Academic journal article Genetics

Evolutionary Conservation of Ceratitis Capitata Transformer Gene Function

Academic journal article Genetics

Evolutionary Conservation of Ceratitis Capitata Transformer Gene Function

Article excerpt

ABSTRACT

Transformer functions as a binary switch gene in the sex determination and sexual differentiation of Drosophila melanogaster and Ceratitis capitata, two insect species that separated nearly 100 million years ago. The TRA protein is required for female differentiation of XX individuals, while XY individuals express smaller, presumably nonfunctional TRA peptides and consequently develop into adult males. In both species, tra confers female sexual identity through a well-conserved double-sex gene. However, unlike Drosophila tra, which is regulated by the upstream Sex-lethal gene, Ceratitis tra itself is likely to control a feedback loop that ensures the maintenance of the female sexual state. The putative CcTRA protein shares a very low degree of sequence identity with the TRA proteins from Drosophila species. However, in this study we show that a female-specific Ceratitis Cctrac DNA encoding the putative full-length CcTRA protein is able to support the female somatic and germline sexual differentiation of D. melanogaster XX; tra mutant adults. Although highly divergent, CcTRA can functionally substitute for DmTRA and induce the female-specific expression of both Dmdsx and Dmfru genes. These data demonstrate the unusual plasticity of the TRA protein that retains a conserved function despite the high evolutionary rate. We suggest that transformer plays an important role in providing a molecular basis for the variety of sex-determining systems seen among insects.

IN the fruit fly Drosophila melanogaster, the X:A ratio serves as the primary signal for somatic sex determination. The primary signal acts via Sex lethal (SxI), which functions as the binary switch gene that controls sexual differentiation. When the X:A ratio is 1 (2X/2A embryos), SxIis switched "on" and signals female identity, whereas a value of 0.5 (X/2A embryos) turns SxI "off," allowing male development to ensue (CLINE and MEYER 1996). During early development, SxI is transiently transcribed from an early promoter (&Zpe) in only the XX embryos in response to the primary signal, while at later stages the transcription of SxI is triggered by a maintenance promoter (&Zpm) in both males and females (KEYES et al. 1992). The early SXL protein establishes a positive feedback loop in the XX embryos by directing the female-specific productive splicing of SxI pre-mRNA transcribed from Sxlpe (CLINE 1984; BELL et al. 1991). This autoregulatory mechanism guarantees the memory of the female sexual state throughout the life cycle of the fly. By contrast, in males, transcription from Sxlre is not activated and therefore the feedback loop is never initiated. SxI controls the sexual differentiation program through the downstream transformer (ira) gene. In females, SXL protein directs the splicing of ire RNA so that the full-length TRA protein is produced only in females (SosNOWSKi et al. 1989; INOUE et al. 1990; VALCÂRCEL et al. 1993). In males, the SxI gene is "off; therefore ira splicing is governed by a default mechanism resulting in small nonfunctional TRA pep tides (BUTLER et al. 1986; BOGGS et al. 1987; MCUEOWN et al. 1987). TRA subsequently controls the downstream components of the cascade, namely the double-sex (dsx) and fruitless (fru) genes. Indeed, it has been shown that TRA is able to direct the female-specific processing of dsx and fru precursor mRNAs (BAKER and RIDGE 1980; McKEOWN et al. 1988; HOSHIJIMA et al. 1991 ; HEINRICHS et al. 1998). This process requires the product of the sex-non-specific transformed (tra2) gene and general splicing factors (AMREIN et al. 1988; INOUE et al. 1992). Female-specific mRNAs are produced from dsx and fru encoding the DSXF and FRUF proteins in the presence of the TRATRA2 complex. While in males, in the absence of TRA protein, dsx and fru give rise to the male-specific DSXM and FRUM isoforms (BuRTis and BAKER 1989; RYNER et al. 1996). The protein isoforms produced from the dsx and fru genes are responsible for the development of sex-specific somatic tissues and behavioral traits. …

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