Convergent regulatory evolution of flower-like inflorescences

CYCLOIDEA-like TCP transcription factors are known to participate in the control of organ symmetry during flower development, such that expression differences dictate radial versus bilateral phenotypes. Recently, this aspect of CYC-like gene regulation has been implicated at a higher level in the reproductive hierarchy, in the development of different flower types within the flower-like inflorescences of the sunflower family, Asteraceae. We have investigated whether the control of floral heteromorphy in such "false flowers", pseudanthia, might be a more general phenomenon among flowering plants. We have specifically studied inflorescences of the asterid eudicots Hydrangea and Viburnum, which are extremely similar in overt appearance despite approximately 110 million years of independent evolution. Both species bear corymbose panicles with tiny flowers to the interior and enlarged flowers at distinct peripheral positions. In Viburnum, the peripheral flowers have elaborated petals, whereas in Hydrangea, the showy organs of such outer flowers are sepals. In both cases, as in Asteraceae, the expression of CYC-like genes during inflorescence development follows a pattern of high-to-low expression in peripheral flowers versus central flowers. This common pattern, executed at different organ categories among these distantly related taxa, suggests that flower-like inflorescences in angiosperms can have unifying tool-kit genes in their causal backgrounds, just as with the symmetry of flowers themselves.