Expression Patterns of Genes Involved in Sexual Development of Turbot (<em>Scophthalmus maximus</em>): A Histological Analysis by <em>In Situ</em> Hybridization on Differentiated Ovarian and Testicular Tissue

Genes involved in sexual development have been identified in many teleost fishes including the turbot, a flatfish with great commercial value for the aquaculture industry, where a principal Sex Determination (SD) Quantitative Trait Loci has been identified and Sox 2 has been proposed as the SD gene. Although RT-PCR, qPCR, and transcriptomic analyses have been carried out in turbot gonads, histological studies are still scarce. We have studied the tissue-specific expression patterns of several genes involved in the sexual development in turbot by in situ hybridization on paraffin sections (SISH) of differentiated ovary and testis of juvenile and adult specimens. Vasa, Foxl2, Amh, Sox2 and Sox17 transcripts were detected both in male and female gonads while Cyp19a1a and Sox19 expression were ovary specific, supporting previous data on gene expression analyses and suggesting a role for those genes in gonad development and/or in reproductive functions in this species. Therefore, SISH provides complementary information to molecular analyses identifying the cell types where sex-related genes are expressed in gonadal tissue and to obtain a more comprehensive view of the gonadal differentiation process in turbot. Comparison with the results obtained in the gonads of other teleost species showed a similar tissue-specific gene expression during sexual development.