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Looks aren’t everything: Hybridization between dalmatian and yellow toadflax

Date: May 11, 2017

Accurate identification of target weeds is key to effective treatment, even in these closely related toadflax species


Dr. Sarah Ward and her undergraduate scholar Amelia Hadid analyzing toadflax plastid DNA using Boswell’s methodology to determine genetic makeup of plants. Plants were field-collected by Sharlene Sing throughout Montana at known or probable toadflax hybridization sites.
Dr. Sarah Ward and her undergraduate scholar Amelia Hadid analyzing toadflax plastid DNA using Boswell’s methodology to determine genetic makeup of plants. Plants were field-collected by Sharlene Sing throughout Montana at known or probable toadflax hybridization sites.

Two closely related invasive Linaria species, Dalmatian toadflax (Linaria Dalmatica) and yellow toadflax (Linaria vulgaris), have successfully invaded a broad range of ecosystems throughout most of continental North America. The management challenge imposed by the landscape scale of many toadflax infestations, particularly in the West, is further complicated by hybridization between these two weeds. Herbicide and biological control treatments for invasive Linaria are highly species-specific, necessitating the development of a molecular diagnostic tool that can accurately confirm when cryptic hybridization has spontaneously occurred in the field, particularly in plants conforming to typical species appearance.


The development of a molecular diagnostic tool began with the extraction of total genomic DNA from individual plants. These were selected from a pool of morphologically unambiguous Dalmatian or yellow toadflax plants. Haplotyping based on species-specific PCR-RFLP markers identified the presence of a single toadflax haplotype, either for Dalmatian toadflax or for yellow toadflax, in the pool of morphologically non-ambiguous plants. Molecular analysis of DNA extracted from an additional pool of hybrid toadflax plants, including field-collected and controlled greenhouse crosses, consistently indicated the presence of both species haplotypes. The 748 bp fragment for yellow toadflax vs. the 608 bp fragment for Dalmatian toadflax was found to be the most reliable and distinct PCR-RFLP polymorphism to distinguish between the two toadflax haplotypes, and confirm hybridization. Because vigorous and fertile hybrids, widespread throughout the Intermountain West, are inconsistently responsive to chemical or biological control, early detection is essential for effective management. Habitat suitability modeling was able to identify areas in Montana, Wyoming and Colorado at greatest risk of invasion by hybrid toadflax, indicating the possibility of extensive spread of hybrid populations. For management purposes, areas identified as having high suitability for hybrid toadflax could be prioritized for monitoring and targeted control.


  • Dalmatian and yellow toadflax co-occur in spite of disparate habitat preferences/tolerances.
  • Hybridization between these species has been confirmed at multiple sites (MT, CO, WY, ID, WA, British Columbia, Alberta).
  • Species-specific markers were developed based on PCR-RFLP polymorphisms in the trnT-D cpDNA region, and matK and trnL-F SNP barcoding regions.
  • Results of plastid DNA analysis using single nucleotide polymorphisms (SNPs) in the matK and trnL-F chloroplast-barcoding regions were consistent with PCR-RFLP diagnoses.
  • Asymmetric gene flow was confirmed both in controlled greenhouse crosses and field populations of hybrid toadflax, indicating a higher proportion of plants with maternally inherited yellow toadflax cpDNA.
  • Species-diagnostic cytoplasmic markers identified the presence of introgressed yellow toadflax genetic material in plants drawn from infestations presumed to be Dalmatian toadflax.
  • There was little overlap in specific environmental covariates predicting habitat suitability for yellow, Dalmatian and hybrid toadflax.
  • Climatic factors were, however, the most significant environmental variable identified in models for each of the three toadflax taxa (mean maximum summer temperature for L. dalmatica, late-season mean maximum temperature for L. vulgaris, and number of frost-free days in the growing season for hybrid toadflax).
  • A warming climate in this study area could significantly impact future distribution of Linaria taxa across the Intermountain West, increasing the possibility that habitats not currently considered vulnerable could plausibly be invaded.
  • Results of suitability modeling indicated that western Montana, the Black Hills region, the Laramie Range in southeastern Wyoming, and parts of the Colorado Front Range and Western Slope are at greatest risk of invasion by hybrid toadflax. 

Available external publications

McCartney, Kevin R.; Kumar, Sunil; Sing, Sharlene E.; Ward, Sarah M. 2019. Using invaded-range species distribution modeling to estimate the potential distribution of Linaria species and their hybrids in the U.S. northern Rockies

McCartney, Kevin R., 2017. Using ecological niche modeling to identify the potential range of novel invasive toadflax genotypes in the U.S. Northern Rockies

Featured Publications

Boswell, Andrew ; Sing, Sharlene E. ; Ward, Sarah M. , 2016
Ward, Sarah M. ; Fleischmann, Caren E. ; Turner, Marie F. ; Sing, Sharlene E. , 2009
Wilson, Linda M. ; Sing, Sharlene E. ; Piper, Gary L. ; Hansen, Richard W. ; De Clerck-Floate, Rosemarie ; MacKinnon, Daniel K. ; Randall, Carol Bell , 2005

Principal Investigators: 
Principal Investigators - External: 
Sarah M. Ward - Colorado State University
External Partners: 
Boswell, Andrew (Co-Investigator) – DuPont Nutrition and Health, Madison WI
Kevin R. McCartney (Co-Investigator) – Grouse Mountain Consultants, Buffalo WY