Touchtone promotes survival of embryonic melanophores in zebrafish

An outstanding problem in the study of vertebrate development is the identification of the genes that direct neural crest precursor cells to adopt and maintain specific differentiated cell fates. In an effort to identify such genes, we have carried out a mutagenesis screen in zebrafish and isolated mutants that lack neural crest-derived melanophores. In this manuscript we describe the phenotype of one such mutant, touchtone(b722) (tct), and the map position of the gene it defines. Analysis of expression of dopachrome tautomerase (dct) and microphthalmia (mitfa) suggests that melanophore precursors are specified normally in homozygous tct mutants. However, differentiated melanophores are pale, small, and about half of them have disappeared by 48 h of development, apparently by cell death. We show that melanophores require Tct function cell autonomously. Signals from the receptor tyrosine kinase receptor C-kit are essential for survival of melanophores in zebrafish and mammals. However, differences in the phenotypes of tct and c-kit homozygous mutants, and an absence of interaction between c-kit and tct heterozygotes, suggest that Tct functions independently of the C-kit pathway. Other neural crest derivatives, including other pigment cell types, appear normal in tct mutants. Interestingly, tct mutant embryos undergo a temporary period of near complete paralyzis during the second day of development, although markers of axons of motor and sensory neurons look normal in this period. A fraction of tct(b722) mutants survive to adulthood, but mutant adults are small, indicating a role for Tct in post-larval growth. The tct gene maps to a small interval near a telomere of chromosome 18. Thus, we have identified a zebrafish gene that when mutated produces semi-viable offspring and that may serve as a model of human diseases that have both pigmentation and neurological symptoms.