Do pollinators determine hybridization patterns in sympatric Satyrium (Orchidaceae) species?

The breakdown of species integrity was studied in three sympatric Satyrium (Orchidaceae) species at a site in the Western Cape province of South Afric a. Hybrids with intermediate morphology were identified using multivariate analysis. Satyrium erectum x S. bicorne and S. erectum x S. coriifolium hyb rids were common, while only one S. bicorne x SI coriifolium hybrid was fou nd (a new record). Reciprocal artificial crossing experiments confirmed tha t genetic compatibility exists between all three species. However, the spec ies have different pollinators: moths in S. bicorne, bees in S. erectum and sunbirds in S. coriifolium. Pollinators, rather than genetic factors, ther efore, appear to be responsible for reproductive isolation between these sp ecies. Breakdown in pollinator specificity results in interspecific pollen transfer and the formation of hybrids. Overlap in floral signals, including nocturnal scent in both S. erectum and S. bicorne, and bright coloration i n both S. erectum and S. coriifolium, facilitates the formation of hybrids between these species pairs. The brown, scented flowers of S. bicorne, on t he other hand, share no obvious floral signals with the bright orange, unsc ented flowers of S. coriifolium, accounting for the rarity of hybrids betwe en this species pair. Differences in floral morphology also result in mecha nical isolation between these two species. Since the three species are symp atric in a very small part of their geographic ranges, it is unlikely that avoidance of hybridization has been an important selective factor in the ev olution of specialized pollination systems.