Inflorescence architecture and development in the megadiverse Eugenia (Myrteae, Myrtaceae): an evolutionary perspective

Abstract This work explores inflorescence development patterns in Eugenia (Myrtaceae), one of the largest genera of woody plants with over 1,200 species. The central hypothesis is that all variation in inflorescence arrangements in Eugenia is homologous. This is addressed as specific aims: 1) to identify the main inflorescence developmental patterns, 2) to test the homology of inflorescence architectures using a time-calibrated phylogeny, and 3) to assess and update traditional terminology. Taxonomically representative samples of Eugenia inflorescences were analysed to provide an evolutionary and comparative framework. Comparative ontogenetic analysis incorporated 24 taxa while mature material analysis encompassed 66 species included in a dated phylogeny. Racemose branching is the ancestral condition of Eugenia. Seven development inflorescence patterns were recognized and combined into five patterns (auxotelic+auxotelic cataphylls, fasciculiform+glomeruliform, umbelliform, racemiform, and thyrsoid) to focus on evolutionary traits. The auxotelic pattern is the ancestral condition of Eugenia recovered in the early nodes and in the backbone with highest probabilities of the reconstructed phylogeny, whereas auxotelic and fasciculiform were the most recurrent patterns in Eugenia sect. Umbellatae. The relative length and number of main axes that arise from an axil are shown to be systematically relevant, while racemiform pattern is exclusive to Eugenia sect. Racemosae. Flexible inflorescence arrangements found in Eugenia reflect a continuous spectrum of morphologies derived from a common ancestral scaffold. The combination of intense floral display in the fasciculiform pattern and flexibility may represent a key-innovation in E. sect. Umbellatae. This work highlights the evolutionary significance of inflorescences and improves existing terminology.