Inbreeding Depression and Pollinator Availability in a Partially Self-Fertile Perennial Herb Blandfordia grandiflora (Liliaceae)

Abstract

In partially self-fertile populations of Blandfordia grandiflora, inbreeding depression and pollinator abundance were examined as factors influencing the evolution of self-fertility. Inbreeding depression at several life-cycle stages was estimated by comparing the performance of crossed and selfed progeny from field-pollinated plants. Seed set was greater and seed abortion was less after crossing than after selfing. Seed weights did not differ. In the glasshouse, crossed progeny outperformed selfed progeny for seed germination, speed of germination, seedling dry weight, relative growth rate and survival. Cumulative inbreeding depression, estimated from seed set, seed germination, seedling dry weight and survival, was 0.69 + 0.03, and was significantly greater than 0.50, the threshold level of inbreeding depression below which selfing should evolve. The availability of avian pollinators was assessed for 3 yr in five different populations. Low numbers of birds were observed on transects (peak flowering in January, 0.6-2.3; end of flowering in March, 0.3-1.0), and a low percentage of flowers were visited by birds (January, 21%; March, 1%). Nectarivorous birds were not important pollinators. Despite variation in abundance of and visitation by birds, natural seed set was similar within and among years (48.958.5%), and seed set of plants from which birds were excluded did not differ from control plants (both about 53%). Selection for reproductive assurance due to the paucity of avian pollinators would have favoured self-fertility in B. grandiflora, despite high inbreeding depression.