Pollinator limitation and inbreeding depression in orchid species with and without nectar rewards

Abstract

Many orchids produce no nectar rewards. Foraging pollinators should visit more flowers per inflorescence in species with nectar, which could increase geitonogamous self-fertilization. If a history of selfing decreases genetic load, then nectar-producing orchids should harbour lower inbreeding depression than nectarless species. Here, I tested this hypothesis by quantifying inbreeding depression and pollinator limitation in populations of three closely related orchid species, one of which provides nectar. I also compared inbreeding depression for nectarless and nectar-producing species of orchids using published studies. All field populations expressed pollinator limitation, but the nectar-providing species was intermediate to the two nectarless species. All populations expressed inbreeding depression, and levels increased in later life-history stages. There was no tendency for nectarless species to express higher inbreeding depression either in experiments or published studies. Nectarless orchids may not express higher levels of inbreeding depression because pollinators fail to visit more flowers in nectar-bearing species, because such visitations do not result in greater selfing, and/or because higher selfing may be ineffective in purging the mutations that cause load.