Amount of introgression in flycatcher hybrid zones reflects regional differences in pre and post-zygotic barriers to gene exchange

Authors

  • T. BORGE,

    1. Zoological Museum, Natural History Museums and Botanical Garden, University of Oslo, Blindern, Oslo, Norway
    2. Department of Evolutionary Biology, Evolutionary Biology Centre, Uppsala University, Uppsala, Sweden
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  • K. LINDROOS,

    1. Department of Medical Sciences, Molecular Medicine, Uppsala University, University Hospital, Uppsala, Sweden
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  • P. NÁDVORNÍK,

    1. Department of Cell Biology and Genetics, Palacký University, Olomouc, Czech Republic
    2. Laboratory of Ornithology, Palacký University, Olomouc, Czech Republic
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  • A. -C. SYVÄNEN,

    1. Department of Medical Sciences, Molecular Medicine, Uppsala University, University Hospital, Uppsala, Sweden
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  • G. -P. SæTRE

    1. Department of Evolutionary Biology, Evolutionary Biology Centre, Uppsala University, Uppsala, Sweden
    2. Department of Biology, Centre for Ecological and Evolutionary Synthesis, University of Oslo, Oslo, Norway
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Glenn-Peter Sætre, Department of Biology, Centre for Ecological and Evolutionary Synthesis, University of Oslo, P. O. Box 1066 Blindern, N-0316 Oslo, Norway.
Tel.: +47 22857291; fax: +47 22854605;
e-mail: g.p.satre@bio.uio.no

Abstract

Introgression is the incorporation of alleles from one species or semispecies into the gene pool of another through hybridization and backcrossing. The rate at which this occurs depends on the frequency of hybridization and the fitness of hybrids and backcrosses compared to ‘pure’ individuals. The collared flycatcher (Ficedula albicollis) and the pied flycatcher (F. hypoleuca) co-exist and hybridize at low to moderate frequencies in a clinal hybrid zone in Central Europe and on the islands of Gotland and Öland off the Swedish east coast. Data on hatching success suggest that hybrids are less fertile in Central Europe compared to on the islands. Direct fitness estimates using molecular markers to infer paternity are consistent with the demographic data. Applying a tag-array-based minisequencing assay to genotype interspecific substitutions and single nucleotide polymorphisms we demonstrate that the amount of introgression from the pied to the collared flycatcher is higher in the two island populations (Gotland and Öland) than in two geographically distinct areas from the Central European hybrid zone (Czech Republic and Hungary). In all areas the amount of introgression from collared to pied flycatchers is very low or seemingly absent. The different patterns of introgression are consistent with regional differences in rates of hybridization and fitness of hybrids. We suggest that barriers to gene exchange may have been partly broken down on the islands due to asymmetric gene flow from allopatry. Alternatively, or in addition, more pronounced reinforcement of prezygotic isolation in Central Europe might have increased post-zygotic isolation through hitchhiking, since genes affecting pre and post-zygotic isolation are both sex-linked in these birds. One of our genetic markers appears to introgress from pied to collared flycatchers at a much higher rate than the other markers. We discuss the possibility that the introgressed marker may be linked to a gene which is under positive selection in the novel genetic background.

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