Plastid DNA Analysis Reveals Cryptic Hybridization in Invasive Dalmatian Toadflax (Linaria dalmatica) Populations

Abstract

Gene flow between Dalmatian toadflax (DT) and yellow toadflax (YT), both aggressive invaders throughout the Intermountain West, is creating hybrid populations potentially more invasive than either parent species. To determine the direction of gene flow in these hybrid populations, species-diagnostic cytoplasmic markers were developed. Markers were based on polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) polymorphisms in the trnT-D chloroplast DNA (cpDNA) region digested with Alu1, and single-nucleotide polymorphisms (SNPs) in the matK and trnL-F chloroplast-barcoding regions. Four hybrid toadflax populations sampled from Colorado, Montana, and Washington contained both DT and YT cytoplasm, with YT predominating; 25 individuals from a fifth hybrid population from Idaho all had identical YT cpDNA haplotypes. Thirteen plants from two Colorado populations, assumed to be DT based on morphology and geographic isolation from any known YT population, were found to have YT cpDNA haplotypes. These results indicate that gene flow between invasive YT and DT populations is more widespread that previously realized and confirms that cryptic introgression of YT alleles has occurred in multiple western U.S. DT populations. The presence of YT genetic material in presumed DT populations may negatively affect host recognition and establishment by biocontrol agents used for toadflax management. Nomenclature: Dalmatian toadflax, Linaria dalmatica (L.) P. Mill.; yellow toadflax, Linaria vulgaris P. Mill.

Using invaded-range species distribution modeling to estimate the potential distribution of Linaria species and their hybrids in the U.S. northern Rockies

“…It is unknown when co-invasion and gene flow between L. dalmatica and L. vulgaris first occurred in our study area of Montana, Wyoming, and Colorado. These interspecific hybrids were confirmed relatively recently (Ward et al 2009), although cryptic hybrids resulting from several generations of backcrossing may be more widespread in invasive Linaria populations than previously realized (Boswell et al 2016). Early-generation hybrids betweenL.…”

“…We confirmed the taxonomic identity of all hybrid populations recorded as occurrence points, based on intermediate morphology as described in Ward et al (2009). For putative hybrid populations for which plant morphology was ambiguous, we used cpDNA and ncDNA markers as described by Boswell et al (2016) and Ward et al (2009) to confirm taxonomic identity. See Figures 1-3 for occurrence maps for each taxon.…”

“…Hybrid Linaria populations incorrectly reported as L. dalmatica represent another potential source of field-sampling bias (SES, personal observation). Some L. dalmatica populations that appear to be taxonomically unambiguous based on plant morphology have been found to contain L. vulgaris DNA, introduced by an earlier hybridization event followed by repeated backcrossing (Boswell et al 2016). The frequency of L. dalmatica and L. vulgaris populations that contain introgressed DNA from the other species is unknown.…”

“…Both species are perennial forbs that generally colonize disturbed areas (Arnold 1982;Vujnovic and Wein 1997), though L. vulgaris has also invaded intact native plant communities in high-elevation wilderness areas (Sutton et al 2007). These Linaria species are obligate outcrossers that can spread via roots and rhizomes (Saner et al 1995;Vujnovic and Wein 1997), and both have high levels of intraspecific genetic diversity within and among invading populations throughout their North American ranges (Boswell et al 2016;Brown 2008;Ward et al 2008).…”

“…Hybrid Linaria populations present additional management challenges, as currently available chemical and biological controls are often less effective on these populations (Sing et al 2016). Accurate taxonomic identification can be complicated by decreasing morphological differences between late-generation backcrossed hybrids and the recurrent parent (Olson et al 2009), leading in the case of Linaria hybrids to deployment of biocontrol agents that are suboptimally matched with the host plants (Boswell et al 2016;Sing et al 2016).…”