Effects of floral display size and biparental inbreeding on outcrossing rates in Delphinium barbeyi (Ranunculaceae)

Floral display size represents a tradeoff between the benefits of increased pollinator visitation and the quantity of pollen received vs. the costs of increased self-pollination and reduced pollination quality. Plants with large floral displays often are more attractive to pollinators, but pollinators visit more flowers per plant. Intraplant foraging movements should increase self-pollination through geitonogamy, lowering outcrossing rates in large plants. Local genetic structure should also increase inbreeding and decrease outcrossing estimates, if pollinators move between neighboring, related plants. These predictions were tested in a population of larkspurs (Delphinium barbeyi) in Colorado. Allozymes were used to estimate outcrossing rates of plants varying in display size. Floral displays varied widely (2-1400 flowers; 1-26 inflorescences per plant), and outcrossing rate decreased significantly with increasing display size. Large, multistalked plants self over twice as frequently as single-stalked plants (46 vs. 21%). Local population structure is significant, and biparental inbreeding depresses outcrossing in plants surrounded by genetically similar neighbors. Protandry, coupled with stereotypical bottom-up pollinator foraging, reduces self-fertilization by autogamy or geitonogamy within inflorescences. Selfing is predominantly (>60%) by geitonogamy between inflorescences in large plants. Geitonogamy may be a significant cost to plants with large floral displays if inbreeding depression and/or pollen and ovule discounting results. If so, floral display size, particularly inflorescence number, may be under contrasting selection for pollination quantity vs. quality.