Sablefish (or black cod, Anoplopoma fimbria) is a burgeoning aquaculture species in Canada and the U.S., with established and emerging markets worldwide. Neomale (XX-genotype male) broodstock are key to the production of faster growing, more profitable, all-female sablefish stocks. Current industry protocols involve dietary treatment with a synthetic androgen, 17-alpha methyltestosterone (MT), during a critical period of early development to induce XX-genotype fish to develop as phenotypic males. At maturity, sperm collected from neomales is used to fertilize eggs from normal female broodstock to produce all-female offspring. However, MT-induced neomales often exhibit gonadal anomalies, including partial female development, abnormal morphology, and non-functional sperm ducts, limiting their utility as broodstock. These anomalies are likely associated with competing androgenic and estrogenic signals during sexual differentiation, as MT overrides but does not suppress estrogen production in XX fish.
Aromatase inhibitors (AIs), which directly suppress estrogen synthesis, may induce more ‘natural’ testicular differentiation in XX fish. The combination of an AI with MT has also been demonstrated in some cases to be more effective than either factor applied alone. We tested two AIs: a non-steroidal (AI-1) and steroidal AI (AI-2), alone or combined with MT, for neomale production. Treated diets were fed to all-XX genotype post-larval sablefish and gonad tissues were sampled for histology immediately post-treatment and at 4 and 12 months. Control males (XY) were reared and sampled in parallel for comparisons to normal testicular differentiation.
Aromatase inhibitor treatments yielded gonads with two lobes, resembling those of XY controls, while MT-treated gonads retained single-lobed gonads, similar to those of XX controls. Combined AI+MT treatments produced mixed phenotypes with one or two lobes. Histologically, gonads of AI-treated XX fish resembled those of XY controls, showing non-meiotic spermatogonia, whereas MT and AI+MT treatments advanced to meiosis (i.e. spermatogenesis) and exhibited spermatocytes and spermatids. Intersex gonads (ovotestes) were commonly observed across treatments, except the AI-2 and control groups.
This study highlights a promising AI-based dietary treatment for inducing more natural testicular differentiation in neomales, potentially overcoming androgen-induced anomalies associated with the use of MT. Future work will refine AI dosage and evaluate the reproductive success of AI-derived neomales for more scalable all-female production of sablefish for aquaculture. This research was supported by NOAA Office of Aquaculture through Internal Competitive Aquaculture Funds (ICAF).