Abstract
Premise of research. The sexuality of flowers is an important reproductive character in angiosperms. An insight into the evolutionary events that led to the appearance of monoecious and dioecious species can be gained by comparing closely related groups with contrasting characters. For this study, we focused on the tribe Chamaedoreeae, within which dioecy appears to have evolved twice from a monoecious ancestor.Methodology. To improve our knowledge of flower structure and ontogeny in this group, SEM and anatomical sectioning were performed on inflorescences and flowers of the dioecious species Chamaedorea tepejilote and the monoecious species Hyophorbe lagenicaulis at different developmental stages.Pivotal results. Our data highlighted that the higher degree of spatial sexual separation seen in the dioecious C. tepejilote, compared to the monoecious H. lagenicaulis, is accompanied by a more accentuated dimorphism between male and female flowers. More specifically, in the case of C. tepejilote, the vestigial reproductive organs (staminodes of the female flower and pistillode of the male flower) are more rudimentary structures, in terms of their developmental differentiation, than their homologs in H. lagenicaulis.Conclusions. Our data suggest that the unisexual flowers already present in the monoecious ancestor of the Chamaedoreeae underwent further modifications either shortly before or since the appearance of dioecy in the genus Chamaedorea. These structural changes were presumably the result of genomic mutations causing earlier developmental arrest of the vestigial reproductive organs and are likely, in turn, to have conferred enhanced resource-allocation efficiency.