Reprogramming defects after nuclear transfer in fish

International audience; Somatic cells bear a major potential as cell resource for biodiversity conservation: they carry the genome of both parents, and they can be collected whatever the age or sex of the donor. In fish, cryobanking of somatic cells is highly valuable because eggs and embryos cannot be cryopreserved. Then, the sole use of cryopreserved sperm for animal restoration requires long back-crossing with fresh eggs from plain strains. With somatic cells, the regeneration of the donor genotype will however rely on the nuclear transfer technology in which the donor cell nucleus is injected into a recipient oocyte. Although this restoration technology was developed in the 60’ for fish, the method is still far from being reliable enough for genome preservation and population restoration. Using the goldfish as a model, we showed that the most critical stage after nuclear transfer was the mid-blastula transition stage, when the 1000 cells embryo triggers its own genome activation. This strongly suggests that although the injected nucleus faced 10 rounds of mitosis, chromatin reprogramming was not reliable enough to allow the proper expression of the early development genes. We demonstrated that several marker genes whose promoter regions are hypomethylated in fertilized early embryos displayed various patterns of hypermethylation which were closer to the donor cell pattern. To some extent, the morphology of the embryo could be correlated to the level of the DNA methylation reprogramming. This demonstration reinforces the need to reprogram the donor cell prior to or during nuclear transfer, and the best strategies when facing fish cells and embryos will be discussed.