The Tomita collection of medaka pigmentation mutants as a resource for understanding neural crest cell development

All body pigment cells in vertebrates are derived from the neural crest. In fish the neural crest can generate up to six different types of pigment cells, as well as various non-pigmented derivatives. In mouse and zebrafish, extensive collections of pigmentation mutants have enabled dissection of many aspects of pigment cell development, including fate specification, survival, proliferation and differentiation. A collection of spontaneous mutations collected from wild medaka (Oryzias latipes) populations and maintained at Nagoya University includes more than 40 pigmentation mutations. The descriptions of their adult phenotypes have been previously published by Tomita and colleagues (summarised in Medaka (Killifish) Biology and Strains, 1975), but the embryonic phenotypes have not been systematically described. Here we examine these embryonic phenotypes, paying particular attention to the likely defect in pigment cell development in each, and comparing the spectrum of defects to those in the zebrafish and mouse collections. Many phenotypes parallel those of identified zebrafish mutants, although pigment cell death phenotypes are largely absent, presumably due to the different selective pressures under which the mutants were isolated. We have identified mutant phenotypes that may represent the Mitf/Kit pathway of melanophore specification and survival. We use in situ hybridisation with available markers to confirm a key prediction of this hypothesis. We also highlight a set of novel phenotypes not seen in the zebrafish collection. These mutants will be a valuable resource for pigment cell and neural crest studies and will strongly complement the mutant collections in other vertebrates.