Evolutionary consequences of extensive morph loss in tristylous decodon verticillatus (Lythraceae): a shift from tristyly to distyly?

The evolution of distyly from tristyly has occurred repeatedly, especially in the Lythraceae. However, the evolutionary forces involved remain unclear since species exhibiting transitional stages between tristyly and distyly have rarely been studied. The self‐compatible, wetland perennial Decodon verticillatus (Lythraceae) may provide this transitional variation since populations commonly lack style morphs, particularly the mid‐styled (M) morph. In dimorphic populations lacking the M morph, anthers positioned at the mid level in both the long‐ (L) and short‐styled (S) morphs have lost their target stigma, setting the stage for either evolutionary repositioning of mid‐level anthers to increase pollen export to L and S stigmas, or increased variability in mid‐level anther position resulting from relaxed selection. We examined these two hypotheses by comparing floral morphology in eight dimorphic and ten trimorphic populations from throughout the species’ range. We found no evidence that loss of the M morph has led to evolutionary modification of mid‐level stamens. While mid‐level stamens of the S morph were 11.0 ± 4.0% (mean ± 1 SE) longer than those of the L morph in dimorphic populations, divergence in stamen length between morphs occurred to the same extent (10.4 ± 2.0%) in trimorphic populations and cannot be attributed to the absence of the M morph. Analyses of variability using median ratio tests revealed no difference in the variability of mid‐level stamen length between dimorphic and trimorphic populations. Mid‐level stamens were not more variable than long‐ and short‐level stamens within dimorphic populations. The consistent divergence in mid‐level stamens between the L and S morphs may reflect morph‐specific differences in the optimal position of mid‐level anthers for maximizing cross‐pollination and avoiding self‐fertilization.