect. Subsequently, 14 putative RTAcs-Bddsx transgenic Ds-Red⁺ G₁ males were identified, and female-specific lethal effects were observed in Ds-Red⁺ G₂ and G₃ lines under cultivation at 29 °C but not at 18 °C. The male ratio can be increased to up to 95% in G₃ line 001, indicating that RTAcs functions well in B. dorsalis.

CONCLUSION

The improved RTAcs-Bddsx system with conditional toxicity represents a novel and promising step toward the practical control of B. dorsalis. © 2021 Society of Chemical Industry.

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.

Transcriptome Analysis of the Oriental Fruit Fly Bactrocera dorsalis Early Embryos

The oriental fruit fly, Bactrocera dorsalis (Hendel), is one of the most devastating and highly invasive agricultural pests world-wide, resulting in severe economic loss. Thus, it is of great interest to understand the transcriptional changes that occur during the activation of its zygotic genome at the early stages of embryonic development, especially the expression of genes involved in sex determination and the cellularization processes. In this study, we applied Illumina sequencing to identify B. dorsalis sex determination genes and early zygotic genes by analyzing transcripts from three early embryonic stages at 0–1, 2–4, and 5–8 h post-oviposition, which include the initiation of sex determination and cellularization. These tests generated 13,489 unigenes with an average length of 2185 bp. In total, 1683, 3201 and 3134 unigenes had significant changes in expression levels at times after oviposition including at 2–4 h versus 0–1 h, 5–8 h versus 0–1 h, and 5–8 h versus 2–4 h, respectively. Clusters of gene orthology (GO) and the Kyoto Encyclopedia of Genes and Genomes (KEGG) annotations were performed throughout embryonic development to better understand the functions of differentially expressed unigenes. We observed that the RNA binding and spliceosome pathways were highly enriched and overrepresented during the early stage of embryogenesis. Additionally, transcripts for 21 sex-determination and three cellularization genes were identified, and expression pattern analysis revealed that the majority of these genes were highly expressed during embryogenesis. This study is the first assembly performed for B. dorsalis based on Illumina next-generation sequencing technology during embryogenesis. Our data should contribute significantly to the fundamental understanding of sex determination and early embryogenesis in tephritid fruit flies, and provide gene promoter and effector gene candidates for transgenic pest-management strategies for these economically important species.

miRNA-1-3p is an early embryonic male sex-determining factor in the Oriental fruit fly Bactrocera dorsalis

Regulation of male sexual differentiation by a Y chromosome-linked male determining factor (M-factor) is one of a diverse array of sex determination mechanisms found in insects. By deep sequencing of small RNAs from Bactrocera dorsalis early embryos, we identified an autosomal-derived microRNA, miR-1-3p, that has predicted target sites in the transformer gene (Bdtra) required for female sex determination. We further demonstrate by both in vitro and in vivo tests that miR-1-3p suppresses Bdtra expression. Injection of a miR-1-3p mimic in early embryos results in 87-92% phenotypic males, whereas knockdown of miR-1-3p by an inhibitor results in 67-77% phenotypic females. Finally, CRISPR/Cas9-mediated knockout of miR-1-3p results in the expression of female-specific splice variants of Bdtra and doublesex (Bddsx), and induced sex reversal of XY individuals into phenotypic females. These results indicate that miR-1-3p is required for male sex determination in early embryogenesis in B. dorsalis as an intermediate male determiner.

Efficiency of RNA interference is improved by knockdown of dsRNA nucleases in tephritid fruit flies

RNA interference (RNAi) in insects is routinely used to ascertain gene function, but also has potential as a technology to control pest species. For some insects, such as beetles, ingestion of small quantities of double-stranded RNA (dsRNA) is able to knock down a targeted gene's expression. However, in other species, ingestion of dsRNA can be ineffective owing to the presence of nucleases within the gut, which degrade dsRNA before it reaches target cells. In this study, we observed that nucleases within the gut of the Queensland fruit fly (Bactrocera tryoni) rapidly degrade dsRNA and reduce RNAi efficacy. By complexing dsRNA with liposomes within the adult insect's diet, RNAi-mediated knockdown of a melanin synthesis gene, yellow, was improved significantly, resulting in strong RNAi phenotypes. RNAi efficiency was also enhanced by feeding both larvae and adults for several days on dsRNAs that targeted two different dsRNase gene transcripts. Co-delivery of both dsRNase-specific dsRNAs and yellow dsRNA resulted in almost complete knockdown of the yellow transcripts. These findings show that the use of liposomes or co-feeding of nuclease-specific dsRNAs significantly improves RNAi inhibition of gene expression in B. tryoni and could be a useful strategy to improve RNAi-based control in other insect species.

Identification of Y-chromosome scaffolds of the Queensland fruit fly reveals a duplicated gyf gene paralogue common to many Bactrocera pest species

Bactrocera tryoni (Queensland fruit fly) are polyphagous horticultural pests of eastern Australia. Heterogametic males contain a sex-determining Y-chromosome thought to be gene poor and repetitive. Here, we report 39 Y-chromosome scaffolds (~700 kb) from B. tryoni identified using genotype-by-sequencing data and whole-genome resequencing. Male diagnostic PCR assays validated eight Y-scaffolds, and one (Btry4096) contained a novel gene with five exons that encode a predicted 575 amino acid protein. The Y-gene, referred to as typo-gyf, is a truncated Y-chromosome paralogue of X-chromosome gene gyf (1773 aa). The Y-chromosome contained ~41 copies of typo-gyf, and expression occurred in male flies and embryos. Analysis of 13 tephritid transcriptomes confirmed typo-gyf expression in six additional Bactrocera species, including Bactrocera latifrons, Bactrocera dorsalis and Bactrocera zonata. Molecular dating estimated typo-gyf evolved within the past 8.02 million years (95% highest posterior density 10.56-5.52 million years), after the split with Bactrocera oleae. Phylogenetic analysis also highlighted complex evolutionary histories among several Bactrocera species, as discordant nuclear (116 genes) and mitochondrial (13 genes) topologies were observed. B. tryoni Y-sequences may provide useful sites for future transgene insertions, and typo-gyf could act as a Y-chromosome diagnostic marker for many Bactrocera species, although its function is unknown.

Efficient somatic and germline genome engineering of Bactrocera dorsalis by the CRISPR/Cas9 system

BACKGROUND

Bactrocera dorsalis (Hendel), a very destructive insect pest of many fruits and vegetables, is widespread in many Asian countries. To facilitate control of this pest, it is essential to investigate its genetics and gene function using targeted gene disruption.

RESULTS

Here, we describe successful targeted mutagenesis of the white and transformer genes in B. dorsalis through use of the clustered regularly interspaced short palindromic repeats/CRISPR-associated 9 (CRISPR/Cas9) system. Co-injection of the white sgRNA and Cas9 mRNA into B. dorsalis embryos caused eye color change, and the white mutations in the germline were heritable. CRISPR-mediated knockout of the sex determination gene transformer (tra) in B. dorsalis resulted in a male-biased sex ratio and adult flies with abnormal outer and interior reproductive organs. Small indels and substitutions were induced by CRIRPR for both genes.

CONCLUSION

Our data demonstrate that somatic and germline genome engineering of the pest B. dorsalis can be performed efficiently using the CRISPR/Cas9 system, opening the door to the use of the CRISPR-mediated method for functional annotations of genes in B. dorsalis and for its population control using, for example, such as gene drive. © 2018 Society of Chemical Industry.

The South American Fruit Fly: An Important Pest Insect With RNAi-Sensitive Larval Stages

RNA interference (RNAi) technology has been used in the development of approaches for pest control. The presence of some essential genes, the so-called “core genes,” in the RNAi machinery is crucial for its efficiency and robust response in gene silencing. Thus, our study was designed to examine whether the RNAi machinery is functional in the South American (SA) fruit fly Anastrepha fraterculus (Diptera: Tephritidae) and whether the sensitivity to the uptake of double-stranded RNA (dsRNA) could generate an RNAi response in this fruit fly species. To prepare a transcriptome database of the SA fruit fly, total RNA was extracted from all the life stages for later cDNA synthesis and Illumina sequencing. After the de novo transcriptome assembly and gene annotation, the transcriptome was screened for RNAi pathway genes, as well as the duplication or loss of genes and novel target genes to dsRNA delivery bioassays. The dsRNA delivery assay by soaking was performed in larvae to evaluate the gene-silencing of V-ATPase, and the upregulation of Dicer-2 and Argonaute-2 after dsRNA delivery was analyzed to verify the activation of siRNAi machinery. We tested the stability of dsRNA using dsGFP with an in vitro incubation of larvae body fluid (hemolymph). We identified 55 genes related to the RNAi machinery with duplication and loss for some genes and selected 143 different target genes related to biological processes involved in post-embryonic growth/development and reproduction of A. fraterculus. Larvae soaked in dsRNA (dsV-ATPase) solution showed a strong knockdown of V-ATPase after 48 h, and the expression of Dicer-2 and Argonaute-2 responded with an increase upon the exposure to dsRNA. Our data demonstrated the existence of a functional RNAi machinery in the SA fruit fly, and we present an easy and robust physiological bioassay with the larval stages that can further be used for screening of target genes at in vivo organisms’ level for RNAi-based control of fruit fly pests. This is the first study that provides evidence of a functional siRNA machinery in the SA fruit fly.

RNAi-Mediated Knockdown of Tssk1 and Tektin1 Genes Impair Male Fertility in Bactrocera dorsalis

The genetic-based sterile insect technique (SIT) is an effective and environmentally safe strategy to diminish populations of agricultural and horticultural insect pests. Functional characterization of genes related to male fertility can enhance the genetic-based SIT. Tssk1 has been involved to control male fertility in both mammals and insects. Moreover, Tektin1 has also been revealed to influence male fertility in both human and mammals. These findings suggested that Tssk1 and Tektin1 identified from Bactrocera dorsalis could be required for male fertility in B. dorsalis. In this study, expression profiles of these two genes were studied at different developmental stages and in various tissues of adult males. Remarkably, it was found that Tssk1 and Tektin1 were highly expressed in the testis of mature adult males of B. dorsalis. Furthermore, Tssk1 and Tektin1 genes were downregulated by using the RNA interference (RNAi) method. Fertility assays including egg laying, hatching, and spermatozoa count were also performed to investigate male fertility of B. dorsalis. Results showed that knockdown of Tssk1 and Tektin1 caused male sterility up to 58.99% and 64.49%, respectively. As expected, the total numbers of spermatozoa were also significantly reduced by 65.83% and 73.9%, respectively. These results suggested that male sterility was happened wing to the low number of spermatozoa. In conclusion, we demonstrate that Tssk1 and Tektin1 are the novel agents that could be used to enhance the genetic-based SIT, or their double-stranded RNA (dsRNA) can be used as biopesticides to control the population of B. dorsalis.

Two of the three Transformer-2 genes are required for ovarian development in Aedes albopictus

In Drosophila melanogaster, transformer-2 (tra2) plays an essential role in the sex-specific splicing of doublesex (dsx) and fruitless (fru), two key transcription factor genes that program sexual differentiation and regulate sexual behavior. In the present study, the sequences and expression profiles of three tra2 (Aalbtra2) genes in the Asian tiger mosquito, Aedes albopictus (Ae. albopictus) were characterized. Phylogenetic analysis revealed that these paralogs resulted from two duplication events. The first occurred in the common ancestor of Culicidae, giving rise to the tra2-α and tra2-β clades that are found across divergent mosquito genera, including Aedes, Culex, and Anopheles. The second occurred within the tra2-α clade, giving rise to tra2-γ in Ae. albopictus. In addition to the conserved RNA recognition motif (RRM), arginine-rich/serine-rich regions (RS domains) and a linker region, a glycine-rich region located between the RRM and RS2 was observed in Tra2-α and Tra2-γ of Ae. albopictus that has not yet been described in the Tra2 proteins of dipteran insects. Quantitative real-time PCR detected relatively high levels of transcripts from all three tra2 paralogs in 0-2 h embryos, suggesting maternal deposition of these transcripts. All three Aalbtra2 genes were highly expressed in the ovary, while Aalbtra2-β was also highly expressed in the testis. RNAi-mediated knockdown of any or all Aalbtra2 genes did not result in an obvious switch of the sex-specificity in dsx and fru splicing in the whole-body samples. However, knockdown of transcripts from all three tra2 genes significantly reduced the female isoform of dsx mRNA and increased the male isoform of the dsx mRNA in both the ovary and the fat body in adult females. Furthermore, knockdown of either Aalbtra2-α or Aalbtra2-γ or all three Aalbtra2 led to a decrease in ovariole number and ovary size after a blood meal. Taken together, these results indicate that two of the three tra2 genes affect female ovarian development.

MicroRNA Let-7 targets the ecdysone signaling pathway E75 gene to control larval-pupal development in Bactrocera dorsalis

MicroRNAs (miRNAs) regulate various biological processes during insect development; however, their role in larval-pupal development in oriental fruit fly, Bactrocera dorsalis (Hendel) remains unknown. In the current study, we address the biological function of a conserved miRNA, Bdo-Let-7 in the regulation of BdE75 gene, which belongs to the ecdysone signaling pathway and participates in the larval-pupal development in B. dorsalis. Using dual luciferase reporter assay in HEK293T cells we show that Bdo-Let-7 miRNA interacts with the 3' untranslated region of BdE75 gene and suppresses its expression. The Bdo-Let-7 and BdE75 are also co-expressed in the larval-pupal stages and in different tissues of B. dorsalis. In in vivo experiments, the injection of Bdo-Let-7 agomir and antagomir in third instar larvae down- and up-regulated the expression of BdE75, respectively. The 20-hydroxyecdysone (20E) injection assay shows that 20E up-regulated the expression of Bdo-Let-7 on the 5th day of the larvae. Moreover, abnormal pupation and eclosion were observed after larval Bdo-Let-7 antagomir injection. Based on these results, we show that Bdo-Let-7 regulates the ecdysone signaling pathway through the exact dose of BdE75 gene, and is indispensable for normal larval-pupal development in B. dorsalis.

Clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated 9-mediated mutagenesis of the multiple edematous wings gene induces muscle weakness and flightlessness in Bactrocera dorsalis (Diptera: Tephritidae)

The clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated 9 (Cas9) system is a versatile, efficient and heritable gene editing tool that can be useful for genome engineering. Bactrocera dorsalis (Hendel) is a major pest of agriculture that causes great economic losses. We used the B. dorsalis multiple edematous wings (Bdmew) gene as the target gene to explore the effectiveness of CRISPR/Cas9 for B. dorsalis genome manipulation. We studied the physiological functions of the Bdmew gene, particularly those related to muscle development. Site-specific genome editing was feasible using direct microinjection of specific guide RNA and the Cas9-plasmid into B. dorsalis embryos. Mutation frequencies ranged from 12.1 to 30.2% in the injected generation. Mosaic G0, with the mew mutation, was heritable to the next generation. The G1 displayed a series of defective phenotypes including muscle weakness, flightlessness, failure to eclose, wing folds and unbalanced movement. These results demonstrated that CRISPR/Cas9 can act as a highly specific, efficient, heritable tool for genome manipulation in B. dorsalis and this has significance for gene function research and genetic control of pests. The Bdmew gene possesses key functions in muscle development of B. dorsalis. Bdmew mutations cause a series of serious defects by interfering with muscle development and may provide a means for controlling B. dorsalis via a gene-based method such as gene drive.

The transformer2 gene of the pumpkin fruit fly, Bactrocera tau (Walker), functions in sex determination, male fertility and testis development

The insect transformer2 (tra2) gene has a prevalent role in cooperating with the sex-determining gene transformer (tra) to direct female differentiation. Here, we report the identification and characterization of Btau-tra2, the tra2 orthologue of the pumpkin fruit fly, Bactrocera tau, an invasive agricultural pest. The Btau-tra2 gene produces three transcript variants. However, only two transcripts can be examined; one is present at all developmental stages in the soma and germline of both sexes and the other one is specific to the embryo and the germline. Knocking down the function of Btau-tra2 produced a male-biased sex ratio and some intersexes. Consistent with a role in sex determination, the obtained intersexual and male sterility phenotypes express a mix of male and female splice variants of the tra and doublesex (dsx) orthologues, indicating that Btau-tra2 has a conserved splicing regulatory function and acts together with/upstream of tra and dsx. In addition, some males obtained from the knock down are fertile but their fertilities are extremely reduced. Moreover, almost all surviving RNA interference (RNAi) males harbour testes having some defects in their external morphologies. Most notably, the body size of a few surviving RNAi flies was two-to threefold increased with respect to the normal size. Our findings suggest that Btau-tra2 is involved in male fertility and may also have an unprecedented role in body size control besides its conserved role in sex determination.

RNA Interference-Mediated Knockdown of Male Fertility Genes in the Queensland Fruit Fly Bactrocera tryoni (Diptera: Tephritidae)

The Queensland fruit fly, Bactrocera tryoni, is Australia’s most important horticultural pest. The Sterile Insect Technique (SIT) has been used to control this species for decades, using radiation to sterilize males before field-release. This method of sterilization can potentially reduce the insects’ abilities to compete for mates. In this study, RNA interference (RNAi) techniques were examined for their potential to sterilize male B. tryoni without adversely affecting mating competitiveness. B. tryoni adults were injected or fed double-stranded RNAs (dsRNAs) targeting spermatogenesis genes (tssk1, topi and trxt); quantitative reverse-transcriptase PCR analyses confirmed that transcript levels were reduced 60–80% for all three genes following injections. Feeding produced a significant gene knockdown for tssk1 and trxt after three days, but interestingly, two genes (trxt and topi) produced an excess of transcripts after 10 days of feeding. Despite these fluctuations in transcript levels, all three dsRNAs impacted the fecundity of treated males, with tssk1- and topi-dsRNA-treated males producing 75% fewer viable offspring than the negative controls. Mating competition assays demonstrated that dsRNA-treated males can actively compete with untreated males. These findings suggest that RNAi technology could serve as an alternative to radiation as a means of sterilizing these insects in an SIT program.

Implementing the sterile insect technique with RNA interference - a review

We review RNA interference (RNAi) of insect pests and its potential for implementing sterile insect technique (SIT)-related control. The molecular mechanisms that support RNAi in pest species are reviewed in detail, drawing on literature from a range of species including Drosophila melanogaster Meigen and Homo sapiens L. The underlying genes that enable RNAi are generally conserved across taxa, although variance exists in both their form and function. RNAi represents a plausible, non-GM system for targeting populations of insects for control purposes, if RNAi effector molecules can be delivered environmentally (eRNAi). We consider studies of eRNAi from across several insect orders and review to what extent taxonomy, genetics, and differing methods of double-stranded (ds) RNA synthesis and delivery can influence the efficiency of gene knockdown. Several factors, including the secondary structure of the target mRNA and the specific nucleotide sequence of dsRNA effector molecules, can affect the potency of eRNAi. However, taxonomic relationships between insects cannot be used to reliably forecast the efficiency of an eRNAi response. The mechanisms by which insects acquire dsRNA from their environment require further research, but the evidence to date suggests that endocytosis and transport channels both play key roles. Delivery of RNA molecules packaged in intermediary carriers such as bacteria or nanoparticles may facilitate their entry into and through the gut, and enable the evasion of host defence systems, such as toxic pH, that would otherwise attenuate the potential for RNAi.

Isolation and Molecular Characterization of the Transformer Gene From Bactrocera cucurbitae (Diptera: Tephritidae)

transformer (tra) is a switch gene of sex determination in many insects, particularly in Dipterans. However, the sex determination pathway in Bactrocera cucurbitae (Coquillett), a very destructive pest on earth, remains largely uncharacterized. In this study, we have isolated and characterized one female-specific and two male-specific transcripts of the tra gene (Bcutra) of B. cucurbitae. The genomic structure of Bcutra has been determined and the presence of multiple conserved Transformer (TRA)/TRA-2 binding sites in Bcutra has been found. BcuTRA is highly conservative with its homologues in other tephritid fruit flies. Gene expression analysis of Bcutra at different developmental stages demonstrates that the female transcript of Bcutra appears earlier than the male counterparts, indicating that the maternal TRA is inherited in eggs and might play a role in the regulation of TRA expression. The conservation of protein sequence and sex-specific splicing of Bcutra and its expression patterns during development suggest that Bcutra is probably the master gene of sex determination of B. cucurbitae. Isolation of Bcutra will facilitate the development of a genetic sexing strain for its biological control.

Sex-Biased Transcriptome of Schistosoma mansoni: Host-Parasite Interaction, Genetic Determinants and Epigenetic Regulators Are Associated with Sexual Differentiation

Background

Among more than 20,000 species of hermaphroditic trematodes, Schistosomatidae are unusual since they have evolved gonochorism. In schistosomes, sex is determined by a female heterogametic system, but phenotypic sexual dimorphism appears only after infection of the vertebrate definitive host. The completion of gonad maturation occurs even later, after pairing. To date, the molecular mechanisms that trigger the sexual differentiation in these species remain unknown, and in vivo studies on the developing schistosomulum stages are lacking. To study the molecular basis of sex determination and sexual differentiation in schistosomes, we investigated the whole transcriptome of the human parasite Schistosoma mansoni in a stage- and sex-comparative manner.

Methodology/ Principal Findings

We performed a RNA-seq on males and females for five developmental stages: cercariae larvae, three in vivo schistosomulum stages and adults. We detected 7,168 genes differentially expressed between sexes in at least one of the developmental stages, and 4,065 of them were functionally annotated. Transcriptome data were completed with H3K27me3 histone modification analysis using ChIP-Seq before (in cercariae) and after (in adults) the phenotypic sexual dimorphism appearance. In this paper we present (i) candidate determinants of the sexual differentiation, (ii) sex-biased players of the interaction with the vertebrate host, and (iii) different dynamic of the H3K27me3 histone mark between sexes as an illustration of sex-biased epigenetic landscapes.

Conclusions/ Significance

Our work presents evidence that sexual differentiation in S. mansoni is accompanied by distinct male and female transcriptional landscapes of known players of the host-parasite crosstalk, genetic determinants and epigenetic regulators. Our results suggest that such combination could lead to the optimized sexual dimorphism of this parasitic species. As S. mansoni is pathogenic for humans, this study represents a promising source of therapeutic targets, providing not only data on the parasite development in interaction with its vertebrate host, but also new insights on its reproductive function.

Parasitic flatworms include more than 20,000 species that are classically hermaphrodites. Among them, the roughly hundred species of Schistosomatidae are intriguing because they are gonochoric. Schistosomes are responsible of the second most important parasitic disease worldwide, and eggs are the main cause of the inflammatory symptoms. Thus, studying the sexual reproduction mechanisms of schistosomes is of particular interest for drug development. Schistosome’s sex is genetically determined by the presence of sex chromosomes: ZZ in males or ZW in females. There is, however, no phenotypic dimorphism in the larval stages: sexual dimorphism appears only in the vertebrate host. In order to understand the molecular mechanisms underlying phenotypic sexual dimorphism, we performed a transcriptome analysis (RNA-Seq) in five different stages of the parasite lifecycle as well as a chromatin status analysis (ChIP-Seq) in the non-differentiated stage cercariae and in the adult differentiated stage, for males and females separately. Our work presents evidence that sexual differentiation in S. mansoni is accompanied by distinct male and female transcriptional landscapes of known players of the host-parasite crosstalk, developmental pathways and epigenetic regulators. Our sex-comparative approach provides therefore new potential therapeutic targets to affect development and sexual reproduction of parasite.

Identification and Characterization of Sex-Biased MicroRNAs in Bactrocera dorsalis (Hendel)

MicroRNAs (miRNAs) are a class of endogenous small non-coding RNAs that regulate various biological processes including sexual dimorphism. The oriental fruit fly Bactrocera dorsalis is one of the most destructive agricultural insect pests in many Asian countries. However, no miRNAs have been identified from the separate sex and gonads to elucidate sex gonad differentiation in B. dorsalis. In this study, we constructed four small RNA libraries from whole body of females, males (except ovaries and testes) and ovaries, testes of B. dorsalis for deep sequencing. The data analysis revealed 183 known and 120 novel miRNAs from these libraries. 18 female-biased and 16 male-biased miRNAs that may be involved in sexual differentiation were found by comparing the miRNA expression profiles in the four libraries. Using a bioinformatic approach, we predicted doublesex (dsx) as a target gene of the female-biased miR-989-3p which is considered as the key switch gene in the sex determination of tephritid insects. This study reveals the first miRNA profile related to the sex differentiation and gives a first insight into sex differences in miRNA expression of B. dorsalis which could facilitate studies of the reproductive organ specific roles of miRNAs.

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.