An unusual role for doublesex in sex determination in the dipteran Sciara

Abstract

The gene doublesex, which is placed at the bottom of the sex determination gene cascade, plays the ultimate discriminatory role for sex determination in insects. In all insects where this gene has been characterized, the dsx pre-mRNA follows a sex-specific splicing pattern producing male- and female-specific mRNAs, encoding the male-DSXM and female-DSXF proteins,which determine male and female development, respectively. The present paper reports the isolation and characterization of the gene doublesex of dipteran Sciara insects. The Sciara doublesex gene is constitutively transcribed during development and adult life of males and females. Sciara had no sex-specific doublesex mRNAs but the same transcripts, produced by alternative splicing of its primary transcript, were present in both sexes, although their relative abundance is sex-specific. However, only the female DSXF protein, but not the male DSXM protein, was produced at similar amounts in both sexes. An analysis of the expression of female and male Sciara DSX proteins in Drosophila showed that these proteins conserved female and male function, respectively, on the control of Drosophila yolk-protein genes. The molecular evolution of gene doublesex of all insects where this gene has been characterized revealed that Sciara doublesex displays a considerable degree of divergence with respect to the rest of dipterans, as suggested by its basal position within the doublesex phylogeny, its gene organization and its splicing pattern. It is proposed that if the gene doublesex were needed for Sciara sex determination, this gene would not play the ultimate discriminatory role for sex determination performed in insects.