Are capitula inflorescences? A reassessment based on flower-like meristem identity and ray flower development

The capitulum of Asteraceae has traditionally been interpreted as a condensed raceme. However, morphological studies challenge this view, indicating that the capitulum does not arise from an inflorescence meristem, but from a determinate floral unit meristem. Terminology, combined with conflicting evidence from developmental genetics, has hindered the formulation of a coherent evolutionary scenario for the origin of the capitulum. In this paper, we aim to refine the floral unit concept and critically re-examine the hypothesis that ray flowers in Asteraceae represent remnants of ancestral thyrsoid branching.

Approaching from evolutionary-developmental morphology, we performed a detailed analysis of capitulum development in 20 highly diverse Asteraceae species, encompassing three subfamilies and 11 tribes, using scanning electron microscopy. We focused on early capitulum development, ray flower formation and changes in meristem geometry.

We show that the capitulum meristem is determinate and exhibits developmental features characteristic of flowers, thereby fulfilling the definition of a floral unit meristem. Continuous meristem expansion changes geometrical conditions and triggers spontaneous fractionation of flower meristems. The early developmental delay of ray flowers and the rare occurrence of bidirectional fractionation in the capitulum meristem can be explained morphologically by local expansion dynamics and mechanical pressure.

Our findings support the hypothesis that the capitulum meristem is distinct from an inflorescence meristem and instead recapitulates developmental properties of a flower meristem at a higher level of organization. This challenges the phylogenetic view that the capitulum evolved through gradual transformation of an ancestral thyrse. Instead, we propose that a single developmental shift (from an indeterminate reproductive meristem to a determinate floral unit meristem) was sufficient to give rise to the capitulum. The early determinacy of the meristem and the insertion of an additional step of fractionation are best explained by heterochronic changes, such as ontogenetic abbreviation and prolongation.