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Abstract
BackgroundThe 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).ResultsRT-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.ConclusionsBdtra 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 materialThe online version of this article (doi:10.1186/s12863-016-0342-0) contains supplementary material, which is available to authorized users.
Highlights
The transformer is a sex determining switch in different orders of insects, including Diptera, as in the family Tephritidae
The common exons 1A and 1B of B. dorsalis tra (Bdtra) were homologous to exon 1 of C. capitata tra (Cctra) [7] and Anastrepha obliqua (Macquart) tra (Aotra) [27] genes
In addition to this work, an intron between two common male-specific exons among other Bactrocera is totally retained with no obvious relationship to the TRA/TRA-2 binding site in B. dorsalis males (China population, [28, 29])
Summary
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). 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. There are several mechanisms that determine the sexual fate among insect models [3, 4]. The primary signals convey genetic instruction to the sex determination switches in order to designate and sustain sexual identity during development and throughout an organism’s life [3, 4]. In very early embryonic development (approximately before the blastoderm cellularization), an upstream regulator is usually required to be established as a stable genetic switch that serves as a device memory for sex-specific cell fate [5,6,7,8,9,10,11,12,13,14,15]
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