Production of functional sperm by subcutaneous auto-grafting of immature testes in rainbow trout.

Sexually mature individuals are indispensable for breeding programs. Salmonids require a long period before reaching sexual maturity, so we aimed to shorten the period required to obtain functional sperm by grafting immature testicular fragments into mature recipients, which we predicted would allow the grafted testicular fragments to skip the long pre-pubertal period. First, we demonstrated successful subcutaneous auto-grafting of testicular fragments in rainbow trout. Unilateral testectomy was performed, and the isolated immature testicular fragment was auto-grafted into the subcutaneous space along the back of recipient fish. The grafted testicular fragments developed synchronously with the recipients' testis remaining in its body cavity, and both eventually produced functional sperm. Next, immature testicular fragments were auto-grafted into the subcutaneous space of sexually mature males. We achieved this, without immune rejection, by isolating and cryopreserving testes from immature fish, and rearing these unilaterally testectomized fish until sexual maturity. The cryopreserved testes were then auto-grafted into the original, now spermiating fish. The grated immature testicular fragments differentiated and produced functional sperm within 5 months after grafting. By combining this grafting method with a technique to avoid immune rejection, we expect to develop a practical method for producing sperm in a shorter period in salmonids.