L. Hufford, PATTERNS OF ONTOGENIC EVOLUTION IN PERIANTH DIVERSIFICATION OF BESSEYA (SCROPHULARIACEAE), American journal of botany, 82(5), 1995, pp. 655-680
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.