Pollination ecology, genetic diversity and selection on nectar spur length in Platanthera lacera (Orchidaceae)

Platanthera lacera (Orchidaceae) is a moth-pollinated, loess prairie orchid producing a raceme of one to many whitish-green flowers. Field studies on a western Illinois population found the crepuscular visiting noctuid moth, Anagrapha falcifera (Noctuidae), to be the most frequent pollinator with occasional visits from Allagrapha aerea (Noctuidae). Visitation rates, assessed by removal of at least one pollinium, were relatively high (84.9%) and fruit production on experimentally outcrossed flowers (94.4%) was higher than open-pollinated plants (71.4%). Experimental pollination showed P. lacera to be highly self-compatible (94.1%) with a low level of autogamy (8.2%). Measurements taken from 598 spurs on 44 plants indicated that nectar spur length varied significantly among plants (10.9-17.1 mm, mean 14.3 mm), but was not under selective pressure from visitation by An. falcifera (mean proboscis length 11.1 mm). The absence of selective pressure on nectar spur length is likely to be explained by occasional pollinating visits from Al. aerea (proboscis length 18 mm) and a limited amount of autogamy. Electrophoretic analysis of 12 enzymes revealed seven polymorphic loci. Mean levels of heterozygosity were H-e = 0.3384, H-o = 0.3229 and F = 0.0458, indicating that P. lacera is primarily an outcrossing species dependent on noctuid moth pollination.