Flower morphology and relationships of Schefflera subintegra (Araliaceae, Apiales): an evolutionary step towards extreme floral polymery

摘要

Gross morphology and the development of flowers in Schefflera subintegra (Araliaceae) are examined. The floral groundplan of this species is found to be very similar to that of Tupidanthus calyptratus representing a case of most extreme floral polymery within Araliaceae. Schefflera subintegra differs from T. calyptratus with respect to a lower floral merism (19-43 versus 60-172 stamens and 15-33 versus 60-138 carpels respectively) and by transformation from polysymmetry to disymmetry of flower in the course of its development. Close relationships between S. subintegra, T. calyptratus, and Schefflera hemiepiphytica have been confirmed by phylogenetic analysis based on nuclear ribosomal internal transcribed spacer sequences. These species form a subclade within the Asian Schefflera clade, with T. calyptratus as a sister taxon to two other species. Apart from more or less pronounced floral polymery, the species of this subclade share calyx and corolla without any traits of individual sepals and petals, and also a massive calyptra. As these data suggest, the extremely polymerous flowers of Tupidanthus apparently evolved in two steps: (1) the saltational multiplication of floral elements together with a loss of individuality of sepals in the calyx and petals in the corolla and (2) further polymerization of androecium and gynoecium. Mutation(s) in CLAVATA-like gene(s) are suggested as a possible mechanism of the saltation event