title: Reproductive isolation and hybridization in sympatric populations of three Dactylorhiza species (Orchidaceae) with different ploidy levels.
abstract: OBJECTIVE
  The potential for gene exchange between species with different ploidy levels has long been recognized, but only a few studies have tested this hypothesis in situ and most of them focused on not more than two co-occurring species. In this study, we examined hybridization patterns in two sites containing three species of the genus Dactylorhiza (diploid D. incarnata and D. fuchsii and their allotetraploid derivative D. praetermissa).
  
  
  METHODS
  To compare the strength of reproductive barriers between diploid species, and between diploid and tetraploid species, crossing experiments were combined with morphometric and molecular analyses using amplified fragment length polymorphism markers, whereas flow cytometric analyses were used to verify the hybrid origin of putative hybrids.
  
  
  RESULTS
  In both sites, extensive hybridization was observed, indicating that gene flow between species is possible within the investigated populations. Bayesian assignment analyses indicated that the majority of hybrids were F(1) hybrids, but in some cases triple hybrids (hybrids with three species as parents) were observed, suggesting secondary gene flow. Crossing experiments showed that only crosses between pure species yielded a high percentage of viable seeds. When hybrids were involved as either pollen-receptor or pollen-donor, almost no viable seeds were formed, indicating strong post-zygotic reproductive isolation and high sterility.
  
  
  CONCLUSIONS
  Strong post-mating reproductive barriers prevent local breakdown of species boundaries in Dactylorhiza despite frequent hybridization between parental species. However, the presence of triple hybrids indicates that in some cases hybridization may extend the F(1) generation.
authors.affiliation: Biology Department, Laboratory of Plant Ecology, University of Leuven, Heverlee, Belgium. koen.dehert@bio.kuleuven.be
authors.forename: Koen
authors.initials: K
authors.lastname: De hert
authors.forename: Hans
authors.initials: H
authors.lastname: Jacquemyn
authors.forename: Sabine
authors.initials: S
authors.lastname: Van Glabeke
authors.forename: Isabel
authors.initials: I
authors.lastname: Roldán-Ruiz
authors.forename: Katrien
authors.initials: K
authors.lastname: Vandepitte
authors.forename: Leen
authors.initials: L
authors.lastname: Leus
authors.forename: Olivier
authors.initials: O
authors.lastname: Honnay
mesh_terms: D001530:Belgium; D051456:Gene Flow; D014644:Genetic Variation; D006824:Hybridization, Genetic; D029595:Orchidaceae / Q000235:genetics*; D011003:Ploidies; D060047:Reproductive Isolation; D013045:Species Specificity