RECONSTRUCTION OF THE EVOLUTION OF REPRODUCTIVE CHARACTERS IN PONTEDERIACEAE USING PHYLOGENETIC EVIDENCE FROM CHLOROPLAST DNA RESTRICTION-SITE VARIATION

We reconstructed the phylogenetic history of Pontederiaceae using chlo roplast DNA restriction-site variation from approximately two-thirds o f the species in this family of aquatic monocotyledons. The molecular phylogeny was used to evaluate hypotheses concerning the evolution of reproductive characters associated with the breeding system. The famil y has four main genera, two of which (Eichhornia and Pontederia) have tristylous, predominantly outcrossing species, while two (Monochoria a nd Heteranthera) have enantiostylous taxa. Self-incompatibility is res tricted to some but not all tristylous species. In Eichhornia and Pont ederia, predominantly selfing species with small monomorphic flowers ( homostyly) have been hypothesized to result from the multiple breakdow n of tristyly. Restriction-site variation provided a well supported ph ylogeny of ingroup taxa, enabling the mapping of reproductive characte rs onto trees. Two contrasting optimization schemes were assessed, dif fering in the relative weights assigned to shifts in character states. The reconstructed sequence of floral character-state change was used to assess competing hypotheses concerning the origin and breakdown of tristyly, and the relationships between tristylous and enantiostylous syndromes. Our results indicate that the class of optimization scheme used was the most critical factor in reconstructing character evolutio n. Despite some topological uncertainties and difficulty in reconstruc ting the primitive floral form in the family, several broad conclusion s were possible when an unordered, unequally-weighted optimization sch eme was used: (1) tristyly originated either once or twice, while the occurrence of enantiostyly in Monochoria and Heteranthera was always f ound to have independent origins; (2) tristyly has repeatedly broken d own leading to selfing, homostylous taxa; and (3) self-incompatibility probably arose after the origin of floral trimorphism, a sequence of events that conflicts with some evolutionary models.