Patterns of Ontogenetic Evolution in Perianth Diversification of Besseya (Scrophulariaceae)

Abstract

Species of Besseya have perianth diversity centered largely in meristic variation, extreme corolla diminution, and corolla tube loss. Flowers of Besseya differ from those of their closest relative Synthyris in having a bilabiate corolla. The avenues of ontogenetic evolution that were important in creating these aspects of perianth diversity were explored by optimizing developmental transformations on cladograms. Three different evolutionary transformations were important in the diversification of calyx ontogenies in Besseya, including the derived reciprocal substitution of ontogenetic states in B. bullii, labile abaxial lobe expression in B. wyomingensis, and a heterotopic novel substitution in B. oblongifolia. The bilabiate corolla of Besseya arose via fractionation and localization of the zonal growth expressed in the more plesiomorphic Synthyris. Besseya plantaginea, B. ritteriana, and B. oblongifolia form a group characterized by diminished expression of the anterior corolla lobe. Four alternative scenarios equally explain the ontogenetic transformations underlying extreme corolla diminution and corolla tube loss. Corolla tube loss has been considered rare or impossible among angiosperms, but some alternative scenarios in Besseya demonstrate how this may have occurred via fractionation of zonal growth. Corolla tube loss may have arisen only indirectly from the sympetalous ancestors of Besseya following extreme corolla diminution and the reestablishment of a “full-size” corolla which had a novel developmental pattern that did not include the distribution of zonal growth present in ancestors.