Sablefish (or black cod, Anoplopoma fimbria) is an emerging aquaculture species typically reared in open water net pens. Escape of farmed fish is an ecological concern associated with net pen aquaculture of sablefish and other marine fishes. To mitigate this risk, we investigated the use of triploidization for the induction of reproductive sterility in sablefish for the first time. Triploidy was induced using a cold shock protocol (-1.5°C, applied 10 min post-fertilization for 120 min). Control diploids and cold shock-induced triploid sablefish were screened for external deformities, co-reared in duplicate tanks for growout, and periodically sampled over 15 months.
We found the control diploid sablefish grew significantly larger (1.48 ± 0.04 kg; mean ± SEM; p < 0.001) than triploids (1.17 ± 0.04 kg) and had greater condition factor (diploid: 1.22 ± 0.004, triploid: 1.16 ± 0.003; p = 0.007), however, the overall specific growth rate (SGR) did not significantly differ between controls (0.58 ± 0.01) and triploids (0.57 ± 0.02). Diploid control females (XX) had ovaries composed of well-developed primary oocytes, while triploid females (XXX) had ovaries that exhibited suppressed development, with mostly empty lamellae (i.e., lacking germ cells) and reduced numbers of smaller primary oocytes. Diploid males (XY) and 35% of the triploid males (XXY) had testes that appeared similar to each other, composed of type-A spermatogonia. Interestingly, we found that 65% of the triploid male (XXY) sablefish developed ovaries that were similar in structure to those of triploid females (XXX). The sex reversal observed in XXY genetic males skewed the overall sex ratio of triploid fish to female biased. The gonadosomatic index (GSI) of ovaries (regardless of genotypic sex, XXX or sex reversed XXY fish) was reduced by 7-fold in triploids relative to diploids, while there was no difference in GSI between testes of triploid (XXY) and diploid (XY) phenotypic males. Gonadal expression of sex related genes, such as dmrt1 and cyp19a1a, retained characteristic sex-specific patterns, with XXY individuals exhibiting patterns that aligned with their phenotypic sex.
These results indicate that triploidy is disruptive to ovarian development in sablefish and likely renders females effectively sterile. Testicular development failed to occur in the majority of XXY fish, which instead developed ovaries, while the testes of XXY males appeared similar to those of XY diploids. It is likely that there are yet uncharacterized differences between testes of diploids and triploids, and that future XXY sperm would be aneuploid and effectively sterile. One possibility for the development of ovaries in 65% of triploid males (XXY) could be a gene dosage effect associated with the two X-chromosomes overriding the male sex-determining gene. More work is needed to fully characterize the XXY triploid-associated sex reversal and determine the utility of triploidy induction for mass sterilization of sablefish for aquaculture.