Ecology and evolution of self-fertilization in a perennial herb, 'Blandfordia grandiflora' R. Br. (Liliaceae)

Abstract

In this study, I investigated the breeding systems of tableland and coastal populations of the perennial herb 'Blandfordia grandiflora' R. Br. (Liliaceae). Tableland plants were self-fertile and coastal plants were self-infertile, and I used a largely comparative approach to gain insight into the ecology and evolution of self-fertility in the tableland populations. Specifically, for tableland and coastal populations I examined the breeding systems, floral morphology and patterns of reproductive allocation, geographic variation in reproductive and vegetative characters throughout about 90% of the species range, availability of pollinators and pollen limitation, and in the tableland populations, the expression and magnitude of inbreeding depression. This approach allowed me to quantify the differences in breeding systems and then to assess the importance of inbreeding depression and reproductive assurance as factors in the evolution of self-fertility in the tableland populations ... The overall findings of the present study suggest that the differences in the breeding systems between tableland and coastal populations of 'B. grandiflora' are related to differences in the availability of avian pollinators. Abundance of and floral visitation by nectarivorous birds were high in coastal populations compared to tableland populations where both were sufficiently low to adversely affect reproductive success. Experimental and natural exclusion of nectarivorous birds from flowering plants in coastal populations significantly decreased seed set by about three-fold. These findings are consistent with the hypothesis that reproductive assurance was an important selective factor in the evolution of self-fertility in tableland populations, despite inbreeding depression that significantly exceeded δ = 0.50, the threshold value below which increased selfing is expected to evolve. Although self-fertility has evolved, floral traits that ensure autonomous self-pollination have not. Before the importation of honeybees, ants may have self-pollinated flowers, thereby providing reproductive assurance without changes in floral morphology that cause autonomous self-pollination. Morphological changes allowing autonomous self-pollination may not have been selectively favoured because they may reduce or prevent occasional floral visits by nectarivorous birds, thereby reducing the reproductive benefits that accrue from outcrossing. Overall, these findings suggest that reproductive assurance was an important selective factor in the evolution of self-fertility.