PATTERNS OF ONTOGENIC EVOLUTION IN PERIANTH DIVERSIFICATION OF BESSEYA (SCROPHULARIACEAE)

Species of Besseya have perianth diversity centered largely in meristi c variation, extreme corolla diminution, and corolla tube loss. Flower s 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 ex plored by optimizing developmental transformations on cladograms. Thre e different evolutionary transformations were important in the diversi fication of calyx ontogenies in Besseya, including the derived recipro cal substitution of ontogenetic states in B. bullii, labile abaxial lo be expression in B. wyomingensis, and a heterotopic novel substitution in B. oblongifolia. The bilabiate corolla of Besseya arose via fracti onation and localization of the zonal growth expressed in the more ple siomorphic Synthyris. Besseya plantaginea, B. ritteriana, and B. oblon gifolia form a group characterized by diminished expression of the ant erior corolla lobe. Four alternative scenarios equally explain the ont ogenetic transformations underlying extreme corolla diminution and cor olla tube loss. Corolla tube loss has been considered rare or impossib le among angiosperms, but some alternative scenarios in Besseya demons trate how this may have occurred via fractionation of zonal growth. Co rolla tube loss may have arisen only indirectly from the sympetalous a ncestors of Besseya following extreme corolla diminution and the reest ablishment of a ''full-size'' corolla which had a novel developmental pattern that did not include the distribution of zonal growth present in ancestors.