Tiny stars: Four cryptic Cacti of Northern Arizona

Evolution of the Xerocarpa clade (Opuntia; Opuntieae): Evidence for the Role of the Grand Canyon in the Biogeographic History of the Iconic Beavertail Cactus and Relatives

“…Pinkava [ 37 ] described the origin and spread of O. basilaris and varieties from northeast to southwest (i.e., from the sands of southeastern Utah to the mountains and lower desert regions of the Sonoran Desert and southern California mountains). This hypothesis of origin and spread of the O. basilaris complex is congruent with our phylogenetic hypothesis of relationships in the group and results from our biogeographic analyses.…”

“…On the contrary, Pinkava [ 37 ] suggested that the O. polyacantha group originated in Mexico, south of most of its current distribution, and then subsequently spread north. Based on our data, it appears most likely that the Polyacantha clade originated, like the O. basilaris clade, in the Colorado Plateau, from where it spread south into the Chihuahuan and Mojave deserts.…”

“…Majure et al [ 45 ] showed that diploids in the Humifusa complex occupied more southerly limits of the distribution of that clade, while polyploids mostly formed around diploid populations and moved further north into previously glaciated parts of their current range. Diploid members of the Macrocentra clade, such as O. macrocentra , are found further south in the Chihuahuan Desert, for example in the Big Bend area of southern Texas, while polyploid derivatives are found further north around Alpine, Texas and then north into New Mexico [ 28 , 37 ]. Diploid relatives of the O. engelmannii complex (e.g., O. cuija/cantabrigiensis ) are found in the states of Querétaro and San Luís Potosí, Mexico, while more northern distributions of close relatives mostly consist of hexaploids, for instance in western United States populations of O. engelmannii s.l., thus, showing a strikingly similar pattern of northward migration and polyploid formation.…”

“…Intriguingly, the Polyacantha and Basilaris clades had not been considered close relatives by Parfitt [ 27 ] or Pinkava [ 30 , 37 ], presumably based on their seemingly divergent morphologies. However, Majure et al [ 9 ] showed that the two clades were sisters, of relatively old age (Pliocene in origin), and shared the putative synapomorphy of dry fruit.…”

“…It is curious that the diploids in the Xerocarpa clade are restricted to both cold (Colorado Plateau) and warm (Chihuahuan, Mojave, and Sonoran) deserts, although essentially all other North American Opuntias appear to be derived from warm deserts (i.e., Chihuahuan Desert) or tropical areas, such as seasonally dry tropical forests (e.g., Nopalea clade sensu Majure et al [ 9 ]). Pinkava [ 37 ] hypothesized that the O. basilaris complex likely originated on the Colorado Plateau, while the O. polyacantha complex originated further south in the Chihuahuan Desert in Mexico, this idea owed mostly to their current distribution. In situ radiations within western North American drylands are poorly studied, and this is especially the case for taxa in and around the Grand Canyon region of the Colorado Plateau, a major geological feature that formed over the past nearly 20 million years (mya; [ 38 ]), but with most erosional depth occurring presumably within the last 5–6 mya [ 39 ].…”