Abstract
Background and aims: Sexual dimorphism, at both the flower and plant level, is widespread in the palm family (Arecaceae), in contrast to the situation in angiosperms as a whole. The tribe Chamaedoreeae is of special interest for studies of the evolution of sexual expression since dioecy appears to have evolved independently twice in this group from a monoecious ancestor. In order to understand the underlying evolutionary pathways, it is important to obtain detailed information on flower structure and development in each of the main clades.Methods: Dissection and light and scanning electron microscopy were performed on developing flowers of Gaussia attenuata, a neotropical species belonging to one of the three monoecious genera of the tribe.Key results: Like species of the other monoecious genera of the Chamaedoreeae (namely Hyophorbe and Synechanthus), G. attenuata produces a bisexual flower cluster known as an acervulus, consisting of a row of male flowers with a basal female flower. Whereas the sterile androecium of female flowers terminated its development at an early stage of floral ontogeny, the pistillode of male flowers was large in size but with no recognizable ovule, developing for a longer period of time. Conspicuous nectary differentiation in the pistillode suggested a possible role in pollinator attraction.Conclusions: Gaussia attenuata displays a number of floral characters that are likely to be ancestral to the tribe, notably the acervulus flower cluster, which is conserved in the other monoecious genera and also (albeit in a unisexual male form) in the dioecious genera (Wendlandiella and a few species of Chamaedorea). Comparison with earlier data from other genera suggests that large nectariferous pistillodes and early arrest in staminode development might also be regarded as ancestral characters in this tribe.