Genetic analysis of melanophore development in zebrafish embryos

Vertebrate pigment cells are derived from neural crest, a tissue that also forms most of the peripheral nervous system and a variety of ectomesenchyma l cell types, formation of pigment cells from multipotential neural crest c ells involves a number of common developmental processes. Pigment cells mus t be specified; their migration, proliferation, and survival must be contro lled and they must differentiate to the final pigment cell type. We previou sly reported a large set of embryonic mutations that affect pigment cell de velopment from neural crest (R. N. Kelsh et al., 1996, Development 123, 369 -389). Based on distinctions in pigment cell appearance between mutants, we proposed hypotheses as to the process of pigment cell development affected by each mutation. Here we describe the cloning and expression of an early zebrafish melanoblast marker, dopachrome tautomerase. We used this marker t o test predictions about melanoblast number and pattern in mutant embryos, including embryos homozygous for mutations in the colourless, sparse, touch down, sunbleached, punkt, blurred, fade out, weiss, sandy, and albino genes . We showed that in homozygous mutants for all loci except colourless and s parse, melanoblast number and pattern are normal. colourless mutants have a pronounced decrease in melanoblast cell number from the earliest stages an d also show poor melanoblast differentiation and migration. Although sparse mutants show normal numbers of melanoblasts initially, their number is red uced later. Furthermore, their distribution indicates a defect in melanobla st dispersal. These observations permit us to refine our model of the genet ic control of melanophore development in zebrafish embryos. (C) 2000 Academ ic Press.