The pigmentary system of developing axolotls. IV. An analysis of the axanthic phenotype.

The axanthic mutant in the Mexican axolotl (Ambystoma mexicanum) was analysed with respect to the differentiation of pigment cells. Transmission electron micrographs revealed the presence of melanophores and cells that are described as unpigmented xanthophores in axanthic skin. Iridophores apparently failed to differentiate in axanthic axolotls (a pattern similar to that observed in melanoid axolotls). Chromatographic analyses of skin extracts confirmed that there are no pteridines (xanthophore pigments) in axanthic skin, suggesting that the axanthic gene may affect pteridine biosynthesis at some point early in the biosynthetic pathway. Why iridophores fail to differentiate in these animals is not known, but this, too, may be related to an inability to synthesize pigments properly. Xanthophore and iridophore pigments both presumably derive from purine precursors. Finally, all axanthic animals were found to be infected by a virus. Electron microscopic results demonstrated the presence of numerous macrophages in the dermis of the skin, occupying positions typical of pigment cells. The virus was localized primarily in macrophages, but was also observed in pigment cells. The virus is, as yet, uncharacterized but is thought to contribute to the low survivability of axanthic adults.