Why are orchid flowers so diverse?
Background: The nearly 30 000 species of orchids produce flowers of unprecedented diversity. However, whether specific genetic mechanisms contributed to this diversity is a neglected topic and remains speculative. We recently published a theory, the orchid code , maintaining that the identity of the different perianth organs is specified by the combinatorial interaction of four DEF-like MADS-box genes with other floral homeotic genes. Scope: Here the developmental and evolutionary implications of our theory are explored. Specifically, it is shown that all frequent floral terata, including all peloric types, can be explained by monogenic gain- or-loss-of-function mutants, changing either expression of a DEF-like or CYC-like gene. Supposed dominance or recessiveness of mutant alleles is correlated with the frequency of terata in both cultivation and nature. Our findings suggest that changes in DEF- and CYC-like genes not only underlie terata but also the natural diversity of orchid speci
