title: Transient hybridization, not homoploid hybrid speciation, between ancient and deeply divergent conifers.
abstract: OBJECTIVE
  Homoploid hybrid speciation is receiving growing attention due the increasing recognition of its role in speciation. We investigate if individuals intermediate in morphology between the two species of the conifer genus Athrotaxis represent a homoploid hybrid species, A. laxifolia, or are spontaneous F1 hybrids.
  
  
  METHODS
  A total of 1055 individuals of Athrotaxis cupressoides and A. selaginoides, morphologically intermediate individuals, and two putative hybrid swarms were sampled across the range of the genus and genotyped with 13 microsatellites. We used simulations to test the power of our data to identify the pure species, F1s, F2s, and backcross generations.
  
  
  RESULTS
  We found that Athrotaxis cupressoides and A. selaginoides are likely the most divergent congeneric conifers known, but the intermediates are F1 hybrids, sharing one allele each from A. cupressoides and A. selaginoides at six loci with completely species specific alleles. The hybrid swarms contain wide genetic variation with stronger affinities to the locally dominant species, A. selaginoides and A. selaginoides backcrosses outnumbering A. cupressoides backcrosses. In addition, we observed evidence for isolated advanced generation backcrosses within the range of the pure species.
  
  
  CONCLUSIONS
  We conclude that, even though they can be large and long-lived, Athrotaxis hybrid swarms are on a trajectory of decline and will eventually be reabsorbed by the parental species. However, this process may take millennia and fossil evidence suggests that such events have occurred repeatedly since the early Quaternary. Given this timeline, our study highlights the many obstacles to homoploid hybrid speciation.
authors.affiliation: Department of Forest Genetics, Forestry and Forest Products Research Institute, 1 Matsunosato, Ibaraki 305-8687, Japan +81 29 829 8261 tmesipteris@gmail.com.
authors.forename: James R P
authors.initials: JR
authors.lastname: Worth
authors.affiliation: School of Biological Sciences, University of Tasmania, Private Bag 55, Hobart, Tasmania 7001, Australia +61 3 6226 1943.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Larcombe
authors.affiliation: Laboratory of Plant Evolution and Biodiversity, Department of General Systems Studies, Graduate School of Arts and Sciences, University of Tokyo, Tokyo, Japan +81 35 454 6368.
authors.forename: Shota
authors.initials: S
authors.lastname: Sakaguchi
authors.affiliation: Menzies Institute for Medical Research, University of Tasmania, Private Bag 24, Hobart, Tasmania 7001, Australia +61 3 6226 4636.
authors.forename: James R
authors.initials: JR
authors.lastname: Marthick
authors.affiliation: School of Biological Sciences, University of Tasmania, Private Bag 55, Hobart, Tasmania 7001, Australia +61 3 6226 1943.
authors.forename: David M J S
authors.initials: DM
authors.lastname: Bowman
authors.affiliation: Laboratory of Plant Evolution and Biodiversity, Department of General Systems Studies, Graduate School of Arts and Sciences, University of Tokyo, Tokyo, Japan +81 35 454 6368.
authors.forename: Motomi
authors.initials: M
authors.lastname: Ito
authors.affiliation: School of Biological Sciences, University of Tasmania, Private Bag 55, Hobart, Tasmania 7001, Australia +61 3 6226 1943.
authors.forename: Gregory J
authors.initials: GJ
authors.lastname: Jordan
mesh_terms: D029779:Cupressaceae / Q000235:genetics*; D018744:DNA, Plant / Q000235:genetics; D020224:Expressed Sequence Tags; D049810:Genetic Speciation; D014644:Genetic Variation; D006824:Hybridization, Genetic; D018895:Microsatellite Repeats; D011003:Ploidies; D017422:Sequence Analysis, DNA; D013648:Tasmania
chemical_list: D018744:DNA, Plant
keywords: Athrotaxis; ancient conifer; homoploid hybrid speciation; hybrid swarm genesis/decline; pollen donor; spontaneous F1s