Reproduction and Sex Control in Turbot

Turbot (Scophthalmus maximus) is a flatfish with increasing aquaculture value in Europe and China. This species shows extreme sexual growth dimorphism, with females growing faster and achieving sexual maturity later than males - hence the interest of industry in producing all-female populations. Sex determination (SD) of turbot shows a major genetic component, the main quantitative trait loci (QTL) being located at linkage group (LG) 5. A microsatellite marker (SmaUSC-E30) at this region shows a high sexing efficiency and is used by industry for precocious sex identification. Minor secondary QTLs and some temperature influence have also been reported. Available data support a ZZ/ZW system of recent evolutionary origin in turbot. The first signs of sex differentiation are detected at 90 days post-fertilization, when the genes amh, cyp19a1a and vasa show differential expression between sexes. From this stage, female gonads undergo quick differentiation, while future male gonads remain more similar to undifferentiated gonads. Different approaches have been evaluated to increase the proportion of females in turbot, including hormone treatments and chromosome set manipulation (triploids, gynogenetics), but the most efficient and safe strategy involves a cross between ZZ males and WW superfemales, obtained through a two-generation pedigree involving sex reversal assisted by the SmaUSC-E30 marker. Triploids represent an interesting product for turbot aquaculture, because of their functional sterility, female-biased sex ratio and null impact of escapees on wild populations. Genomic strategies will be likely key to identify the SD master gene, as essential information for efficient sex monitoring and brood stock management at turbot farms.