Expression and fate of CAT reporter gene microinjected into fertilized medaka (Oryzias latipes) eggs in the form of plasmid DNA, recombinant phage particles and its DNA.

Fertilized medaka (Oryzias latipes) eggs were cytoplasmically injected with the chloramphenicol acetyltransferase (CAT) gene encompassed in supercoiled and linear plasmid DNA, as well as in intact recombinant phage particles and DNA isolated from the phage. Expression for the CAT plasmid DNA was highest at the gastrula/neurula stage, while for the DNA of the phage, it peaked in the 1-week old embryo; then expression declined but was still detectable in early adulthood (4 weeks post injection). Following the fate of exogenous DNA, an extensive replication was observed in early embryogenesis, and DNA was still found 4 weeks after injection, suggesting a possibility of integration. The system is useful as a transient expression system for the analysis of early developmental genes in particular, but also as a test system for the analysis of cloned genes of interest for the farming of economically important fish species. The fact that DNA transferred in intact phage particles or its DNA is functionally active opens the possibility to introduce larger DNA pieces (20 kb), e.g., for the functional test of larger and more distant control regions.