Floral developmental evidence for the systematic relationships of Tropaeolum (Tropaeolaceae)

The floral ontogeny of three species of Tropaeolum was studied using scanni ng electron microscopy to find morphological evidence for discussing the sy stematic position of the family. The initiation of the androecium is highly unusual: there are always eight stamens which arise (1) either in a spiral sequence starting with the stamen opposite sepal four, running in a direct ion opposite to the sequence of the sepals, and with reversals in the direc tion of the spiral, or (2) as a sequence of paired and unpaired stamens. Th e floral symmetry changes twice during the development of the flower, from polysymmetrical at sepal and petal initiation, through oblique monosymmetry at stamen initiation, and ending with median monosymmetry in later develop mental stages. The occurrence of median monosymmetry is a late-developmenta l event and is caused by the initiation of a hypanthial spur, and the unequ al growth of the petals and styles. The origin for the unusual sequence of stamen initiation reflects a trend affecting the whole flower which is link ed with the changing patterns of floral symmetry. Octandry is enhanced by m ultiple causes, such as the loss of two stamens in an originally diplostemo nous androecium and the regulating pressure of the gynoecium. The change in symmetry during ontogeny is significant for discussing the systematic posi tion of Tropaeolaceae in comparison with the glucosinolate-producing taxa a nd the Sapindales. The combination of an androecium with eight stamens and oblique monosymmetry is either a single event in evolution and links Tropae olum with the Sapindales, or it has evolved at least twice, once in the Sap indales, and once in a clade comprising Tropaeolaceae, Akaniaceae and Brets chneideraceae. Morphological data support a sister group relationship of th e three latter families, which is in line with macromolecular studies. (C) 2001 Annals of Botany Company.