Pollinator specificity drives strong prepollination reproductive isolation in sympatric sexually deceptive orchids

Few studies have quantified the full range of pre– and postzygotic barriers that limit introgression between closely related plant species. Here we assess the strength of four isolating mechanisms operating between two morphologically similar and very closely related sympatric orchid taxa, Chiloglottis valida and C. aff. jeanesii. Each taxon sexually attracts its specific wasp pollinator via distinct floral volatile chemistry. Behavioral experiments with flowers and synthetic versions of their floral volatiles confirmed that very strong pollinator isolation is mediated by floral odor chemistry. However, artificially placing flowers of the two taxa in contact proximity revealed the potential for rare interspecific pollination. Although we found hybrid vigour in F1 hybrids produced by hand-crossing , genetic analysis at both nuclear and chloroplast loci showed significant and moderate to strong genetic differentiation between taxa. A Bayesian clustering method for the detection of introgr...