Regulation of pigment cell specification during embryonic development in the zebrafish

Pigment cells of the zebrafish, Danio rerio, offer an exceptionally tractable system for studying the genetic and cellular bases of cell fate decisions. The pigment cells of zebrafish include the black melanophores, the iridescent iridophores and the yellow xanthophores. Each of these cell types develops from the neural crest, a pluripotent cell population found in vertebrates and some basal chordates. The central aim of this dissertation is to understand the mechanisms used to specify distinct pigment cell types from a homogenous neural crest cell population. Using genetic markers specific to each pigment cell type, I determined that xanthophores become distinct from a partially restricted melanophore/iridophore bi-potent precursor at 24 hpf. This bi-potent precursor resolves into distinct melanophores and iridophores between 28 and 42 hpf. I labeled and tracked these mitfa+ precursor cells with a photoconvertible protein, EosFP to confirm that both melanophores and iridophores can develop from the bi-potent cell. mitfa is a basic helix-loop-helix transcription factor that activates multiple downstream pathways necessary for melanophore development and exerts a repressive effect on iridophore development. Foxd3, a forkhead transcription factor, helps resolve the bi-potent pigment precursor by repressing mitfa expression and promoting iridophore development. Using cell culture and in vivo assays, I demonstrate that Foxd3 represses mitfa by directly binding to its proximal promoter region. Taken together, our data reveal a Foxd3/mitfa transcriptional switch that governs whether a bi-potent neural crest precursor will attain either an iridophore or melanophore fate.