RA-signalling pathway during sex differentiation and puberty in the European sea bass (Dicentrarchus labrax)

Abstract

Retinoic acid (RA) is a vitamin A derivative metabolite, known to activate the onset of meiosis in higher vertebrates through a spatio-temporal balance between its synthesis and degradation. This signalling pathway needs a fine tuning for the transcription of several genes, including a meiosis controlling master gene. However, in teleosts this paradigm is challenged, considering the insufficient evidence of this gene’s existence. Studies in fish point at the role of RA on body patterning, organ development and nutrition, but remain scarce regarding reproduction. Recent investigations in zebrafish suggest the pivotal role of RA on germ cell fate and the onset of meiosis, although the ultimate mechanism of action remains unclear. The European sea bass is a gonochoristic species with a sex specific timing of meiosis, established as a model for the study of fish reproduction. The present work is focussed at the assessment of possible sex-related differences in the expression of several genes involved in the RA signalling pathway during the first two years of age in the sea bass, including sex differentiation and puberty. Among these genes it is worth mentioning: crbp4, crabp (binding proteins), aldh1a2, aldh1a3 (oxidizing enzymes), cyp26a1 (RA catabolizing enzyme), stra6 (retinol receptor), and rarα, rxrα, ppar> (RA nuclear receptors). In males, cyp26a1, stra6, rarα, rxrα, crbp4 and crabp expression levels exhibited a significant decrease by the onset of puberty. Regarding aldh1a2, aldh1a3 and ppar>, the highest values were attained when sex differentiation was completed, and then decreased to lowest levels by the onset of puberty. Females had a different pattern since the expression of most genes increased along the experimental period. The study suggests that an increase in the availability of RA is needed to trigger the onset of meiosis. This is supported by the decrease of cyp26a1 levels by the time of puberty in males during the second year of life. Conversely, in females, cyp26a1 levels remained high since puberty is attained during the third year. The study contributes to the general knowledge of the prominent role of RA and the different components of its signalling pathway in fish reproduction