On the mechanism of floral shifts in speciation:: gained pollination efficiency from tongue- to eye-attachment of pollinia in Platanthera (Orchidaceae)

This work explores the mechanism of floral shifts that may result in speciation. The model organisms chosen were the moth-pollinated pollinator-limited orchid species Platanthera bifolia and R chlorantha. R bifolia exhibits tongue-attachment of pollinia on pollinators, a character state that has been found previously to be ancestral. The close relative P chlorantha exhibits the derived state of eye-attachment of pollinia on pollinators. We reasoned that differences between the species in pollination efficiency could give insights into the mechanism of floral shifts and thus cladogenesis. Four populations per species were investigated. In three populations, where the species were growing intermixed and were sharing pollinators, there was significant difference in the pollen export and import efficiency per visit-night (night with pollen export and/or import) between the two species. P bifolia exported pollinia more efficiently but imported pollen less efficiently than did P. chlorantha. Pollen import was 1.7-4 times faster in R chlorantha (eye-attachment) than it was in R bifolia (tongue-attachment). R chlorantha had a lower risk of interference between pollen import and export. An increase in fitness through greater speed and efficiency of pollen import due to an enlargement of the stigmatic surface and a reduction in the risk of sexual interference may therefore be mechanisms of the floral shift from tongue- to eye-attachment of pollinia on pollinators. (C) 2004 The Linnean Society of London.