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.

Development of the posterior lateral line system in Thunnus thynnus, the Atlantic blue-fin tuna, and in its close relative Sarda sarda.

The lateral line system of amphibians and fish comprises a large number of individual mechanosensory organs, the neuromasts, and their sensory neurons. The pattern of neuromasts varies markedly between species, yet the embryonic pattern is highly conserved from the relatively basal zebrafish, Danio rerio, to more derived species. Here we examine in more detail the development of the posterior lateral line (PLL) in embryos and early larvae of one of the most derived fish species, the blue-fin tuna Thunnus thynnus, and of its close relative, the Atlantic bonito Sarda sarda. We show that the basic features of embryonic PLL development, including the migratory properties of the PLL primordium, the patterning of neuromasts and their innervation, are largely conserved between zebrafish and tuna. However, Thunnus and Sarda embryos differ from Danio in three respects: the larger size of the neuromast cupula, the capability of mature neuromasts to migrate dorsally, and the presence of a single, precisely located terminal neuromast.

Characterization of thyroid hormones and thyroid hormone receptors during the early development of Pacific bluefin tuna (Thunnus orientalis).

We studied the profiles of 3,5,3'-l-triiodothyronine (T3), thyroxine (T4), and thyroid hormone receptors (TRs) in Pacific bluefin tuna (Thunnus orientalis) during embryonic and post-embryonic development. Both T3 and T4 were detected in embryos just before hatching, and it was found that the levels of both were increased in postflexion fish. The thyroid follicles were increased in both size and number in postflexion fish compared with preflexion fish. A TRbeta cDNA clone was generated by RACE. Two TRalpha cDNA clones were also partially identified and analyzed by real-time RT-PCR in this study. The TR mRNA levels in embryos were determined, and these were found to be lower than those in preflexion fish. Therefore, we considered that thyroid hormones function during early post-embryonic development as well as during embryonic development. Moreover, there was a peak in the TR mRNA level during postflexion stages, as seen during metamorphosis in Japanese flounder and Japanese conger eel. It is possible that thyroid hormones control the early development of scombrid fish through TRs, as they do for Pluronectiformes and Anguilliformes.