title: Differential pollinator effectiveness and importance in a milkweed (Asclepias, Apocynaceae) hybrid zone.
abstract: OBJECTIVE
  Exceptions to the ideal of complete reproductive isolation between species are commonly encountered in diverse plant, animal, and fungal groups, but often the causative ecological processes are poorly understood. In flowering plants, the outcome of hybridization depends in part on the effectiveness of pollinators in interspecific pollen transport. In the Asclepias exaltata and A. syriaca (Apocynaceae) hybrid zone in Shenandoah National Park, Virginia, extensive introgression has been documented. The objectives of this study were to (1) determine the extent of pollinator overlap among A. exaltata, A. syriaca, and their hybrids and (2) identify the insect taxa responsible for hybridization and introgression.
  
  
  METHODS
  We observed focal plants of parental species and hybrids to measure visitation rate, visit duration, and per-visit pollinia removal and deposition, and we calculated pollinator effectiveness and importance.
  
  
  RESULTS
  Visitation rates varied significantly between the 2 yr of the study. Overall, Apis mellifera, Bombus sp., and Epargyreus clarus were the most important pollinators. However, Bombus sp. was the only visitor that was observed to both remove and insert pollinia for both parent species as well as hybrids.
  
  
  CONCLUSIONS
  We conclude that Bombus may be a key agent of hybridization and introgression in these sympatric milkweed populations, and hybrids are neither preferred nor selected against by pollinators. Thus, we have identified a potential mechanism for how hybrids act as bridges to gene flow between A. exaltata and A. syriaca. These results provide insights into the breakdown of prezygotic isolating mechanisms.
authors.affiliation: The George Washington University, Department of Biological Sciences, 2023 G St. NW, Suite 340, Washington, District of Columbia 20052-0086, USA. teresa.stoepler@gmail.com
authors.forename: Teresa M
authors.initials: TM
authors.lastname: Stoepler
authors.forename: Andrea
authors.initials: A
authors.lastname: Edge
authors.forename: Anna
authors.initials: A
authors.lastname: Steel
authors.forename: Robin L
authors.initials: RL
authors.lastname: O'Quinn
authors.forename: Mark
authors.initials: M
authors.lastname: Fishbein
mesh_terms: D000818:Animals; D029062:Apocynaceae / Q000235:genetics* / Q000502:physiology*; D035264:Flowers / Q000033:anatomy & histology / Q000502:physiology; D006824:Hybridization, Genetic; D007313:Insecta / Q000502:physiology*; D054817:Pollination / Q000502:physiology*