| Summary: | <p>Abstract</p> <p>Background</p> <p>The gene <it>transformer-2</it>, which is involved in sex determination, has been studied in <it>Drosophila, Musca</it>, <it>Ceratitis</it>, <it>Anastrepha </it>and <it>Lucilia</it>. All these members of Diptera belong to the suborder Brachycera. In this work, it is reported the isolation and characterisation of genes <it>transformer-2 </it>of the dipterans <it>Sciara ocellaris </it>and <it>Bradysia coprophila </it>(formerly <it>Sciara coprophila</it>), which belong to the much less extensively analysed <it>Sciaridae </it>Family of the Suborder Nematocera, which is paraphyletic with respect to Suborder Brachycera.</p> <p>Results</p> <p>The <it>transformer-2 </it>genes of the studied <it>Sciara </it>species were found to be transcribed in both sexes during development and adult life, in both the soma and germ lines. They produced a single primary transcript, which follows the same alternative splicing in both sexes, giving rise to different mRNAs isoforms. In <it>S. ocellaris </it>the most abundant mRNA isoform encoded a full-length protein of 251 amino acids, while that of <it>B. coprophila </it>encoded a protein of 246 amino acids. Both showed the features of the SR protein family. The less significant mRNA isoforms of both species encoded truncated, presumably non-functional Transformer-2 proteins. The comparison of the functional <it>Sciara </it>Transformer-2 proteins among themselves and those of other insects revealed the greatest degree of conservation in the RRM domain and linker region. In contrast, the RS1 and RS2 domains showed extensive variation with respect to their number of amino acids and their arginine-serine (RS) dipeptide content. The expression of <it>S. ocellaris </it>Transformer-2 protein in <it>Drosophila </it>XX pseudomales lacking the endogenous <it>transformer-2 </it>function caused their partial feminisation.</p> <p>Conclusions</p> <p>The <it>transformer-2 </it>genes of both <it>Sciaridae </it>species encode a single protein in both sexes that shares the characteristics of the Transformer-2 proteins of other insects. These proteins showed conserved sex-determination function in <it>Drosophila</it>; i.e., they were able to form a complex with the endogenous <it>Drosophila </it>Transformer protein that controls the female-specific splicing of the <it>Drosophila doublesex </it>pre-mRNA. However, it appears that the complex formed between the <it>Drosophila </it>Transformer protein and the <it>Sciara </it>Transformer-2 protein is less effective at inducing the female-specific splicing of the endogenous <it>Drosophila doublesex </it>pre-mRNA than the <it>Drosophila</it>Transformer-Transformer2 complex. This suggests the existence of species-specific co-evolution of the Transformer and Transformer-2 proteins.</p>
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