Assessment of the ra-signalling pathway during gonadal development and maturation in the european sea bass (Dicentrarchus labrax)

Abstract

In higher vertebrates, the retinoic acid (RA) signalling pathway stimulates meiosis by regulating the transcription of several genes that result in a spatio-temporal balance between the synthesis and degradation of RA. The goal of this research is to gain a better understanding of the role of the RA-signalling pathway in fish meiosis using the European sea bass as a model. The study is focussed on the expression of nine RA-related genes by the time of sex differentiation and the onset of meiosis in males and females. Moreover, in females it was also extended to the first vitellogenesis and final maturation. The selected genes include a retinol receptor (stra6), oxidizing proteins that synthesize RA (aldh1a2, aldh1a3), binding proteins (crabp, crbp4), nuclear receptors (rarα, rxrα, ppar¿), and the RA catabolizing enzyme (cyp26a1). The results show that in males, stra6, crbp4, rarα, rxrα, cyp26a1, and crabp reached lowest levels by the onset of puberty. The highest expression of aldh1a2, aldh1a3, and pparγ was attained after sex differentiation, decreasing to their lowest levels by the time of puberty. In females, a steady increase of stra6, rarα, rxrα, and cyp26a1 up to the start of vitellogenesis, was followed by a progressive decrease during maturation. A similar scenario applied for aldh1a2, aldh1a3, pparγ and crabp, although in this case, a final increase occurred by the time of maturation. The decrease in the expression of cyp26a1 in both sexes marked the onset of meiosis, indicating that an increase in the availability of RA is required to trigger the process. In addition, in females, an increase in the levels of both aldhs and thus of RA was also observed by the time of final oocyte maturation, coinciding with the resumption of meiosis, further supporting the need of high RA levels for the process. Regardless of these evidences, more studies are required to elucidate the influence of RA in fish meiosis and the network of genes that might be involved.