Exine patterns, pollen size dimorphism, and breeding systems in species of Villarsia (Menyanthaceae). Species of the genus Villarsia display a diversity of breeding systems. In ten of the sixteen known species, distylous flowers are associated with a conventional diallelic self-incompatibility (SI) system or with derivative breeding systems. In these species striking dimorphisms in shape and size of the stigma have been reported. Two of the four monomorphic species studied are self-compatible and two are self-incompatible. It has been hypothesized that floral monomorphism here has evolved from distylous ancestors by two different evolutionary pathways. Our present study shows that stigma dimorphisms may be associated with pollen size dimorphism both in species with a conventional distylous breeding system and in species with derivative breeding systems. No differences were observed between exine sculpturing of the two morphs. In all distylous and in three of the four monomorphic species examined, the exine is striate to rugulose. The exception is monomorphic, self-incompatible V. submersa. Here the exine was found to be mesh-like. By its exine pattern, V. submersa is unique in the genus and even in the family. The exine pattern in V. submersa, in addition to distinct ecological and morphological traits that are atypical for Villarsia, adds evidence favoring taxonomic separation of this species from the genus. The finding of an exceptional exine pattern in V. submersa strongly suggests an independent origin of monomorphism with SI in this species, i.e., a third pathway.

Key words: breeding systems, distyly, exine, Menyanthaceae, pollen size dimorphism, Villarsia