Oculocutaneous albinism in the i⁶ mutant of the medaka fish is associated with a deletion in the tyrosinase gene

Three mutant alleles (i¹, i⁴, and i⁵) of the tyrosinase gene in the i locus of the medaka fish Oryzias latipes have hitherto been described, all being associated with transposable element insertion. We have recently identified another allele causing a complete albino phenotype in homozygous carriers and named it i⁶. Sequence comparison between the tyrosinase gene for the i⁶ allele (Tyr-i⁶) and the wild-type gene previously obtained (Tyr-i⁺) revealed three deletions of 8, 44, and 245 bp. The first two deletions reside in an intron and are differences in the number of tandem tetranucleotide repeats that are polymorphic even among wild-type genes, and, thus, not likely to be responsible for the i⁶ albino phenotype. The largest deletion spans over the last 180 bp of the second intron and the first 65 bp of the third exon. Because of this deletion, the Tyr-i⁶ gene lacks the branch point sequence and the acceptor site for the second intron, both being considered to be necessary for normal RNA splicing. Therefore, the 245-bp deletion is likely to be responsible for the albino phenotype. With a mutant gene of this type, unlike ones bearing transposable element insertions, the possibility of reversion mutations to the wild-type would be negligible. Therefore, fish having the i⁶/i⁶ genotype should serve as superior recipients for the tyrosinase gene in rescue experiments.