ALBINISM DUE TO TRANSPOSABLE ELEMENT INSERTION IN FISH

The i locus of the medaka fish, Oryzias latipes, is responsible for ty rosinase expression, and several mutant alleles have been identified. The genotype i(1)/i(1) exhibits a complete albino phenotype, having pa le orange-red skin and red eyes. This mutant lacks in vivo tyrosinase activity. The genotype i(4)/i(4) on the other hand, shows a quasi-albi no phenotype with skin as bright as that of i(1)/i(1) but with red-win e-colored eyes. At the light microscope level, reduced pigmentation is observed both in the skin and eyes of this mutant. The tyrosinase gen es for the i(1) and the i(4) alleles were cloned and sequenced, and co mpared with that of the wild-type tyrosinase gene. The i(1) allele was found to contain a 1.9-kb transposable element in the Ist exon, and t he i(4) allele was found to contain a 4.7-kb transposable element in t he 5th exon. Both i(1) and i(4) are alleles that were found in a comme rcial breeding population. The insertion of a transposable element thu s appears to constitute a natural cause of mutations that cause albini sm in this organism.