Breakthrough in aquaculture: Functional Pacific bluefin tuna sperm was produced in small surrogate fish within 8 months of age by Tokyo University of Marine Science and Technology

The research group of Professor Goro Yoshizaki's at the Department of Marine Biosciences at Tokyo University of Marine Science and Technology announced that they succeeded in the production of functional Pacific bluefin tuna sperm in an interspecific hybrid of the small-bodied scombrid fish at just eight months of age. Use of the interspecific hybrid as surrogate parent fish increased survival rates during the juvenile stage and shortened the time to maturity. Pacific bluefin tuna usually take three to five years to mature. They confirmed that the produced sperm fertilized interspecific hybrid eggs and initiated embryonic development. The findings are expected to contribute to improving the efficiency of bluefin tuna aquaculture and were published in the October 3 issue of the international journal Nature Communications.

Pacific bluefin tuna (Thunnus orientalis) is a heavily depleted fish species, which has become a concern, and strict regulations are currently applied to the size and quantity of fish that can be caught. The International Union for Conservation of Nature and Natural Resources (IUCN) has designated this species as near threatened.

To address this concern, full-cycle aquaculture and release of artificially hatched fish into the wild are expected to be effective for restoration of Pacific bluefin tuna stock; however, it requires a minimum of three years before the fish mature, and large costs are incurred for the large tanks and other materials. The research group has been studying an approach of transplanting germline stem cells from donors into host (surrogate parent) larvae to produce the next generation from donors mainly in fishes of the family Salmonidae.

In May of this year, they succeeded in producing salmon eggs by transplanting salmon germline stem cells into sterile rainbow trout. Through the study, the research group has also shown that newly hatched larvae do not reject xenogeneic germline stem cells, that germline stem cells injected into the peritoneal cavity migrate spontaneously to the host's ovaries or testes, and that germline stem cells have sexual plasticity, undergoing oogenesis in the ovary and spermatogenesis in the testis.

In this study, the research group applied these techniques to Pacific bluefin tuna. Previous studies have shown that this technology tends to work successfully in closely related species. Therefore, they transplanted Pacific bluefin tuna germline stem cells into three different small-bodied mackerels as hosts selected from members of the family Scombridae to which Pacific bluefin tuna belong: chub mackerel (Scomber japonicus), striped bonito (Sarda orientalis), and Eastern little tuna (Euthynnus affinis).

As a result, the frequency that Pacific bluefin tuna germline stem cells migrated to the gonads was 1.3% in chub mackerel, 12% in striped bonito, and 19.2% in Eastern little tuna, with Eastern little tuna being the most efficient host species in this regard. However, Eastern little tuna had issues with a low survival rate during the larval stage and poor growth during the early developmental stage.

To combat this issue the group used heterosis. The cryopreserved sperm of the Atlantic little tuna of the genus Euthynnus (E. alletteratus) was confirmed to fertilize Eastern little tuna eggs, and these two species were hybridized. The resulting hybrid little tuna had significantly higher growth and survival rates than Eastern little tuna, could be kept in aquariums, and retained genomes from both parents.

Subsequently, Pacific bluefin tuna germline stem cells were transplanted to the hybrid little tuna host. In just eight months, 68 mature males weighing about one kilogram were obtained. Sperm collected from these hybrid males was stained with monoclonal antibodies specific to Pacific bluefin tuna sperm, and the results showed that nine hybrid males (13.2%) produced Pacific bluefin tuna sperm. These hybrids were mated in the aquarium by inducing reproductive behavior through hormone treatment, and fertilized eggs were collected.

DNA analysis of fertilized eggs after development confirmed that eggs with the Pacific bluefin tuna genome underwent embryonic development, although the success rate was low. The time to maturity, which previously took 3−5 years, was reduced to 8 months, and Pacific bluefin tuna sperm could be obtained in small-bodied hybrid little tuna weighing about 1 kilogram. Conventionally, the only way to do this was to keep mature Pacific bluefin tuna males and females in tanks that hold large tuna weighing 50 kilograms or more and wait for maturation and mating to occur naturally. This technology is expected to be applicable to Thunnus species other than Pacific bluefin tuna. Although there are different issues regarding eggs, the group will consider the use of hosts other than hybrid little tuna during their future studies.

Yoshizaki commented, "Moving forward, our important task is to improve the efficiency at the industrial level. We would also like to try egg production. If tuna can be born from small surrogates through application of this technology to the mother fish, it will enable a continuous production of tuna species. Breeding usually takes 20 years or five generations of tuna, but it can be done in 40 months with little tuna. Even if we only shorten the paternal side, it will improve the efficiency of the breeding process, and we expect the technology to contribute to the creation of better tuna strain."

Journal Information
Publication: Nature Communications
Title: Bluefin tuna sperm production is hastened by surrogacy in small Euthynnus
DOI: 10.1038/s41467-024-52393-4