Production of Donor-Derived Eggs After Ovarian Germ-Cell Transplantation into the Gonads of Adult, Germ Cell-Less, Triploid Hybrid Fish†.

In animals, spermatogonial transplantation in sterile adult males is widely developed; however, despite its utility, ovarian germ-cell transplantation is not well developed. We previously showed that the interspecific hybrid offspring of sciaenid was a suitable model for germ-cell transplantation studies as they have germ cell-less gonads. However, all these gonads have testis-like characteristics. Here, we tested whether triploidization in hybrid embryos could result in germ cell-less ovary development. Gonadal structure dimorphism and sex-specific gene expression patterns were examined in 6-mo-old triploid hybrids (3nHybs). Thirty-one percent of 3nHybs had germ cell-less gonads with an ovarian cavity. cyp19a1a and foxl2, ovarian differentiation-related genes, were expressed in these gonads, whereas dmrt1 and vasa were not expressed, suggesting ovary-like germ cell-less gonad development. Some (26%) 3nHybs had testis-like germ cell-less gonads. Ovarian germ cells collected from homozygous green fluorescent protein (GFP) transgenic blue drum (Nibea mitsukurii) were transplanted into 6-mo-old 3nHybs gonads via the urogenital papilla or oviduct. After 9 mo, the recipients were crossed with wild-type blue drum. Among the six 3nHyb recipients that survived, 1 female and 1 male produced fertile eggs and motile sperm carrying gfp-specific DNA sequences. Progeny tests revealed that all F1 offspring possessed gfp-specific DNA sequences, suggesting that these recipients produced only donor-derived eggs or sperm. Histological observation confirmed donor-derived gametogenesis in the 3nHyb recipients' gonads. Overall, triploidization reduces male-biased sex differentiation in germ cell-less gonads. We report, for the first time, donor-derived egg production in an animal via direct ovarian germ-cell transplantation into a germ cell-less ovary.