EVIDENCE FOR POLLINATION ECOTYPES IN THE YELLOW-FRINGED ORCHID, PLATANTHERA CILIARIS

Platanthera ciliaris is a butterfly-pollinated, terrestrial orchid with a loose terminal raceme of 10-50 orange flowers, characterized by a long nectariferous spur. In the southeastern United States, P. ciliaris occurs in the Appalachian mountains and coastal-plain physiographic provinces, but it is found rarely in the intervening Piedmont. In 1983 and 1984, detailed observations of two populations within these disjunct areas revealed that the butterfly species that serve as the primary pollinators differ sharply. In the mountains, Papilio troilus (spicebush swallowtail) was the most frequent and effective visitor, whereas in the coastal plain, P. palamedes (palamedes swallowtail) was the predominant pollinator. Proboscis lengths of P. troilus (mean = 23.3 mm) were significantly shorter than those of P. palamedes (mean = 28.7 mm). Floral characters, most notably spur length, also differed significantly between mountain (mean = 23.8 mm) and coastal-plain (mean = 25.6 mm) plants. In both 1983 and 1984, levels of pollinator service, as assessed by rates of removal and insertion of pollinia, were higher in the mountains (0.81 and 0.86) than in the coastal plain (0.63 and 0.67). In addition, fruit-set was significantly greater in mountain (83.9% in 1983, 86.5% in 1984) than in coastal-plain (63.8% in 1983, 65.5% in 1984) populations. We hypothesize that selection pressure exerted through pollinator proboscis lengths has resulted in pollination ecotypes of P. ciliaris. The short-spurred mountain plants appear to be in equilibrium with their short-tongued butterfly pollinators, receiving high levels of effective pollination and achieving high levels of fruit-set. Coastal-plain plants produce flowers with longer spurs which, nevertheless, are shorter than optimal for insuring that very long-tongued butterflies make contact with their pollinia. Thus, effective pollinator service and fruit-set are reduced. Correlation analyses in 1984 showed a positive relationship between spur length and fruit-set only in the coastal plain. The presence of long-tongued nectar thieves in the coastal plain may also contribute to lower effective pollinator service and fruit-set. Results of reciprocal-transplant studies attempting to determine the genetic basis of floral characters, including spur length, were inconclusive but suggest that differences between mountain and coastal-plain plants are not due solely to phenotypic plasticity.