Transcriptomic study of gonadal sex differenciation in turbot (Scophthalmus maximus) using a species-specific microarray enriched with reproduction-related genes

Abstract

The turbot (Scophthalmus maximus) is a flatfish with increasing consumer demand world-wide and therefore its aquaculture production is steadily growing. Turbot exhibits a marked sexual growth dimorphism in favor of females and thus there is interest to understand its sex determination and differentiation in order to help the production of all-female stocks. Here, we studied gene expression during sex differentiation in turbot by transcriptional analysis of gonads of sexually undifferentiated, sexually differentiating and juvenile males and females by using a species-specific microarray enriched with reproduction-related genes. We identified groups of genes preferentially expressed at different stages during sex differentiation and studied the expression patterns of 18 canonical reproduction-related genes. We found that in turbot cyp19a1a and dmrt3 expression levels can be used as reliable markers of female and male differentiation, allowing the accurate prediction of phenotypic sex at 90 and 140 days post fertilization (dpf), respectively, as verified by molecular and histological analysis. With fish thus sexed, we identified a suite of 45 and 12 novel differential expressed genes (DEG) associated with ovarian and testicular differentiation, respectively. Some of these genes were previously related not only to sex differentiation, but also to other aspects of reproduction control, general metabolism, immune response and the circadian clock system. Further investigation of these genes should be addressed. In juveniles, we found ~4.000 DEG between ovaries and testes and, with the aim of identifying possible sex determining candidate genes, we mapped these DEG to the previously-identified sex- and growth-QTL markers. We found that that a larger but not significantly different amount of male-biased sex-differential transcripts were located in the linkage group 8 of turbot map. Transcripts mapped near growth-related QTLs were only found in females but not in males. In summary, this study describes the first transcriptomics of turbot gonads during sex differentiation, identifies new candidate genes and establishes two markers of early phenotypic sex