Thistle be a mess: Untangling the taxonomy of Cirsium (Cardueae: Compositae) in North America

Abstract

Cirsium (i.e., "thistles") are one of the most taxonomically challenging groups of Compositae in North America. This study represents the first attempt to infer a broadly sampled phylogeny of Cirsium in North America. The two main objectives are to: (i) test whether currently hypothesized species variety complexes (C. arizonicum, C. clavatum, C. eatonii, and C. scariosum) constitute monophyletic lineages, and (ii) recircumscribe any taxa that are identified as problematic. Phylogeny reconstructions based on DNA sequence data from two nuclear ribosomal regions and four plastid markers were used to infer evolutionary lineages and test species' delimitations. Eight species varietal complexes were resolved as polyphyletic. We recircumscribed these complexes and in doing so found evidence to support the recognition of six new taxa. We hypothesize that the extensive taxonomic difficulty within Cirsium is the result of several factors: (i) previously undescribed taxa, (ii) inadequate representation of taxa from herbarium specimens, (iii) phenotypic convergence, (iv) hybridization, and (v) incipient speciation. While we can provide evidence to support the recircumscription of some taxa, others remain unresolved. Therefore, we are working on a phylogenomic study of North American Cirsium to answer remaining taxonomic questions as well as provide a robust framework for biogeographic studies.

Hiding in plain sight: Two new species of alpine thistles, Cirsium culebraensis and C. funkiae (Cardueae) from the southern Rocky Mountains (United States)

“…To resolve this polyphyly, varieties within the C. eatonii complex were separated out as distinct species. Previously described C. clokeyi (Ackerfield et al, 2020;Siniscalchi & Ackerfield, unpublished data).…”

“…eriocephalum and C. eatonii var. hesperium were resolved as polyphyletic as well (Ackerfield et al, 2020;Siniscalchi & Ackerfield, unpublished data). Three unique evolutionary lineages were recovered for C. eatonii var.…”

“…Molecular phylogenetic inferences utilizing both Sanger and hybridization sequencing (Hyb-Seq) data to test the monophyly of this varietal complex demonstrated that C. eatonii, as circumscribed by Keil (2006), was polyphyletic. Each variety in turn corresponded to a unique evolutionar y lineage exhibiting a distinct phenotype and geographic range (Ackerfield et al, 2020;Siniscalchi & Ackerfield, unpublished data). To resolve this polyphyly, varieties within the C. eatonii complex were separated out as distinct species.…”

Collections‐based systematics and biogeography in the 21st century: A tribute to Dr. Vicki Funk

“…With the timing of the early radiation of this five‐tribe lineage dated to be shortly before the Eocene–Oligocene extinction event (c. 34 Ma), early lineages were likely lost, hence obscuring the details of their early diversification history. Ackerfield et al (2020) have tackled the phylogeny of the thistle genus ( Cirsium L.), one of the most taxonomically challenging groups of Compositae in North America. This first phylogenetic attempt of the North American thistles suggests that the messy difficulties within Cirsium may be due to (i) previously undescribed taxa, (ii) inadequate representation of taxa from herbarium specimens, (iii) phenotypic convergence, (iv) hybridization, and (v) incipient speciation.…”

Chromosome Evolution in Korean <i>Carduus</i>-<i>Cirsium</i> Taxa (Asteraceae)

“…belongs to tribe Cardueae Cass. ; it is a taxonomically and morphologically difficult group that includes approximately 250 perennial, biennial, or rarely annual species distributed in the northern hemisphere (Bureš et al 2004, Shi et al 2011, Im et al 2017, Ackerfield et al 2020a. The genus is characterized by spiny leaves, terminal or axillary discoid capitulum, five-lobed purple to white corolla, synantherous stamen, branched style, and plumose pappus compared with other genera in the tribe (Shi et al 2011, Ackerfield et al 2020a.…”