Floral Developmental Evidence for the Systematic Relationships of Tropaeolum(Tropaeolaceae)

The floral ontogeny of three species of Tropaeolum was studiedusing scanning electron microscopy to find morphological evidencefor discussing the systematic position of the family. The initiationof the androecium is highly unusual: there are always eightstamens which arise (1) either in a spiral sequence startingwith the stamen opposite sepal four, running in a directionopposite to the sequence of the sepals, and with reversals inthe direction of the spiral, or (2) as a sequence of pairedand unpaired stamens. The floral symmetry changes twice duringthe development of the flower, from polysymmetrical at sepaland petal initiation, through oblique monosymmetry at stameninitiation, and ending with median monosymmetry in later developmentalstages. The occurrence of median monosymmetry is a late-developmentalevent and is caused by the initiation of a hypanthial spur,and the unequal growth of the petals and styles. The originfor the unusual sequence of stamen initiation reflects a trendaffecting the whole flower which is linked with the changingpatterns of floral symmetry. Octandry is enhanced by multiplecauses, such as the loss of two stamens in an originally diplostemonousandroecium and the regulating pressure of the gynoecium. Thechange in symmetry during ontogeny is significant for discussingthe systematic position of Tropaeolaceae in comparison withthe glucosinolate-producing taxa and the Sapindales. The combinationof an androecium with eight stamens and oblique monosymmetryis either a single event in evolution and links Tropaeolum withthe Sapindales, or it has evolved at least twice, once in theSapindales, and once in a clade comprising Tropaeolaceae, Akaniaceaeand Bretschneideraceae. Morphological data support a sistergroup relationship of the three latter families, which is inline with macromolecular studies. Copyright 2001 Annals of BotanyCompany Tropaeolum, Tropaeolaceae, Glucosinolate clade, Sapindales, oblique monosymmetry, androecium, octandry, floral development, phylogeny