Convergent evolution of inflorescence-like units and syncephalia in Asteraceae

Capitula in Asteraceae are arranged in flowering shoot systems usually interpreted as compound inflorescences or synflorescences. If they form dense conglomerates, they are termed syncephalia likewise interpreted as inflorescences. However, ontogenetic evidence indicates that inflorescence-like patterns can also arise from flowering shoot systems and complex floral units. We here disentangle confusion surrounding the different organizational levels and explain convergent evolution in inflorescence-like units and syncephalia.

We examined meristem organization, developmental trajectories, and flowering sequences in the capitulum arrangement of 22 species representing seven tribes. Eight species were subject to further developmental investigations. The ontogenetic concept of inflorescence diversity was used as reference framework.

Matricaria dioscoidea, used as reference species, produces solitary capitula in branched flowering shoot systems. Three genera arrange capitula in inflorescence-like patterns proved to be flowering shoot systems (Liatris) or inflorescences (Ligularia, Achillea). Flowering sequences turned out to be a key character to distinguish flowering shoot systems from inflorescences. The remaining species had syncephalia with densely aggregated capitula arranged in inflorescences (9 sp.) and complex floral units (5 sp.). For the latter, we introduce the term caput to distinguish them from head aggregates. Three syncephalia could not be clearly interpreted without additional developmental data. Aggregation can be combined with a reduction in flower number per capitulum and the formation of a common receptacle. Pseudanthia formation, i.e. the reiteration of flower-like patterns known from capitula, mainly occurs through extrafloral bracts acting as attractive elements.

Our findings demonstrate that both inflorescence-like units and syncephalia are convergent structures. Initiation and flowering sequences provide more reliable criteria for identifying the underlying morphology than branching patterns, which recur on different hierarchical levels. The formation of capita through repeated fractionation of a determinate meristem reveals a distinct evolutionary pathway beyond inflorescence aggregation driven by heterochrony.