Evolution of flower shape in Veroniceae (Scrophulariaceae)

Evolution of flower form was studied in the tribe Veroniceae (Scrophulariaceae) by quantitatively comparing developmental patterns of nine species representing all major clades within the tribe and of two outgroup species belonging to Digitaleae and Verbesceae. For the first time, a method was devised to extract systematic characters from quantitative developmental data; developmental and mature morphology characters were used to estimate the phylogeny of the Veroniceae. Measurements of all flower organ types from 50 buds of all developmental stages were taken for each species, and compared using bivariate plots of organ size against gynoecium size as an approximation of age. An intraspecific comparison of morphologically distinct strains of one species showed very similar growth patterns; higher standard deviations during earlier stages indicated variation in timing of growth spurts that are compensated by later growth. Comparison of two congeneric species revealed that different mechanisms (delayed onset, decreased rate, early offset of growth) result in the miniaturization of the flower of one species. At the generic level, similarities of early developmental growth patterns and organ initiation sequences indicated groups of genera and tribal membership. Overall growth patterns were also compared using summary trajectories produced with multigroup principal components analysis (M-PCA). Variation due to size increase fell out along M-PC 1, while corolla tube proportions controlled component scores along M-PC 2; M-PC 3 scores indicated shape changes that were influenced by all variables, and showed the greatest congruence with probable phylogeny. To extract character states from the different growth patterns of each organ type for phylogenetic analysis, curve intercepts at four developmental stages were grouped with a clustering program. Developmental and mature morphological data resulted in similar cladograms which generally concurred with analyses of data from the literature. The distributions of flower shapes and developmental patterns on the cladograms indicate that the ancestor of the Veroniceae had comparatively short corolla tube and lobes; reversals to the long tubes occurred four times, and to large corolla lobes twice within the tribe.