An efficient expression vector for transgenic medaka construction.

The transparency and external fertilization of the eggs of medaka (Oryzias latipes) make them ideally suitable for investigating molecular interactions that occur during vertebrate development. Genetically engineered medaka is a potential tool for such studies. It requires several types of suitable expression vectors. To obtain abundant and ubiquitous expression of foreign genes in medaka embryos, we have designed an expression vector that contains the proximal promoter and enhancer elements and polyadenylation signal of the medaka beta-actin gene. The utility of this "all-medaka" expression vector was examined using the Escherichia coli lacZ gene as a reporter gene. Most of the injected embryo showed high gene expression, and several embryos showed ubiquitous expression even at six days after injection. Of nine individuals derived from the injected embryos and grown until adult stage, one produced expression-positive F1 fish. The transgene was identified in these F1 using polymerase chain reaction (PCR). These data revealed that the expression vector based on the expression cassette from the medaka beta-actin gene should be useful for making transgenic medaka. The cloned gene in this cassette vector is stably transmittable and efficiently expressible.