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Genetic analysis of admixture and patterns of introgression in foundation cottonwood trees (Salicaceae) in southwestern Colorado, USA

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Abstract

Cottonwoods are well known as foundation riparian trees that support diverse communities and drive ecosystem processes. Although hybridization naturally occurs when the distributions of two or more cottonwood species overlap, few cottonwood hybrid zones have been genetically characterized. We use genetic and genomic analyses to characterize patterns of admixture and introgression for a newly described hybrid zone at the intersection of three species (Populus L. Salicaceae—Populus deltoides, Populus fremontii, and Populus angustifolia) in southwestern Colorado, USA. Analysis of nuclear and chloroplast microsatellite marker data detected substantial genetic variation among individuals, revealing that (1) hybridization is occurring between two, not three, species (P. deltoides and P. angustifolia); (2) gene flow is bidirectional; (3) hybrids are not abundant (admixture detected in only 34 of 270 trees), with most being early-generation F1 hybrids; (4) cytonuclear disequilibria exists and F1 hybrids tend to retain P. deltoides—like chloroplasts; and (5) roughly 30 % of the nuclear markers deviated from a neutral pattern of introgression, suggesting that selection may play a role in shaping the genetic structure of the hybrid zone in this region. Overall, our results show that despite strong selection maintaining species divergence, transfer of allelic variants across species boundaries can occur. Our study assesses the fine-scale genetic structure of hybridization between P. angustifolia and P. deltoides and lays the foundation for examining how geographic differences in hybrid zone dynamics arise and may influence subsequent ecological and evolutionary processes.

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Acknowledgments

The authors thank M. Zinkgraf for help in the field; R. Cronn, K. Mayer, T. Max, and E. Olson for help in the lab; and H. Bothwell for providing unpublished data. Our research was supported by NSF–FIBR DEB-0425908.

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All nuclear and cpDNA microsatellite data will be submitted to Dryad.

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  1. Department of Biological Sciences, Michigan Technological University, Houghton, MI, 49931, USA

    Erika I. Hersch-Green

  2. Environmental Genetics & Genomics Laboratory, Northern Arizona University, Box 5640, Flagstaff, AZ, 86011, USA

    Gerard J. Allan

  3. Department of Biological Sciences, Northern Arizona University, Box 5640, Flagstaff, AZ, 86011, USA

    Gerard J. Allan & Thomas G. Whitham

  4. Merriam-Powell Center for Environmental Research, Flagstaff, AZ, 86011, USA

    Thomas G. Whitham

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  1. Erika I. Hersch-Green
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Hersch-Green, E.I., Allan, G.J. & Whitham, T.G. Genetic analysis of admixture and patterns of introgression in foundation cottonwood trees (Salicaceae) in southwestern Colorado, USA. Tree Genetics & Genomes 10, 527–539 (2014). https://doi.org/10.1007/s11295-014-0701-9

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