Early development of primordial germ cells in Pacific bluefin tuna Thunnus orientalis

Bluefin tuna is one of the most important aquaculture species in several countries; however, information regarding the primordial germ cell (PGC) development and migration in this species is scarce. This information is vital for application in reproductive biotechnology, for example, induced sterility through targeted cell ablation or PGC manipulation. Teleost PGC can be visualized by injecting an RNA transcribed from the fusion product of a fluorescent protein gene attached to the 3' untranslated region (3'UTR) of zebrafish nanos3 (zf-nos3) into eggs. In this study, we identified the PGC and its migratory pathway during early embryogenesis and larvae development by injecting the GFP-zf-nos3 3'UTR mRNA into the Pacific bluefin tuna (PBT, Thunnus orientalis). PBT PGCs were initially found around the marginal and dorsal regions of the blastodisc at 50%-epiboly stage. The PGCs were aligned as two elongated lines at the posterior part of the embryonic body during the early segmentation period, and eventually formed a single tight cluster underneath somites 10 to 15 of the embryonic body until the late segmentation period. Although the aggregated PGCs stayed at the same position during hatching, they started migrating anteriorly and were split into two populations at 3 days after hatching (DAH). Until 15 DAH, these PGCs settled in two bilateral lines at the apex of the peritoneal cavity. Histological analysis of PBT larvae revealed that at 3 and 5 DAH, the PGCs were not enclosed by the somatic cells, whereas at 15 DAH, they were entirely covered by the somatic cells, indicating the development of the primordial gonads. These results are essential for future experiments in germ line control technologies for bluefin tuna.