Abstract
The sex determination cascades in insects are diversified at the top of the cascade, where different primary molecular signals are employed, while at the bottom of the cascades, particularly the doublesex genes, are highly conserved. Here, we identified the doublesex ortholog (Btau-dsx) of Bactrocera tau, a pumpkin fruit fly, and found that Btau-dsx is composed of six exons and five introns with an additional short "m" exon located in the second intron. Btau-dsx is different from its orthologs in most dipteran insects: Its pre-mRNA is sex-specifically spliced to yield three (two male and one female) instead of two transcript variants. The two deduced proteins produced by the male-specific transcripts are a functional (Btau-DSXM1) and a truncated (Btau-DSXM2) protein, while the female-specific transcript produces the functional Btau-DSXF protein. These three proteins contain all conserved domains except Btau-DSXM2 which has no OD2 domain. The female-specific transcript is detected in both fertilized and unfertilized eggs and in both somatic and germ cells of the adult females, while the male-specific transcript is detected only in fertilized eggs and in the abdominal tissues and testes of adult males. The presence of the Btau-dsxM1 transcript in fertilized eggs at the early syncytium stage suggests that in XY embryos, the Y-linked M factor gene may function quite soon after fertilization to alter the splicing pattern of Btau-dsx pre-mRNA from the female-specific to the male-specific mode. Injection of Btau-dsxF dsRNA into recently emerging females can reduce the expression of vitellogenin (Btau-Vg) and causes some defects in the ovaries, indicating that Btau-dsxF works upstream of Btau-Vg.
Highlights
The most clearly known mechanism of sex determination among insects is obtained from studies of Drosophila melanogaster, whose sex determination cascade contains four key genes, sex lethal (Sxl), transformer, transformer2 and doublesex
The Btau-dsx gene was transcribed in both sexes but its pre-mRNA was spliced in sex-specific manners, resulting in three transcripts: one female (Btau-dsxF) and two male (Btau-dsxM1 and Btau-dsxM2)
We identified the doublesex (Btau-dsx) gene from the pumpkin fruit fly, Bactrocera tau (Walker), an invasive insect pest distributed worldwide
Summary
The most clearly known mechanism of sex determination among insects is obtained from studies of Drosophila melanogaster, whose sex determination cascade contains four key genes, sex lethal (Sxl), transformer (tra), transformer (tra2) and doublesex (dsx). These genes function in a hierarchical order, from the top one to the bottom gene (Cline, 1993; Bopp et al, 2014). When Sxl protein is present, the downstream gene tra produces the female-specific protein, TRAF, which combines with TRA2, a non sex-specific protein, generating the TRA/TRA2 complex protein This protein complex regulates dsx splicing in the female mode to produce DSXF, which directs female embryonic development. In XY embryos that do not have the SXL protein, both tra and dsx genes produce male-specific proteins, TRAM and DSXM, which induce embryos to develop as male flies (Baker & Wolfner, 1988; Burtis & Baker, 1989; Hoshijima et al, 1991; Salz & Erickson, 2010)
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