The mechanism of delayed selfing in Collinsia verna (Scrophulariaceae)

Collinsia verna, blue-eyed Mary, has floral attributes of an outcrossing sp ecies, yet most flowers readily self-pollinate under greenhouse conditions. Here we describe the mechanism of self-pollination in C. verna via changes in relative positions of the stigma and anthers and late timing of recepti vity, resulting in delayed selfing. Each flower contains four anthers that dehisce sequentially over similar to 1 wk. Pollen that is not collected by pollinators accumulates in the keel petal and retains high viability (>80% pollen germination) up to the time of corolla abscission. The stigmatic sur face does not become receptive until after the third anther dehisces. This overlap in the sexual phases is concurrent with a change in herkogamy durin g floral development. In most flowers (70%), the stigma has moved to the fr ont of the keel and is positioned near the anthers when the third anther de hisces. Under field conditions, fruiting success of plants within pollinato r exclosures was similar to 75% of the fruiting success in open-pollinated plants (33% fruiting success via autogamy vs. 44% fruiting success, respect ively). Collinsia verna plants in pollinator exclosures exhibit variation i n autogamy rates within natural populations (range 0-80%). In addition, onl y half of naturally pollinated, receptive flowers examined had pollen tubes growing in their styles. In contrast, shortly after corolla abscission, ne arly all flowers examined (96%) had pollen tubes in their styles. Thus we f ind that in C. verna, autogamy occurs late in floral development, which has the potential to provide substantial reproductive assurance, and that indi viduals vary in their ability to set fruit through this mechanism. We sugge st that delayed selfing mechanisms may be overlooked in other species and t hat variable pollinator availability may pray a significant role in the mai ntenance of mixed mating in species with delayed selfing, such as C. verna.