GYNOECIAL STRUCTURE OF VITALES AND IMPLICATIONS FOR THE EVOLUTION OF PLACENTATION IN THE ROSIDS

Premise of research.Evolutionary relationships of the Vitales (the economically important Vitaceae and its sister family Leeaceae) within the rosids have been difficult to resolve. Gynoecial structure, especially the placentation type, of the two families has been variously interpreted. A survey of gynoecial structure is undertaken within these two families, and they are interpreted in light of phylogenetic comparison of placentation types among all angiosperms.Methodology.Gynoecial structure and architecture in 21 species were studied with light and scanning electron microscopy. Ancestral character reconstruction of gynoecia with axile, parietal, basal, apical, free-central, marginal, or laminar placentae across 640 taxa representing all 58 orders of angiosperms was inferred using maximum likelihood to help interpret the evolution of the gynoecium in the Vitales.Pivotal results.The syncarpous ovary is bicarpellate in Vitaceae and tricarpellate in Leeaceae; in both the carpels are congenitally fused to form the synascidiate zone. Placentae are located basally on the septum with generally two ovules per carpel. Distally, septa are incomplete, resulting in the ovary being incompletely bilocular in Vitaceae and incompletely trilocular in Leeaceae. Further morphological variations are a result of differential growth of the septa. In some species of Cyphostemma septa are further reduced, and the ovary is clearly unilocular. Ancestral character reconstruction using maximum likelihood across 640 angiosperm taxa infers marginal placentation to be the ancestral condition, while that of the Superrosidae is axile, with basal placentation inferred to be derived within the clade.Conclusions.The apically incompletely septate gynoecia and basally septate gynoecia of Vitaceae and Leeaceae are also found in 31 other angiosperm families. These results underscore the labile nature of placentation despite its long use as a character of taxonomic significance. The study provides a basis for inferring the directionality of placentation evolution in the context of a wider investigation of rosid relationships.