Evolutionary origin of the Asteraceae capitulum: Insights from Calyceraceae

 Premise of the study: Phylogenies based on molecular data are revealing that generalizations about complex morphological structures often obscure variation and developmental patterns important for understanding the evolution of forms, as is the case for infl orescence morphology within the well-supported MGCA clade (Menyanthaceae + Goodeniaceae + Calyceraceae + Asteraceae). While the basal families share a basic thyrsic/thyrsoid structure of their infl orescences, Asteraceae possesses a capitulum that is widely interpreted as a racemose, condensed infl orescence. Elucidating the poorly known infl orescence structure of Calyceraceae, sister to Asteraceae, should help clarify how the Asteraceae capitulum evolved from thyrsic/thyrsoid infl orescences.  Methods: The early development and structure of the infl orescence of eight species (fi ve genera) of Calyceraceae were studied by SEM, and patterns of evolutionary change were interpreted via phylogenetic character mapping.  Key results: The basic infl orescence structure of Calyceraceae is a cephalioid (a very condensed botryoid/thyrsoid). Optimization of infl orescence characters on a DNA sequence-derived tree suggests that the Asteraceae capitulum derives from a simple cephalioid through two morphological changes: loss of the terminal fl ower and suppression of the cymose branching pattern in the peripheral branches.  Conclusions: Widely understood as a condensed raceme, the Asteraceae capitulum is the evolutionary result of a very reduced, condensed thyrsoid. Starting from that point, evolution worked separately only on the racemose developmental control/pattern within Asteraceae and mainly on the cymose developmental control/pattern within Calyceraceae, producing head-like infl orescences in both groups but with very different diversifi cation potential. We also discuss possible remnants of the ancestral cephalioid structure in some Asteraceae.