Pollinator mediated interactions among sympatric sister species in the plant genus Mimulus

Background/Question/Methods

Sister species in the plant genus Mimulus rarely co-occur at local scales. When they do co-occur, they tend to differ in mating system, with one species relying on pollinators for seed set and the other automatically self-pollinating.  Thus, present day pollinator mediated interactions among sympatric sister species in Mimulus may be uncommon.  In this talk, we focus on an exception to this general pattern, presenting evidence of local scale co-occurrence in the sister species pair M. douglasii and M. congdonii --both are visited by bee fly pollinators in Napa Co.  Interestingly, they both display mixed mating whereby a single individual produces two flower types: large, open flowers visited by bee flies and tiny, closed self-pollinated flowers.  Using a set of experimental pollination arrays we assess the role of pollinator-mediated interactions (facilitative or competitive) in driving selection on flowering time and mating system in this sister species pair.

Results/Conclusions

Across their ranges, Mimulus douglasii and M. congdonii exhibit significant local scale aggregation.  In Napa Co. their flowering seasons are largely overlapping, with M. congdonii flowering slightly earlier. In experimental mixed species arrays, M. douglasii experienced selection for convergence in flowering time with M. congdonii relative to single species arrays.  Additionally, selection for increased production of closed self-pollinated flowers was greater in single species relative to mixed species arrays suggesting that pollen may be limited in allopatry. We cautiously suggest a role for pollinator mediated facilitation in this co-flowering sister pair, and the potential for evolution of flowering time to be driven by pollinator mediated interactions between closest relatives.