The bees don't know and the flowers don't care: the effect of heterospecific pollen on reproduction in co-occurringThysanotusspecies (Asparagaceae) with similar flowers

Abstract

Species integrity relies on the maintenance of reproductive isolation, particularly between closely related species. Further, it has been hypothesized that the presence of heterospecific pollen on flower stigmas adversely affects plant reproduction with increasing effect in closely related species. Using two pairs of co-occurring buzz-pollinated Thysanotus spp. in the Mediterranean climate region of Perth, Western Australia, we quantified the effect of heterospecific pollen on fruit and seed set. We first determined the mating systems of the two focal species using self- and outcross pollen, followed by separate treatments with heterospecific pollen within each species pair. Of the two species receiving pollen, Thysanotus triandrus had a mixed mating system, but with significantly lower fruit and seed set from self-pollen relative to outcross pollen. Thysanotus tenellus was autogamous with no difference in fruit or seed set between autogamous, self- or outcross pollinations. Heterospecific pollen had no effect on fruit or seed set of either focal species. These outcomes point to post-pollination reproductive isolation, consistent with a floral morphology that causes low specificity of pollen placement and thus a poor capacity for pre-pollination discrimination. Negative effects of heterospecific pollen, therefore, do not appear to play a role in the reproduction in this group of buzz-pollinated flowers.