An apparent lack of prezygotic reproductive isolation among 42 sympatric species of Bromeliaceae in southeastern Brazil

The Atlantic rainforest in southeastern Brazil is one the main centers of diversification of Bromeliaceae. Empirical estimates of the relative importance of different pre- and postzygotic barriers to gene flow between recently diverged species are important for understanding speciation processes. While many bromeliad species easily hybridize in cultivation, few records of natural hybrids between closely related sympatric species suggest the existence of strong prezygotic barriers. A community of 42 sympatric bromeliad species, represented by nine genera (Aechmea, Billbergia, Edmundoa, Neoregelia, Nidularium, Quesnelia, Racinaea, Tillandsia, and Vriesea), were surveyed from 2004-2005 to assess the role of microhabitat isolation, phenological isolation and ethological isolation as potential prezygotic barriers. These bromeliads are abundant in the understory and frequently grow densely aggregated in multispecific patches that may facilitate promiscuous pollination. There was a large overlap of flowering periods even between congeneric species. Most species shared a similar assemblage of flower visitors; the hummingbird Rhamphodon naevius was the major visitor for 35 species. Only one putative hybrid was observed. This uncommonness of hybridization is puzzling, given that the prezygotic isolation mechanisms investigated are potentially weak to effectively avoid interspecific pollination.