Stepwise evolution of floral pigmentation predicted by biochemical pathway structure

Developmental pathways play a major role in influencing the distribution of naturally occurring phenotypes. For example, pathway structure and regulation could make some phenotypes inaccessible or restrict the routes through which phenotypes evolve. In this study, we examine floral anthocyanin pigments across the Solanaceae family and test whether patterns of phenotypic variation are consistent with predicted constraints based on the structure of the flavonoid biosynthetic pathway. We find that anthocyanin evolution occurs in a stepwise manner whereby transitions between the production of red mono hydroxylated pelargonidin pigments and blue trihydroxylated delphinidin pigments first passes through an intermediate step of producing purple dihydroxylated cyanidin pigments. Although the transitions between these three pigment types differ in frequency, we infer that these shifts are often reversible, suggesting that the functionality of the underlying biochemical pathway is generally conserved. Furthermore, our study finds that some pigment combinations are never observed, pointing to additional constraints on naturally occurring phenotypes. Overall, our findings provide insights into how the structure of an angiosperm-wide biochemical pathway has shaped macroevolutionary variation in floral pigmentation.