In situdifferentiation of iridophore crystallotypes underlies zebrafish stripe patterning

Skin color patterns are ubiquitous in nature, evolve rapidly, and impact social behavior¹, predator avoidance², and protection from ultraviolet irradiation³. A leading model system for vertebrate skin patterning is the zebrafish^4-7; its alternating blue stripes and yellow interstripes depend on guanine crystal-containing cells called iridophores that reflect light. It was suggested that the zebrafish’s alternating color pattern arises from a single type of iridophore migrating differentially to stripes and interstripes^7-9. When we tracked iridophores, however, we found they did not migrate between stripes and interstripes but instead differentiated and proliferated in place based on their micro-environment. RNA seq analysis further revealed stripe and interstripe iridophores had different transcriptomic states, while cryogenic scanning electron microscopy and micro-X-ray diffraction showed they had different guanine crystal organizations and responsiveness to norepinephrine, all indicating that stripe and interstripe iridophores are different cell types. Based on these results, we present a new model of skin patterning in zebrafish in which distinct iridophore crystallotypes containing specialized, physiologically responsive, subcellular organelles arise in stripe and interstripe zones byin situdifferentiation. In this model, pattern phenotype depends not only on interactions among pigment cells that affect their arrangements, but also on factors that specify subcellular organization and physiological responsiveness of specialized organelles.