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Abstract

The striking beauty and diversity of colour patterns fascinate not only biologists. Despite their importance as targets for both, natural and sexual selection, little is known about the development and evolution of colour patterns in vertebrates. The pattern of the adult zebrafish is composed of a series of blue and golden horizontal stripes covering the body and the anal- as well as tail fin. Closely related Danio species display very different patterns. Pigment cells– melanophores, iridophores and xanthophores are distributed in three superimposed monolayers under the skin. Whereas in zebrafish melanophores are only present in the dark stripes, both xanthophores and iridophores are spread over the entire body, albeit in different shapes and densities in light and dark stripes. The pigment cells originate from neural crest-derived stem cells associated with the dorsal root ganglia of the peripheral nervous system. Clonal analysis revealed that, surprisingly, progenitors of pigment cells remain multipotent and share the lineage with neurons and glia of the peripheral nervous system well into metamorphosis. They remain plastic and their growth rate is highly variable. Iridophores and xanthophores are capable of proliferating and spreading in the skin as differentiated pigmented cells, whereas pigmented melanophores hardly ever divide or migrate. Collective migration and homotypic competition lead to an even spacing of pigment cells in the skin. This mode of colouring the skin is probably common to fish, whereas different patterns emerge by species specific cell interactions among the different pigment cell types. These interactions are mediated by channels involved in direct cell contact between the pigment cells, as well as unknown cues provided by the tissue environment. Colour patterns provide a novel paradigm for pattern formation by contact-dependent cell-cell interactions. It is a nascent field holding great promise for evolutionary developmental biology.