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

Female semiochemicals stimulate male courtship but dampen female sexual receptivity

Chemical communication plays a vital role in mate attraction and discrimination among many insect species. Here, we document a unique example of semiochemical parsimony, where four chemicals act as both aphrodisiacs and anti-aphrodisiacs in different contexts in Bactrocera dorsalis. Specifically, we identified four female-specific semiochemicals, ethyl laurate, ethyl myristate, ethyl cis-9-hexadecenoate, and ethyl palmitate, which serve as aphrodisiacs to attract male flies and arouse male courtship. Interestingly, these semiochemicals, when sexually transferred to males during mating, can function as anti-aphrodisiacs, inhibiting the receptivity of subsequent female mates. We further showed that the expression of elongase11, a key enzyme involved in the biosynthesis of these semiochemicals, is under the control of doublesex, facilitating the exclusive biosynthesis of these four semiochemicals in females and guaranteeing effective chemical communication. The dual roles of these semiochemicals not only ensure the attractiveness of mature females but also provide a simple yet reliable mechanism for female mate discrimination. These findings provide insights into chemical communication in B. dorsalis and add elements for the design of pest control programs.

RNAi-mediated knockdown of α-Spectrin depresses reproductive performance in female Bactrocera dorsalis

The female reproductive potential plays a crucial role in reproduction, population dynamics and population maintenance. However, the function of endogenous genes in undifferentiated germ cells has been largely unknown in Bactrocera dorsalis. In this study, the conservative analysis showed that α-Spectrin shared a similarity in B. dorsalis and other dipteral flies. Further, the differential expression of α-Spectrin was examined in B. dorsalis by RT-qPCR, and the expression pattern of α-Spectrin protein was identified in female adult ovaries by using immunostaining. During the development of ovary, the change on the number of undifferentiated germ cells was also characterized and analyzed. To understand the function of α-Spectrin in B. dorsalis ovary, the RNAi-based knockdown was conducted, and the RNAi efficiency was examined by RT-qPCR, western blot and bioassay. The results revealed that the α-Spectrin dsRNA could strikingly decrease the expression level of α-Spectrin in ovaries and diminish oviposition and ovary size as a consequence of downregulation of α-Spectrin. Overall, our study facilitates reproductive research on the function of conservative genes in B. dorsalis ovary, which may provide a new insight into seeking novel target genes for pest management control.

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.

Functional analysis of five trypsin-like protease genes in the oriental fruit fly, Bactrocera dorsalis (Diptera: Tephritidae)

Insect midgut proteases catalyze the release of free amino acids from dietary proteins and are essential for insect normal development. To date, digestive proteases as potential candidates have made great progress in pest control. To clarify the function of trypsin-like protease genes in the digestive system of Bactrocera dorsalis, a serious pest of a wide range of tropical and subtropical fruit and vegetable crops, five trypsin genes (BdTry1, BdTry2, BdTry3, BdTry4 and BdTry5) were identified from transcriptome dataset, and the effects of feeding condition on their expression levels were examined subsequently. RNA interference (RNAi) was applied to further explore their function on the growth of B. dorsalis. The results showed that all the BdTrys in starving midgut expressed at a minimal level but up-regulated upon feeding (except BdTry3). Besides, RNAi by feeding dsRNAs to larvae proved to be an effective method to cause gene silencing and the mixed dsRNAs of the five BdTrys slowed larvae growth of B. dorsalis. The current data suggest that trypsin genes are actively involved in digestion process of B. dorsalis larvae and thereafter play crucial roles in their development.

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

Transformer (tra) is a switch gene in the somatic sex-determination hierarchy that regulates sexual dimorphism based on RNA splicing in many insects. In tephritids, a Y-linked male determining gene (M) controls sex in the sex-determination pathway. Here, homologues of Drosophila tra and transformer-2 (tra-2) genes were isolated and characterized in Bactrocera dorsalis (Hendel), one of the most destructive agricultural insect pests in many Asian countries. Two male-specific and one female-specific isoforms of B. dorsalis transformer (Bdtra) were identified. The presence of multiple TRA/TRA-2 binding sites in Bdtra suggests that the TRA/TRA-2 proteins are splicing regulators promoting and maintaining, epigenetically, female sex determination by a tra positive feedback loop in XX individuals during development. The expression patterns of female-specific Bdtra transcripts during early embryogenesis shows that a peak appears at 15 h after egg laying. Using dsRNA to knock-down Bdtra expression in the embryo and adult stages, we showed that sexual formation is determined early in the embryo stage and that parental RNAi does not lead to the production of all male progeny as in Tribolium castaneum. RNAi results from adult abdominal dsRNA injections show that Bdtra has a positive influence on female yolk protein gene (Bdyp1) expression and fecundity.

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.

Lethal and sublethal effects of cyantraniliprole on Bactrocera dorsalis (Hendel) (Diptera: Tephritidae)

BACKGROUND

The oriental fruit fly, Bactrocera dorsalis (Hendel), is one of the most globally important insect pests. Studies were conducted with the novel anthranilic diamide insecticide cyantraniliprole to determine its lethal and sublethal effects on B. dorsalis.

RESULTS

An ingestion toxicity bioassay showed that cyantraniliprole was active against B. dorsalis, and the 72 h feeding LC50 was 3.22 µg g(-1) in adult diet for a susceptible strain. Sublethal doses of cyantraniliprole (1.30 µg g(-1) adult diet) induced a hormesis effect on B. dorsalis. The mating competitiveness of B. dorsalis treated with cyantraniliprole at 3.27 µg g(-1) adult diet was significantly lower when compared with the controls. The lower dose (1.30 µg g(-1) adult diet) of cyantraniliprole improved the total mating times of both mating pairs in treated groups and also the mating competitiveness of the treated males when compared with the higher dose and controls. Cyantraniliprole-treated females of the mated pairs with the lower dose laid more eggs. On the fifth day, female receptivity in the treated group was significantly reduced when compared with the controls.

CONCLUSION

These results indicate that cyantraniliprole is effective against B. dorsalis. The inhibition and stimulation effect of cyantraniliprole on the adult's mating performance at different concentrations was proved.

In silico cloning and annotation of genes involved in the digestion, detoxification and RNA interference mechanism in the midgut of Bactrocera dorsalis [Hendel (Diptera: Tephritidae)]

As the second largest organ in insects, the insect midgut is the major tissue involved in the digestion of food and detoxification of xenobiotics, such as insecticides, and the first barrier and target for oral RNA interference (RNAi). In this study, we performed a midgut-specific transcriptome analysis in the oriental fruit fly, Bactrocera dorsalis, an economically important worldwide pest, with many populations showing high levels of insecticide resistance. Using high-throughput sequencing, 52 838 060 short reads were generated and assembled to 25 236 unigenes with a mean length of 758 bp. Interestingly, 34 unique sequences encoding digestion enzymes were newly described and these included aminopeptidase and trypsin, genes associated with Bacillus thuringiensis resistance and fitness cost. Second, 41 transcripts were annotated to particular detoxification genes such as glutathione S-transferases, carboxylesterases and cytochrome P450s, and the subsequent phylogenetic analysis indicated homology with tissue-specific and insecticide resistance-related genes of Drosophila melanogaster. Third, we identified the genes involved in the mechanism of RNAi and the uptake of double-stranded RNA. The sequences encoding Dicer-2, R2D2, AGO2, and Eater were confirmed, but SID and SR-CI were absent in the midgut transcriptome. In conclusion, the results provide basic molecular information to better understand the mechanisms of food digestion, insecticide resistance and oral RNAi in this important pest insect in agriculture. Specific genes in these systems can be used in the future as potential targets for pest control, for instance, with RNAi technology.

Molecular characterizations of two cytochrome P450 genes encoding CYP6A41 and CYP6EK1 from the oriental fruit fly, Bactrocera dorsalis (Diptera: Tephritidae)

Two P450 genes encoding CYP6A41 and CYP6EK1 were cloned from the oriental fruit fly using polymerase chain reaction (PCR) and rapid amplification of cDNA ends (RACE) techniques. CYP6A41 and CYP6EK1 contained open reading frames of 1,530 and 1,524 nucleotides that encode 510 and 508 amino acid residues, respectively. The putative proteins shared 44% identity with each other. Phylogenetic analysis showed that CYP6A41 and CYP6EK1 were most closely related to Ceratitis capitata CYP6A10 and CYP6A subfamily. Expression patterns of the two genes in different geographical populations (Yunnan, Hainan, Dongguang, and Guangzhou), developmental stages (eggs, larvae, pupae, and adults), and tissues (midguts, fat bodies, and Malpighian tubules) were analyzed by real-time quantitative PCR (RT-qPCR) methods. The results showed that the expression levels of CYP6EK1 were significantly different among the four populations, but were not different for CYP6A41. Both the expressions of CYP6A41 and CYP6EK1 were development specific and had significantly higher levels in the larval stage. The expression of CYP6A41 did not vary among the midgut, fat body, or Malpighian tubules; however, CYP6EK1 expression was higher in the Malpighian tubules. The results suggest that CYP6A41 and CYP6EK1 might be involved in detoxification of xenobiotic compounds that were harmful to larval flies or development. Moreover, high expression of CYP6EK1 in the Malpighian tubules also implied participation in detoxification.