INSERTION OF A NOVEL TRANSPOSABLE ELEMENT IN THE TYROSINASE GENE IS RESPONSIBLE FOR AN ALBINO MUTATION IN THE MEDAKA FISH, ORYZIAS-LATIPES

In the medaka fish (Oryzias latipes) many mutants for body color have been isolated. A typical example is the recessive oculocutaneous albin o mutant i, which has amelanotic skin and red-colored eyes with no tyr osinase activity. To cast light on the molecular basis of the albino m echanism, we performed Southern blot analysis of genomic DNA from the mutant with an authentic tyrosinase gene probe; the results demonstrat e that an extra 1.9 kb fragment is present inside the first exon. The insertion is responsible for the oculocutaneous albinism. About 80 cop ies of this fragment are present in the genomes of albino-i and wild-t ype fish; these repeated sequences are here designated Tol1 elements a nd the particular element found in the tyrosinase gene of albino-i is denoted Tol1-tyr. The nucleotide sequence of Tol1-tyr shows that the f ragment (i) carries terminal inverted repeats of 14 bp, and (ii) is fl anked by duplicated 8 bp segments of the host chromosome. These are pr operties of DNA-mediated transposable elements. Comparison of the nucl eotide sequence of Tol1-tyr with other sequences in DNA databases, wit h special attention to sequences of transposable elements known to dat e, did not reveal any similarity. Thus, Tol1 constitutes a hitherto un known family of DNA transposable elements.