title: Timeframe of speciation inferred from secondary contact zones in the European tree frog radiation (Hyla arborea group).
abstract: BACKGROUND
  Hybridization between incipient species is expected to become progressively limited as their genetic divergence increases and reproductive isolation proceeds. Amphibian radiations and their secondary contact zones are useful models to infer the timeframes of speciation, but empirical data from natural systems remains extremely scarce. Here we follow this approach in the European radiation of tree frogs (Hyla arborea group). We investigated a natural hybrid zone between two lineages (Hyla arborea and Hyla orientalis) of Mio-Pliocene divergence (~5 My) for comparison with other hybrid systems from this group.
  
  
  RESULTS
  We found concordant geographic distributions of nuclear and mitochondrial gene pools, and replicated narrow transitions (~30 km) across two independent transects, indicating an advanced state of reproductive isolation and potential local barriers to dispersal. This result parallels the situation between H. arborea and H. intermedia, which share the same amount of divergence with H. orientalis. In contrast, younger lineages show much stronger admixture at secondary contacts.
  
  
  CONCLUSIONS
  Our findings corroborate the negative relationship between hybridizability and divergence time in European tree frogs, where 5 My are necessary to achieve almost complete reproductive isolation. Speciation seems to progress homogeneously in this radiation, and might thus be driven by gradual genome-wide changes rather than single speciation genes. However, the timescale differs greatly from that of other well-studied amphibians. General assumptions on the time necessary for speciation based on evidence from unrelated taxa may thus be unreliable. In contrast, comparative hybrid zone analyses within single radiations such as our case study are useful to appreciate the advance of speciation in space and time.
authors.affiliation: Department of Ecology & Evolution, University of Lausanne, Biophore Building, 1015, Lausanne, Switzerland. Christophe.Dufresnes@unil.ch.
authors.forename: Christophe
authors.initials: C
authors.lastname: Dufresnes
authors.affiliation: Department of Ecology & Evolution, University of Lausanne, Biophore Building, 1015, Lausanne, Switzerland. Alan.Brelsford@unil.ch.
authors.forename: Alan
authors.initials: A
authors.lastname: Brelsford
authors.affiliation: Faculty of Sciences and Mathematics, University of Niš, Višegradska 33, 18000, Niš, Serbia. jelka@ibiss.bg.ac.rs.|Institute for Biological Research "S. Stanković", University of Belgrade, Despota Stefana 142, 11000, Belgrade, Serbia. jelka@ibiss.bg.ac.rs.
authors.forename: Jelka
authors.initials: J
authors.lastname: Crnobrnja-Isailović
authors.affiliation: Vertebrates Department, National Museum of Natural History, Tsar Osvoboditel blvd. 1, 1000, Sofia, Bulgaria. ntzankov@gmail.com.
authors.forename: Nikolay
authors.initials: N
authors.lastname: Tzankov
authors.affiliation: Natural History Museum of Crete, University of Crete, Knosos Av, P.O. Box 2208, 71409, Irakleio, Crete, Greece. lyberis@nhmc.uoc.gr.
authors.forename: Petros
authors.initials: P
authors.lastname: Lymberakis
authors.affiliation: Department of Ecology & Evolution, University of Lausanne, Biophore Building, 1015, Lausanne, Switzerland. Nicolas.Perrin@unil.ch.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Perrin
mesh_terms: D000818:Animals; D004272:DNA, Mitochondrial / Q000235:genetics; D049810:Genetic Speciation; D006824:Hybridization, Genetic; D011898:Ranidae / Q000145:classification* / Q000235:genetics*; D060047:Reproductive Isolation; D017422:Sequence Analysis, DNA
chemical_list: D004272:DNA, Mitochondrial