Genotypic female Atlantic salmon (Salmo salar) immersed in an exogenous androgen overexpress testicular-related genes and develop as phenotypic males

In Atlantic salmon ( Salmo salar ) aquaculture, it can be more profitable to produce all-female populations. For this species, all-female populations can be created by crossing normal female broodstock (XX) with masculinised female broodstock (XX, known as neomales). Neo-males are typically produced via the application of an exogenous androgen treatment during the labile period (developmental window where phenotypic sex can be easily reversed). The present study determined the masculinisation rate (at 81 and 240–241 days after treatment, dat) following a single 2-h immersion in 17α-methyldihydrotestosterone (MDHT, 400 μg/L) applied at 19 days post hatch (dph) to an all-female Atlantic salmon population with and without dimethyl sulfoxide (DMSO, 300 ppm) as an adjuvant. The expression of several sex differentiation-related genes was also determined in the control and MDHT groups at 0, 1, 3, 7, 16, 33 and 56 dat. Survival and growth were not significantly different between treatment groups. At 81 dat, the proportion of males in the control, DMSO, MDHT and MDHT+DMSO groups was 5%, 5%, 100% and 100%, respectively. At 240–241 dat, the proportion of males in these groups was 6%, 0%, 82% and 88%, respectively. A small proportion of intersex fish were also observed in the control, MDHT and MDHT+DMSO groups at this time, as were sterile fish in the MDHT and MDHT+DMSO groups. When compared to the control group, significant effects of MDHT on gene expression were not detected before 16 dat. Cytochrome P450, family 19, subfamily A, polypeptide 1a ( cyp19a1a ) expression was downregulated from 33 dat, whereas anti-müllerian hormone ( amh ) and gonadal soma-derived factor ( gsdf ) expression was upregulated from 56 dat in the MDHT group. Forkhead box L2a ( foxl2a ), r-spondin 1 ( rspo1 ) and sexually dimorphic on the Y chromosome ( sdY ) expression was unaffected by MDHT treatment during the experimental period. This study provides new insights into the changes in gene expression and gonadal morphology induced in genotypic female Atlantic salmon following an immersion in MDHT. The changes in gene expression observed could be involved in mediating morphological sex reversal, and the temporal scale at which they occurred may have implications for refining the age at which sex reversal treatments should be applied in this species. The findings also showed that similar rates of masculinisation are achieved with MDHT alone or in combination with DMSO. • 17α-methyldihydrotestosterone (MDHT) immersion masculinised 82–88% of females. • Expression of sex-related genes was unaffected for 16 days following MDHT treatment. • cyp19a1a , amh and gsdf expression was affected from 33 days after MDHT treatment. • A low incidence of transient sex reversal was apparent in MDHT-treated groups. • 9% of genotypic females in the control treatment developed as male or intersex.