title: Post-pollination barriers and their role in asymmetric hybridization in Rhinanthus (Orobanchaceae).
abstract: OBJECTIVE
  Several barriers against hybrid formation exist, and their combined action can affect the evolutionary outcomes of hybridization. To explain the asymmetrical introgression observed between two bumblebee-pollinated plant species (Rhinanthus angustifolius and R. minor), we focused on post-pollination barriers and ethological isolation of hybrids.
  
  
  METHODS
  We studied pollen competition in conspecific and heterospecific crosses on both species and analyzed germination rates of hybrid and pure seeds. We also measured bumblebee visitation rates to hybrids relative to their parents using potted Rhinanthus placed in populations of each parental species.
  
  
  RESULTS
  In mixed pollinations, there was a conspecific siring advantage in both species, but no difference in pollen tube growth rates in either cross type. F(1) seeds with a R. angustifolius maternal plant germinated poorly, while those with R. minor as the maternal parent germinated better than pure seeds. Interestingly, bumblebees treated hybrids almost as equal to the background species and more often rejected the nonresident Rhinanthus. In a R. angustifolius background, bumblebees preferred R. angustifolius, but visited hybrids more often than R. minor. In contrast, visitation rates were similar on a R. minor background.
  
  
  CONCLUSIONS
  Our results suggest that hybridization rates in Rhinanthus remain low because of several leaky barriers that make R. minor the maternal parent of most F(1) offspring. Preference for R. angustifolius and the equal treatment of F(1) and background species by bumblebees induce a visitation pattern that directs gene flow toward R. angustifolius when this species predominates.
authors.affiliation: Biodiversity Research Centre, Earth and Life Institute, Université catholique de Louvain, Croix du Sud 4-5, Box L7.07.04, B-1348 Louvain-la-Neuve, Belgium. laurentnatalis@gmail.com
authors.forename: Laurent C
authors.initials: LC
authors.lastname: Natalis
authors.forename: Renate A
authors.initials: RA
authors.lastname: Wesselingh
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D001516:Bees / Q000502:physiology; D035264:Flowers / Q000235:genetics / Q000502:physiology; D051456:Gene Flow / Q000235:genetics; D018525:Germination / Q000235:genetics; D006824:Hybridization, Genetic; D031670:Orobanchaceae / Q000145:classification / Q000235:genetics* / Q000502:physiology; D011058:Pollen / Q000235:genetics* / Q000502:physiology; D054817:Pollination / Q000235:genetics* / Q000502:physiology; D060047:Reproductive Isolation; D012639:Seeds / Q000235:genetics / Q000502:physiology; D013045:Species Specificity; D013997:Time Factors