The making of Delphinieae flowers through rewiring the interactions between duplicated and diversified floral organ identity and symmetry genes

Species of the tribe Delphinieae (Ranunculaceae) have long been the focus of morphological, ecological and evolutionary studies for their highly specialized, nearly zygomorphic spiral flowers with nested petal and sepal spurs and reduced petals. The mechanisms underlying the development and evolution of the Delphinieae flowers, however, remain unclear. Here, by conducting extensive phylogenetic, comparative transcriptomic, expression and functional studies, we clarify the evolutionary histories, expression patterns and functions of floral organ identity and symmetry genes in Delphinieae. We find that duplication and/or diversification of APETALA3-3 (AP3-3), AGAMOUS-LIKE6 (AGL6), CYCLOIDEA (CYC) and DIVARICATA (DIV) lineage genes were tightly associated with the origination of the Delphinieae flowers. Specifically, an AGL6-lineage member (such as the Delphinium ajacis AGL6-1a) represses sepal spur formation and petal development in the lateral and ventral parts of the flower while determining petal identity redundantly with AGL6-1b. Two CYC2-like genes, CYC2b and CYC2a, however, define the dorsal and lateral/ventral identities of the flower, respectively, and form complex regulatory links with AP3-3, AGL6-1a and DIV1. Duplication and diversification of floral symmetry genes, as well as co-option of the duplicated copies into the pre-existing floral regulatory network, therefore, have been the key to the origin of the Delphinieae flowers