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

Erika I. Hersch-Green, Gerard J. Allan, Thomas G. Whitham

Research output: Contribution to journalArticlepeer-review

14 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)527-539
Number of pages13
JournalTree Genetics and Genomes
Issue number3
StatePublished - Jun 2014


  • Cottonwoods
  • Cytonuclear disequilibrum
  • Hybridization
  • Introgression
  • Populus
  • selection

ASJC Scopus subject areas

  • Forestry
  • Molecular Biology
  • Genetics
  • Horticulture


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