Sex differentiation in Atlantic cod (Gadus morhua L.): morphological and gene expression studies

Gonads were examined histologically in juveniles from 14 to 39？mm total body length (TL). Number and size of germ cells were determined in a subset of the samples. Relevant genes were cloned, and mRNA levels determined by qPCR of amh, cyp19a1a; dax1 (nr0b2); shp (nr0b2a) and sox9b in a mixed-sex and an all-female population ranging from 12–49？mm TL.Individuals between 14–20？mm TL could be separated in two subgroups based on gonad size and germ cell number. Ovarian cavity formation was observed in some individuals from 18–20？mm TL. The mixed sex population displayed bimodal expression patterns as regards cyp19a1a (starting at 12？mm TL) and amh (starting at 20？mm TL) mRNA levels. After approximately 30？mm TL, cyp19a1a and amh displayed a gradual increase in both sexes. No apparent, sex-dependent expression patterns were found for dax1, shp or sox9b transcripts. However, shp levels were high until the larvae reached around 35？mm TL and then dropped to low levels, while dax1 remained low until 35？mm TL, and then increased sharply.The morphological sex differentiation in females commenced between 14–20？mm TL, and ovarian cavities were evident by 18–20？mm TL. Testis development occurred later, and was morphologically evident after 30？mm TL. This pattern was corroborated with sexually dimorphic expression patterns of cyp19a1a from 12–13？mm TL, and a male-specific increase in amh from 20？mm TL.