Oculocutaneous albinism in the i(6) mutant of the medaka fish is associated with a deletion in the tyrosinase gene

Three mutant alleles (i(1), i(4), and i(5)) 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 ide ntified another allele causing a complete albino phenotype in homozygous ca rriers and named it i(6). Sequence comparison between the tyrosinase gene f or the i(6) allele (Tyr-i(6)) and the wild-type gene previously obtained (T yr-i(+)) revealed three deletions of 8, 44, and 245 bp, The first two delet ions reside in an intron and are differences in the number of tandem tetran ucleotide repeats that are polymorphic even among wild-type genes, and, thu s, not likely to be responsible for the i(6) albino phenotype, The largest deletion spans over the last 180 bp of the second intron and the first 65 b p of the third exon, Because of this deletion, the Tyr-i(6) gene lacks the branch point sequence and the acceptor site for the second intron, both bei ng considered to be necessary for normal RNA splicing. Therefore, the 245-b p deletion is likely to be responsible for the albino phenotype, With a mut ant gene of this type, unlike ones bearing transposable element insertions, the possibility of reversion mutations to the wild-type would be negligibl e. Therefore, fish having the i(6)/i(6) genotype should serve as superior r ecipients for the tyrosinase gene in rescue experiments.