The Pacific bluefin tuna (Thunnus orientalis) dead end gene is suitable as a specific molecular marker of type A spermatogonia.

We developed a spermatogonial transplantation technique to produce donor-derived gametes in surrogate fish. Our ultimate aim is to establish surrogate broodstock that can produce bluefin tuna. We previously determined that only type A spermatogonia (ASG) could colonize recipient gonads in salmonids. Therefore, it is necessary to develop a precise molecular marker that can distinguish ASG in order to develop efficient spermatogonial transplantation methods. In this study, the Pacific bluefin tuna (Thunnus orientalis) dead end (BFTdnd) gene was identified as a specific marker for ASG. In situ hybridization and RT-PCR analysis with various types of spermatogenic cell populations captured by laser microdissection revealed that localization of BFTdnd mRNA was restricted to ASG, and not detected in other differentiated spermatogenic cells. In order to determine if BFTdnd can be used as a molecular marker to identify germ cells with high transplantability, transplantation of dissociated testicular cells isolated from juvenile, immature, and mature Pacific bluefin tuna, which have different proportions of dnd-positive ASG, were performed using chub mackerel as the surrogate recipient species. Colonization of transplanted donor germ cells was only successful with testicular cells from immature Pacific Bluefin tuna, which contained higher proportions of dnd-positive ASG than juvenile and mature fish. Thus, BFTdnd is a useful tool for identifying highly transplantable ASG for spermatogonial transplantation.