title: Concordant morphological and molecular clines in a contact zone of the Common and Spined toad (Bufo bufoandB. spinosus) in the northwest of France.
abstract: BACKGROUND
  Hybrid zones are regions where individuals of two species meet and produce hybrid progeny, and are often regarded as natural laboratories to understand the process of species formation. Two microevolutionary processes can take place in hybrid zones, with opposing effects on population differentiation. Hybridization tends to produce genetic homogenization, reducing species differences, whereas the presence of mechanisms of reproductive isolation result in barriers to gene flow, maintaining or increasing differences between taxa.
  
  
  RESULTS
  Here we study a contact zone between two hybridizing toad species,Bufo bufoandB. spinosus, through a combination of molecular (12 polymorphic microsatellites, four nuclear and two mitochondrial SNP markers) and morphological data in a transect in the northwest of France. The results show largely concordant clines across markers, defining a narrow hybrid zone of ca. 30 km wide. Most hybrids in the centre of the contact zone are classified as F2or backcrossed individuals, with no individuals assigned to the F1hybrid class.
  
  
  CONCLUSIONS
  We discuss the implications of these results for our understanding of the evolutionary history of these species. We anticipate that the toad contact zone here described will become an important asset in the study of hybrid zone dynamics and evolutionary biology because of its easy access and the abundance of the species involved.
authors.affiliation: Naturalis Biodiversity Center, P.O. Box 9517, Leiden, 2300 RA The Netherlands.
authors.forename: Jan W
authors.initials: JW
authors.lastname: Arntzen
authors.affiliation: Museo Nacional de Ciencias Naturales, CSIC, c/José Gutiérrez Abascal, 2, Madrid, 28006 Spain.
authors.forename: Tania
authors.initials: T
authors.lastname: Trujillo
authors.affiliation: Naturalis Biodiversity Center, P.O. Box 9517, Leiden, 2300 RA The Netherlands.
authors.forename: Roland
authors.initials: R
authors.lastname: Butôt
authors.affiliation: Plant Cluster, Institute of Biology, Sylvius Laboratory, Leiden University, P.O. BOX 9505, Leiden, 2300 RA The Netherlands.
authors.forename: Klaas
authors.initials: K
authors.lastname: Vrieling
authors.affiliation: Plant Cluster, Institute of Biology, Sylvius Laboratory, Leiden University, P.O. BOX 9505, Leiden, 2300 RA The Netherlands.
authors.forename: Onno
authors.initials: O
authors.lastname: Schaap
authors.affiliation: Museo Nacional de Ciencias Naturales, CSIC, c/José Gutiérrez Abascal, 2, Madrid, 28006 Spain.
authors.forename: Jorge
authors.initials: J
authors.lastname: Gutiérrez-Rodríguez
authors.affiliation: Naturalis Biodiversity Center, P.O. Box 9517, Leiden, 2300 RA The Netherlands ; Instituto de Investigación en Recursos Cinegéticos (IREC-UCLM-CSIC-JCCM), Ronda de Toledo, Ciudad Real, s/n, 13005 Spain ; CIBIO-InBIO, Centro de Investigação em Biodiversidade e Recursos Genéticos, Campus Agrário de Vairão, Universidade do Porto, Vairão, 4485-661 Portugal ; Present address: Ecology, Evolution and Development Group, Department of Wetland Ecology, Doñana Biological Station, CSIC, c/Americo Vespucio, Seville, s/n, 41092 Spain.
authors.forename: Iñigo
authors.initials: I
authors.lastname: Martínez-Solano
keywords: Cline analysis; Hybridization; Microsatellites; Morphology; SNPs; mtDNA