The Laboratory of Cellular Engineering (College of Pharmaceutical Sciences) reported on in vitro differentiation of fertile sperm in endangered endemic cyprinid honmoroko (Gnathopogon caerulescens) inhabiting Lake Biwa in Japan, which enables production of fertile sperm from non-spawning fish, and even from juveniles, in a short period of time.

Many endemic fish species are threatened with extinction due chiefly to human activities, suggesting the necessity of conservation strategies and restoration of endemic fish after extinction. In mammals, sperm cryopreservation is the main method of preserving male genetic material. In small endangered fish species, however, the number of mature fish is limited and only a small volume of sperm can be recovered in a short spawning period, which hinders the use of sperm for preservation purposes. The Laboratory demonstrated that fertile sperm can be generated from non-spawning males and even from juveniles of the endangered small cyprinid honmoroko (G. caerulescens). The entire process of spermatogenesis was recapitulated in vitro using cryopreserved spermatogonia of non-spawning adult and juvenile fish. Fertility was demonstrated by artificial insemination. The Laboratory also demonstrated that zebrafish eggs can be used to test fertility of the sperm of G. caerulescens. It takes only 1 month to produce sperm in vitro, whereas 6 months is required in nature. In addition, this in vitro differentiation of sperm would be useful for investigating the mechanism of spermatogenesis.

These results suggest that the combination of cryopreservation of spermatogonia and in vitro sperm differentiation will provide a new and promising strategy for the preservation of paternal genetic materials in endangered endemic fish.