Tyrp1 is the mendelian determinant of the Axolotl (Ambystoma mexicanum) copper mutant

Several dozen Mendelian mutants have been discovered in axolotl (Ambystoma mexicanum) populations, including several that affect pigmentation. Four recessive mutants have been described in the scientific literature and genes for three of these have been identified. Here we describe and genetically dissect copper, a mutant with an albino-like phenotype known only from the pet trade. We performed a cross segregating copper and wildtype color phenotypes and used bulked segregant RNA-Seq to identify a region on chromosome 6 that was enriched for single-nucleotide polymorphisms (SNPs) between the color phenotypes. This region included Tyrosinase-like Protein 1 (Tyrp1), a melanin synthesis protein that when mutated, is associated with lighter than black melanin coloration in animal models and oculocutaneous albinism in humans. Inspection of RNA-Seq reads identified a single nucleotide deletion that is predicted to change the coding frame, introduce a premature stop codon in exon 6 and yield a truncated Tyrp1 protein in copper individuals. Using CRISPR-Cas9 editing, we show that wildtype Tyrp1 crispants exhibit copper pigmentation, thus confirming Tyrp1 as the copper locus. Our results suggest that commercial and hobbyist axolotl populations may harbor useful mutants for biological research.