Characterizing a rare shift in the pollination mode of cottonsedge, Eriophorum virginicum: Wind to insects

Background/Question/Methods

Few plant species have made the evolutionary shift from wind to insect-pollination. Those that have share with the bog-dwelling cottonsedge, Eriophorum virgincium,floral characteristics such as a brightly colored and aromatic inflorescence, compared to the brown, relatively scentless inflorescences of most wind-pollinated species.  This sedge lies in a wind-pollinated clade, so if it is insect-pollinated, as indicated by some of its floral characteristics, it would represent a novel addition to the few known evolutionary shifts from wind to insect pollination. However, nothing is known about its pollination mode.  Three distinct possibilities exist: (A) high affinity for wind pollination, (B) high affinity for insect pollination, and (C) reliance on both wind and insect pollination modes, possibly as an intermediate stage in the evolutionary shift.  To test these hypotheses, we compared pollination success of plants open to wind plus insects versus plants enclosed in mesh insect-exclusion bags that were open only to wind (stamens were removed to prevent self-pollination). We further investigated the effectiveness of wind pollination by characterizing changes with time since stamen dehiscence and the rate of pollen release into the wind (measured using sticky slides) and pollen viability (assessed by fluorescence microscopy of pollen tubes following hand pollinations).

Results/Conclusions

First, experimentally excluding insects did not significantly affect seed production, indicating either that insects do not substantially increase cross-pollination beyond that effected by wind or that stamen removals did not prevent self-pollination. The latter is likely because similar seed set occurred through autonomous self-pollination or apomixis in additional inflorescences from which wind-borne pollen was excluded using microfiber bags. Second, time to peak pollen release into the wind was approximately eight hours post stamen dehiscence, yet pollen viability – indicated by the number of pollen grains attached to stigmas and pollen tube length – dropped to near zero four hours post stamen dehiscence.  These contrasting results suggest that, at the very least, wind is highly inefficient as a pollen vector of cottensedge.  We observed insects, including syrphid flies and bees, moving sequentially between cottonsedge inflorescences and carrying its pollen. Our evidence for the inefficiency of wind pollination in this species, combined with certain floral traits more consistent with insect pollination syndromes, implies that these insects may contribute substantially to cross-pollination of cottonsedge. Given its wind-pollinated ancestry, this would indicate a novel instance of the rare evolutionary shift from wind to insect pollination and shed light on how and why this shift occurs.