Cloning and sequence analysis of a vasa homolog in the European sea bass ( Dicentrarchus labrax): Tissue distribution and mRNA expression levels during early development and sex differentiation

Abstract

Vasa is a protein expressed mainly in germ cells and conserved across taxa. However, sex-related differences and environmental influences on vasa expression have not been documented. This study characterized the cDNA of a vasa homolog in the European sea bass, Dicentrarchus labrax ( sb-vasa), a gonochoristic fish with temperature influences on gonadogenesis. The 1911 bp open reading frame predicted a 637-amino acid protein with the eight conserved domains typical of Vasa proteins. Comparisons of the deduced amino acid sequence with those of other vertebrates and invertebrates revealed the highest homology (68–85%) with those of other teleosts. An updated tree with the full-length sequences for Vasa proteins in 66 species belonging to six different phyla was constructed, establishing the evolutionary relationships of Vasa amino acid sequences. European sea bass vasa was highly expressed in gonads with little or no expression in other tissues. Real time RT-PCR quantification of the temporal expression of sb-vasa from early development throughout sex differentiation showed that mRNA levels were high in unfertilized eggs, decreased during larval development and increased again during the period of germ cell proliferation. Rearing of fish at high temperature resulted in further increased sb-vasa levels, most likely reflecting temperature effects on both somatic and gonadal growth. Differences in expression were also found well before sex differentiation and persisted until the end of the first year, with higher levels present in females. These differences in expression demonstrate the implication of vasa during the initial stages of fish sex differentiation and gametogenesis and suggest that, through its helicase activity, it might be implicated in the translational regulation of mRNAs involved in the specification and differentiation of gonadal-specific cell types.