Multiple origins of lipid-based structural colors contribute to a gradient of fruit colors in Viburnum (Adoxaceae)

Structural color is poorly known in plants relative to animals. In fruits, only a handful of cases have been described, including in Viburnum tinus where the blue color results from a disordered multilayered reflector made of lipid droplets. Here, we examine the broader evolutionary context of fruit structural color across the genus Viburnum. We obtained fresh and herbarium fruit material from 30 Viburnum species spanning the phylogeny and used transmission electron microscopy, optical simulations, and ancestral state reconstruction to (1) identify the presence/absence of photonic structures in each species, (2) understand the mechanism producing structural color in newly identified species, (3) relate the development of cell wall structure to reflectance in V. dentatum, and (4) describe the evolution of cell-wall architecture across Viburnum. We identify at least two (possibly three) origins of blue fruit color in Viburnum, both of which produce large photonic structures made of lipid ...