Crossability Barriers in Neotropical Heliconia

Artificial hybridization among species of neotropical Heliconiawas studied at two sites in Costa Rica, centralAmerica. At LasCruces Tropical Botanical Garden individuals in cultivationwere used as parents in crosses primarily between species withpendent inflorescences that normally are distributed allopatrically.At Finca La Selva normally sympatric species with either pendentor erect inflorescences were crossed in their natural habitats.Observation of pollen tube growth by means of fluorescence microscopyand seed set were used to determine the extent of crossability.Crossability barriers between the majority of species are strongand foreign pollen tubes are inhibited at the stigmatic surface,within the stylar tissue or within the ovary. The site of inhibitionis consistent for each pair of species, and is dependent onthe parentage and the direction of the cross. Although additionalisolating mechanisms, such as pollinator specificity and phenologicalseparation, are present in Heliconia, pre-fertilization crossabilitybarriers act as the ultimate mechanism to prevent hybridization.The type of barrier (stigmatic, stylar or ovarian) that existsbetween two species is not dependent upon the geographical distributionof the parental species or the specific types of pollinatorsthat visit them, but in some cases may indicate taxonomic relationships. Heliconia spp, isolating mechanisms, crossability barriers, progamic phase, hybridization, Costa Rica, hummingbirds, taxonomy, pollinator sharing