ELIMINATING REPRODUCTIVE GENES: A GOOD APPROACH TO STERILITY IN FISH?

Fueled by the ecological and regulatory urgency to biologically contain farmed fish, genetically engineered or not, the quest for efficient methods to induce sterility in farmed fish has intensified. A logical approach for generating sterile fish is to functionally silence/disrupt the hormones along the brain-pituitary-gonadal (BPG) axis that regulate reproduction. This paper reviews approaches to disrupt or silence key reproductive hormones and their functional consequences. Interfering with the early migration of the reproductively most-relevant GnRH3 disrupted the early establishment of the GnRH3 system, but only temporarily, and the fish ended up being reproductively competent. Using gene targeting and knockout platforms, such as TALEN and CRISPR, completely eliminated key reproductive hormones in zebrafish, such as the two kisspeptins and two GnRHs in the brain and luteinizing hormone (LH) and follicle stimulating hormone (FSH) in the pituitary. However, very surprisingly, eliminating most of these hormones did not lead to loss of reproductive function. With the exception of LH, the mutated fish displayed full gametogenesis and produced viable gametes and offspring that continued to reproduce despite carrying the reproductive deletions. The LH-mutated fish underwent gametogenesis but not final oocyte maturation and ovulation. Interestingly, using laser ablation to entirely remove GnRH3 neurons during early development did lead to the arrest of gametogenesis at its early stages and to the production sterile fish. Collectively, these findings challenge the current and prevailing wisdom of simple control of reproduction by the brain and pituitary hormones that were silenced. It also suggests that fish have developed back-up strategies and redundancies in which yet-to-be-discovered hormones and mechanisms compensate for the lack of the known reproductive regulators, thereby ensuring reproductive competence in the absence of the main hormonal players. Furthermore, this data suggests that silencing the genes expressing reproductive hormones in the brain and pituitary may not be a promising approach for generating sterile fish. However, contrary to the findings in the brain and the pituitary, silencing of certain reproductive genes in early germ cells leads to a total lack of gametogenesis and the production of 100% sterile fish. Disrupting or silencing these gonadal genes, using transgenic and non-transgenic approaches, holds great promise for producing reproductively sterile fish in aquaculture.