title: Genetic and morphological evidence for introgression between three species of willows.
abstract: BACKGROUND
  Hybridization and introgression are said to occur relatively frequently in plants, and in particular among different species of willows. However, data on the actual frequency of natural hybridization and introgression is rare. Here, we report the first fine-scale genetic analysis of a contact zone shared between the three basket willow species, Salix dasyclados, S. schwerinii and S. viminalis in the vicinity of the Lake Baikal in Southern Siberia. Individuals were sampled in fourteen populations and classified as pure species or hybrids based on a set of morphological characters. They were then genotyped at 384 nuclear SNP and four chloroplast SSR loci. The STRUCTURE and NewHybrids softwares were used to estimate the frequency and direction of hybridization using genotypic data at the nuclear SNP loci.
  
  
  RESULTS
  As many as 19 % of the genotyped individuals were classified as introgressed individuals and these were mainly encountered in the centre of the contact zone. All introgressed individuals were backcrosses to S. viminalis or S. schwerinii and no F1 or F2 hybrids were found. The rest of the genotyped individuals were classified as pure species and formed two clusters, one with S. schwerinii individuals and the other with S. viminalis and S. dasyclados individuals. The two clusters were significantly genetically differentiated, with F ST = 0.333 (0.282-0.382, p < 0.001). In contrast, for the chloroplast haplotypes, no genetic differentiation was observed as they were completely shared between the species. Based on morphological classification only 5 % of the individuals were classified as introgressed individuals, which was much less than what was detected using genotypic data.
  
  
  CONCLUSIONS
  We have discovered a new willow hybrid zone with relatively high frequency of introgressed individuals. The low frequency of F1 hybrids indicates that ongoing hybridization is limited, which could be because of the presence of reproductive barriers or simply because the conditions are not favorable for hybridization. We further conclude that in order to get a complete picture of the species composition of a hybrid zone it is necessary to use a combination of morphological characters and genetic data from both nuclear and chloroplast markers.
authors.affiliation: Swedish University of Agricultural Sciences, Department of Plant Biology, Uppsala BioCenter, Linnean Centre for Plant Biology, P.O. Box 7080, SE-75007, Uppsala, Sweden. johan.fogelqvist@slu.se.
authors.forename: Johan
authors.initials: J
authors.lastname: Fogelqvist
authors.affiliation: Siberian Institute of Plant Physiology & Biochemistry, Irkutsk-33, P.O. Box 317, 664033, Irkutsk, Russia. allaverh@list.ru.
authors.forename: Alla V
authors.initials: AV
authors.lastname: Verkhozina
authors.affiliation: Siberian Institute of Plant Physiology & Biochemistry, Irkutsk-33, P.O. Box 317, 664033, Irkutsk, Russia. alex@sifibr.irk.ru.
authors.forename: Alexander I
authors.initials: AI
authors.lastname: Katyshev
authors.affiliation: Swedish University of Agricultural Sciences, Department of Plant Biology, Uppsala BioCenter, Linnean Centre for Plant Biology, P.O. Box 7080, SE-75007, Uppsala, Sweden. pascal.pucholt@slu.se.
authors.forename: Pascal
authors.initials: P
authors.lastname: Pucholt
authors.affiliation: Swedish University of Agricultural Sciences, Department of Plant Biology, Uppsala BioCenter, Linnean Centre for Plant Biology, P.O. Box 7080, SE-75007, Uppsala, Sweden. christina.dixelius@slu.se.
authors.forename: Christina
authors.initials: C
authors.lastname: Dixelius
authors.affiliation: Swedish University of Agricultural Sciences, Department of Plant Biology, Uppsala BioCenter, Linnean Centre for Plant Biology, P.O. Box 7080, SE-75007, Uppsala, Sweden. anki.wastljung@slu.se.
authors.forename: Ann Christin
authors.initials: AC
authors.lastname: Rönnberg-Wästljung
authors.affiliation: Uppsala University, Department of Ecology and Genetics, Evolutionary Biology Centre, Science for Life Laboratory, Norbyvägen 18D, 752 36, Uppsala, Sweden. martin.lascoux@ebc.uu.se.
authors.forename: Martin
authors.initials: M
authors.lastname: Lascoux
authors.affiliation: Swedish University of Agricultural Sciences, Department of Plant Biology, Uppsala BioCenter, Linnean Centre for Plant Biology, P.O. Box 7080, SE-75007, Uppsala, Sweden. sofia.berlin@slu.se.
authors.forename: Sofia
authors.initials: S
authors.lastname: Berlin
mesh_terms: D018744:DNA, Plant / Q000235:genetics; D005838:Genotype; D006824:Hybridization, Genetic; D018895:Microsatellite Repeats; D020641:Polymorphism, Single Nucleotide; D032108:Salix / Q000033:anatomy & histology* / Q000145:classification / Q000235:genetics*; D012800:Siberia
chemical_list: D018744:DNA, Plant