Genetic and experimental studies on a new pigment mutant in Xenopus laevis

White lethal (wl) is a recessive mutation affecting the differentiation of the three types of chromatophores in Xenopus laevis and eventually leading to the death of the mutants around stage 50. Melanophores appear at st. 33 but differentiate abnormally, remaining pale grey, and do not proliferate after st. 41. The rare xanthophores present contain only a few differentiated pterinosomes, and the iridophores consist of noniridescent white dots. When the albino gene (ap) is combined with wl, melanophores do not differentiate. Reciprocal heterotopic and orthotopic trunk neural crest grafts have shown that the defect is intrinsic to the neural crest cells but is not due, in the case of melanophores, to a tyrosinase deficiency as revealed by the dopa reaction. The mode of action of the gene, the abnormal pattern, and lethality are discussed.