Production of functionally sterile triploid Nibe croaker Nibea mitsukurii induced by cold-shock treatment with special emphasis on triploid aptitude as surrogate broodstock

Among the pelagic egg spawning marine teleosts, spermatogonial transplantation techniques were first developed in the Nibe croaker Nibea milsukurii; however, production of donor-derived offspring had not yet been achieved. In salmonids, when triploid individuals were used as recipients, the resulting surrogate broodstock produced only donor-derived gametes, as the triploids were incapable of producing their own gametes. Therefore, in the present study, we determined the optimal parameters for cold-shock inducing triploidy through the suppression of meiosis II resulting in retention of the second polar body in fertilized Nibe croaker eggs. It was determined that cold-shock treatment, which was carried out at 10 degrees C for a period of 15 min beginning 5 min after fertilization, following pre-shock treatment at 22 degrees C achieved a 100% triploidization rate as well as hatching rates of which were high enough for practical application in the pelagic egg spawning marine teleost (16.4 +/- 10.4%). Average chromosome numbers of cold-shock treated larvae counted using mitotic figures found on the fin membrane of 1 day post-hatch larvae were 67.3 +/- 0.7, which was approximately 1.5 times larger than that of control larvae (46.8 +/- 1.1), suggesting that triploidy had been induced. No differences in survival rates of hatched larvae during the rotifer-feeding stage existed between treated and non-treated groups. Gonadosomatic index and gametogenesis were impaired in both sexes of triploid Nibe croaker. Although triploids produced small amounts of gametes, no progeny from crosses between triploids and diploids survived later than 24 h subsequent to hatching. These results indicated that the triploid Nibe croaker developed in this study could be effectively used as surrogate recipients in spermatogonial transplantation. Statement of relevance: In intraperitoneal germ-cell transplantation techniques in salmonids, donor-derived diploid germ cells proliferated and differentiated within triploid recipient gonads, and the surrogate recipients produced only donor-derived gametes, which were capable of generating viable offspring when used in fertilization. To date, germ-cell transplantation techniques have been developed for use in perciforms, including sciaenids, however, triploid recipients have not yet been utilized in marine fishes. In this study, triploid Nibe croaker was produced at high triploidization rates (similar to 100%) using cold-shock treatments. Analysis of the gonadal development and gametogenesis of triploid Nibe croaker in two consecutive spawning seasons revealed that the triploid Nibe croaker was functionally sterile in both sexes and that the triploids would be suitable for use as surrogate recipients. (C) 2016 Elsevier B.V. All rights reserved.