Does stigma curvature promote delayed selfing? An experimental investigation in Triodanis perfoliata (Campanulaceae)

Self-fertilisation that is delayed until after opportunities for outcrossing have ceased has been argued to provide both the reproductive assurance benefits of selfing and the genetic advantages of outcrossing. In the Campanulaceae, presentation of pollen on stylar hairs and progressive stigma curvature have been hypothesised to facilitate delayed selfing, but experimental tests are lacking. Stigma curvature is common in Campanula, a genus largely characterised by self-incompatibility, and therefore is unlikely to have initially evolved to promote self-fertilisation. In derived self-compatible species, however, stigma curvature might serve the secondary function of delayed selfing. We investigated delayed selfing in Triodanis perfoliata, a self-compatible relative of Campanula. Using floral manipulation experiments and pollen tube observations, we quantified the extent and timing of self-pollination. Further, we hypothesised that, if stigma curvature provides the benefit of delayed selfing in Triodanis, selection should have favoured retention of self-pollen through the loss of a stylar hair retraction mechanism. Results of a stigma removal experiment indicated that autonomous selfing produces partial seed set, but only some selfing was delayed. Pollen tube observations and a flower senescence assay also supported the finding of partial delayed selfing. Scanning electron microscopy revealed that pollen-collecting hairs retract during anthesis, which may limit the extent of delayed selfing. Delayed selfing appeared to be only partially effective in T. perfoliata. The stylar hair retraction in this species would seem to contradict selection for selfing. We suggest that caution and rigour are needed in interpreting floral traits as adaptive mechanisms for delayed selfing.