Antirrhinum and Asteridae--evolutionary changes of floral symmetry

Lamiales s.l., a prominent clade in the Asteridae, commonly have pentamerous monosymmetric flowers with the upper (odd) stamen reduced or lacking. Of the five largest families of the Lamiales s.l. (Gesneriaceae, Scrophulariaceae, Bignoniaceae, Acanthaceae, and Verbenaceae/Lamiaceae), perhaps the most phylogenetically basal, the Gesneriaceae, is the only one with odd staminodes or stamens occurring in all genera. In addition, Gesneriaceae have relatively large odd staminodes often with a differentiation in filament and reduced anther. They also have the largest proportion of genera (approx. 8%) with more or less polysymmetric flowers with five fertile stamens. In the more advanced families of the Lamiales s.l. the pattern varies. In Bignoniaceae an odd staminode is also commonly present. The traditional Scrophulariaceae is the most diverse family with some tribes constantly having an odd staminode (e.g. Antirrhineae) and others missing it (e.g. Manuleae, Pedicularieae), reflecting its probable polyphyly. In Acanthaceae, and still more in Verbenaceae/Lamiaceae with their extremely monosymmetric flowers an odd staminode is more often missing than present. From the systematic distribution of the different floral forms it is most likely that monosymmetric flowers were already present at the base of the Lamiales s.l. but 'reversal' to polysymmetry was still easy in the basal groups with only weak expression of monosymmetry. The evolution of more pronounced monosymmetry proceeded in different lineages and loss of the odd stamen occurred in various clades. The developmentally most intriguing groups are those with loss of the perianth and reduction of floral organ numbers to two or one, Callitriche being the most extreme genus.