Andromonoecy in an Old World Papilionoid legume, Erophaca baetica

Andromonoecy (i.e. the occurrence on individual plants of hermaphroditic and male flowers) is a rare sexual system among the angiosperms, regarded by some authors as a transitional stage from hermaphroditism to monoecy. Having discovered the occurrence of andromonoecy in Erophaca baetica (a Mediterranean shrubby legume with two subspecies), a novelty for Old World papilionoid legumes, we investigated the morpho-functional correlates and the geographical distribution of this phenomenon in the species. The relative frequencies of hermaphrodite and male flowers were determined in two field and 111 herbarium populations. Biomass allocation within flowers, pollen production and viability, pollen tube growth, nectar production and the temporal pattern of male flower production were also studied in two nearby southern Spanish populations. Virtually all of the studied populations were andromonoecious. Male flowers tended to appear at apical positions within the inflorescence, and became more abundant by the end of the flowering season. Male flowers were externally similar to hermaphroditic flowers (although with less biomass and smaller parts) and released equivalent amounts of pollen and nectar; however, their pollen germinated significantly better. Erophaca is the first example of an andromonecious Papilionoid in the Old World. Since the main difference among floral morphs in this species is functional (i.e. pollen germination rate) rather than morphological, andromonoecy is not readily noticeable, and very careful inspection may be required to reveal it. The potential effect of andromonoecy in enhancing outcrossing rate in this species is discussed.