Which plants the current name includes
Sclerocactus glaucus was formerly also described as Echinocactus glaucus and Pediocactus glaucus. Historical circumscriptions of the species covered a larger area than the current one. The Utah populations were separated into S. wetlandicus and S. brevispinus, and the northern Colorado populations received the name S. dawsoniae in 2024. Older data on S. glaucus in the broad sense can be assigned to present-day species using the locality and material on which the account was based. [McGlaughlin and Naibauer 2023] [2-WWW]
In its current, narrower circumscription, S. glaucus occurs in Grand Valley near Grand Junction and along the Gunnison River near Delta. Genetic analysis separated these two population groups as units within one species. The distinction between them is smaller than the difference between them and S. dawsoniae, located farther north. [McGlaughlin and Naibauer 2023]
Growth form and variation
In the material comparison cited in the genetic study, plants of present-day S. glaucus were taller and wider on average and had more spines per areole than S. dawsoniae. The differences concern quantitative traits whose values overlap partly. [McGlaughlin and Naibauer 2023]
Identification therefore combines stem structure, spination and geographical origin. The authors of the analysis emphasise the agreement between the molecular separation, distributions and morphological variation. Herbarium specimens and populations of documented origin are particularly valuable because they link an observed trait to a specific genetic lineage. [McGlaughlin and Naibauer 2023]
How DNA differences were read
McGlaughlin and Naibauer used ddRAD, a method for comparing thousands of DNA fragments selected with enzymes that cut the genome at specific sites. The taxonomic analysis included 208 plants and 20 991 polymorphic DNA positions from 3537 loci. Here, a locus means a genome fragment under study, and a single nucleotide polymorphism, or SNP, is a difference of one base between the sequences compared. [McGlaughlin and Naibauer 2023]
The material included S. glaucus, the northern populations later named S. dawsoniae, and S. parviflorus, S. wetlandicus and S. brevispinus. Reconstruction of relationships and independent analyses of genetic similarity consistently distinguished S. glaucus and S. dawsoniae. Agreement among several analytical approaches strengthens the recognition of two lineages under the sampling design used. [McGlaughlin and Naibauer 2023]
In this dataset, the genetic differentiation coefficient FST between Grand Valley and Gunnison River was 0.08. Between each of these groups and S. dawsoniae, it was 0.25–0.26. The coefficient describes the partitioning of genetic variation between groups through differences in the frequencies of DNA variants. A higher value indicates stronger differentiation among the populations studied. [McGlaughlin and Naibauer 2023]
Structure within the species
A separate dataset of 150 plants was prepared for a more detailed assessment of structure, including 35 from Grand Valley, 40 from Gunnison River and 75 from the northern group. It used 3812 SNPs from 2476 loci. Its marker selection differed from that of the taxonomic analysis, so the two stages present two related levels of comparison. [McGlaughlin and Naibauer 2023]
Grand Valley and Gunnison River remained genetically distinguishable. Retaining information about population origin preserves this geographical dimension of variation. In the assignment analysis, five plants showed a small contribution from the other main lineage, at a level of 5–8%. The authors interpret this result as a trace of historical gene flow between the S. glaucus and S. dawsoniae lineages. [McGlaughlin and Naibauer 2023]
Diversity and conservation
In the material analysed, S. glaucus retained higher nucleotide diversity than S. dawsoniae. Nucleotide diversity describes the average number of differences between sequences. At the same time, heterozygosity, the proportion of differing variants in a pair of gene copies, and the inbreeding coefficient were similar. The individual measures therefore reveal different properties of the same genetic resource. [McGlaughlin and Naibauer 2023]
A US federal decision of 29 May 2025, effective from 30 June 2025, removed the entity protected as the Colorado hookless cactus from the federal list of threatened species. The decision covers both species following their separation, S. glaucus and S. dawsoniae. Older articles refer to the legal status before this change. [22-WWW]
Related articles
Materials from the author's archive
The species also appears in Konrad Szychowski's private material archive. [Bydgoski Klub Miłośników Kaktusów 2002, file p. 7]
Synonyms and names in the literature
Echinocactus glaucus · Ferocactus glaucus · Pediocactus glaucus · Sclerocactus whipplei var. glaucus · Sclerocactus franklinii [2-WWW]