Abstract
Animals have evolved a fascinating diversity in their color patterns, which serves as an essential component of their survival strategies. Color pattern formation in zebrafish (Danio rerio) is a great model system to study general pattern formation involving different cell types. The conspicuous pattern of alternating blue and yellow/silver stripes displayed by adult zebrafish is composed of three kinds of pigment cells - black melanophores, yellow xanthophores, and silvery iridophores. It is known that almost all vertebrate pigment cells originate during early embryogenesis from the neural crest, a vertebrate specific transient population of multi-potent migratory cells giving rise to the peripheral nervous system, some craniofacial structures, pigment cells and others. Mutants specifically lacking one class of pigments cells show that color pattern formation in zebrafish requires interactions among all three kinds of pigment cells. Uncovering the origin and behaviors of these individual cell types is of great interest for researchers studying the mechanism of color pattern formation in zebrafish. In particular, very little is known about the origin, role and behavior of xanthophores during the process. By employing genetic tools, such as the Cre-LoxP and the Gal4-UAS system, in combination with high-resolution live imaging techniques, I examined the cellular identity of the progenitor cells during metamorphic stages of development. The analysis of pigment cell dynamics during stripe pattern formation in vivo had led to the discovery of several novelties about xanthophores. Firstly, with the help of genetic cell ablation and long term imaging of clonal xanthophores, I confirmed the notion of a dual cellular origin of adult xanthophores: while most adult xanthophores originate from existing larval xanthophores, others come from multi-potent postembryonic progenitors. Secondly, xanthophores are the first cells to cover the entire skin during metamorphosis, and iridophores and melanophores induce local reorganizations of xanthophores. Xanthophores disperse in response to melanophores in the dark stripe regions, whereas they compact their shapes and densely cover the metamorphic iridophores in the light stripe regions. These local cellular reorganizations of xanthophores, repulsion by melanophores, and attraction by iridophores, lead to a sharpening and coloration of the striped pattern. Lastly, I explored the in vivo mechanisms of these xanthophore behaviors, and found that local, heterotypic interactions with dense iridophores regulated xanthophore cell shape transition and density in the light stripe. Genetic analysis revealed a cell-autonomous requirement of gap junctions composed of Cx41.8 and Cx39.4 in xanthophores for an iridophore-dependent cell shape transition and increased in density. Initial melanophore-xanthophore interactions are independent of these gap junctions; however, they are subsequently required to induce the stellate shapes of xanthophores in the dark stripes. In conclusion, the color pattern formation in zebrafish involves a novel mechanism of patterning, dependent on cell shape transitions of xanthophores and iridophores. These shape transitions are dependent on local cell-cell interactions, and are mediated by gap junctions. This analysis of stripe pattern formation gives us an insight into the origins and interactions of diverse cell-types in a genetically tractable vertebrate system.
