Escapement is an ecological concern for marine fish reared in net-pen systems. To mitigate this risk, we have initiated research to develop approaches for reproductive sterilization. Sablefish (or black cod, Anoplopoma fimbria) is being used as a model for this line of research because of our sound understanding of its sex determination system and reproductive development, and availability of genetic/genomic information.
This study sought to induce triploidy for the first time in sablefish and assess its effects on early reproductive development. Protocols for triploidy induction using hydrostatic pressure or thermal shock were developed, as well as methods for determination of ploidy in whole larvae or blood samples. Three methods for ploidy determination were developed and validated: 1) measurement of cell size by flow cytometry, 2) counting nucleolus organizing regions, and 3) measurement of erythrocyte cell size in blood smears. Diploid (control) and putative triploid sablefish were reared until gonadal sex differentiation could be assessed by histology and ploidy and genotypic sex determined from blood samples, and then fish periodically sampled over a year.
Diploid control (XX-genotype) females 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 a portion of the triploid males (XXY) had testes that appeared similar to each other, composed of type-A spermatogonia. Interestingly, however, we found that >50% of the triploid male (XXY) sablefish developed ovaries. At one-year post-weaning, the gonadosomatic index (GSI) of phenotypic females (regardless of genotypic sex) was reduced by ~10 fold in triploids relative to diploids, while there was no difference in GSI between triploid and diploid phenotypic males.
Results from this study indicate that triploidy induction is disruptive to early ovarian development in sablefish and likely renders females effectively sterile. The observed ovarian development in over half the triploid males (XXY-genotype fish) could be due to a gene dosage effect associated with the X-chromosomes overriding the male sex-determining gene in some individuals. Future research will focus on scaling up triploid sablefish production and comprehensively assessing their survival and performance in each phase of aquaculture.