Transplantation of blastula nuclei to non-enucleated eggs in the medaka, Oryzias latipes

Studies of nuclear transplantation were conducted to establish methods for the production of clones of fish, using a small laboratory fish, medaka, Oryzias latipes. As the first step of the study, single-blastula nuclei of an inbred strain with the wild-type body color were transplanted into non-enucleated unfertilized eggs of an outbred orange red strain. Of 845 operated eggs, 45 hatched into fry exhibiting the wild-type body color, one of the donor markers. Twenty-seven of these nuclear transplants grew to the adult stage and clearly exhibited external secondary sexual characteristics. Fourteen were females and 13 were males. The allozyme analysis of phosphoglucomutase, measurements of relative DNA content by microfluorometry and chromosome counts consistently indicated that the nuclear transplants were triploids that originated from both the diploid donor nuclei and the haploid recipient pronuclei. In the crossing experiments between the nuclear transplants and the orange-red strain, most of the male nuclear transplants were sterile, whereas one male produced a viable offspring with wild-type body color. All of the female nuclear transplants were sterile. Macroscopic observations of their gonads showed that the testes appeared normal and the ovaries appeared degenerated. These features of the reproductive potential and the morphology of gonads also indicated that the nuclear transplants were triploids. These results demonstrated that a basic technique for nuclear transplantation in medaka was established.