The Formation of Biogenic Guanine Crystals Closely Resembles Melanosome Morphogenesis

Animals precisely control the morphology and assembly of highly reflective guanine crystals to produce diverse optical phenomena used in coloration and vision. However, little is known about how organisms regulate crystallization to produce optically useful crystal morphologies which express highly reflective crystal faces. Guanine crystals form inside iridosome vesicles within specialized chromatophore cells called iridophores. By following the formation of iridosomes in developing scallop eyes we show that, despite their different physical properties, the morphogenesis of iridosomes bears a striking resemblance to melanosome morphogenesis in vertebrates. A key finding is that pre-assembled intravesicular fibrillar sheets template crystal nucleation, growth, and orientation, dictating the formation of optically functional, but thermodynamically disfavored plate-like habits. The common control mechanisms for melanin and guanine formation inspire new approaches for manipulating the morphologies and properties of molecular materials. ### Competing Interest Statement The authors have declared no competing interest.