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, especi ally in the Lythraceae. However, the evolutionary forces involved rema in unclear since species exhibiting transitional stages between tristy ly and distyly have rarely been studied. The self-compatible, wetland perennial Decodon verticillatus (Lythraceae) may provide this transiti onal variation since populations commonly lack style morphs, particula rly the mid-styled (M) morph. In dimorphic populations lacking the M m orph, anthers positioned at the mid level in both the long- (L) and sh ort-styled (S) morphs have lost their target stigma, setting the stage for either evolutionary repositioning of mid-level anthers to increas e pollen export to L and S stigmas, or increased variability in mid-le vel anther position resulting from relaxed selection. We examined thes e two hypotheses by comparing floral morphology in eight dimorphic and ten trimorphic populations from throughout the species' range. We fou nd no evidence that loss of the M morph has led to evolutionary modifi cation of mid-level stamens. While mid-level stamens of the S morph we re 11.0 +/- 4.0% (mean +/- 1 SE) longer than those of the L morph in d imorphic populations, divergence in stamen length between morphs occur red to the same extent (10.4 +/- 2.0%) in trimorphic populations and c annot be attributed to the absence of the M morph. Analyses of variabi lity using median ratio tests revealed no difference in the variabilit y of mid-level stamen length between dimorphic and trimorphic populati ons. Mid-level stamens were not more variable than long- and short-lev el stamens within dimorphic populations. The consistent divergence in midlevel stamens between the L and S morphs may reflect morph-specific differences in the optimal position of mid-level anthers for maximizi ng cross-pollination and avoiding self-fertilization.