It's not very scientific to sort plants by how interesting they look. After all, why should a plant's place in science—the accidents of its evolution—also tickle our human sense of beauty? Why should a plant with scientific interest also match the desires of home growers of cacti and succulents?
What does surprise me is how often plants which stick out in the science of plant naming—plants from a monospecific genus—are also interesting to look at.
A monospecific genus is a genus with only one species. Sometimes this is an isolated population with millions of years of adaptation to a particular evolutionary niche—long enough to diverge from other genera. In other cases, plants from a monospecific genus may be highly variable in size and shape (or habit) and may occur over a wide region. The physical differences between populations are simply too slight, too insignificant to warrant different species names (aka specific epithets).
Ortegocactus is a monospecific genus thought to be isolated for much of its recent evolution. In habitat, Ortegocactus macdougallii is known from one small area—the limestone scree and escarpments of a hill known as El Cántaro, 4km west of San José Lachiguiri, Oaxaca, Mexico.The exact position of Ortegocactus in the cactus evolutionary tree has been debated. Recent molecular phylogenetic work by Butterworth places it close to genus Neolloydia and Cochemiea and to the series Ancistracanthae within the enormous genus Mammillaria, now seen to be polyphyletic.
See AlsoOrtegocactus at Desert-Tropicals.com
Ortegocactus at Cactus-Art.Biz
Ortegocactus at WelcomeToCactusLand.Com
Ortegocactus at CactusCristateMania.blogspot.com
San José Lachiguiri at the Encyclopedia of Mexico Municipalities, State of Oaxaca
Butterworth, Charles A. and Wallace, Robert S. 2004. Phylogenetic studies of Mammillaria (Cactaceae)—insights from chloroplast sequence variation and hypothesis testing using the parametric bootstrap. American Journal of Botany. 91:1086-1098.
Addendum: quote from Butterworth's paper and commentary:
Clade A [a group which appears to have evolved from a single population] includes sampled members of the genera Coryphantha, Escobaria, and Pelecyphora, which form sister lineages to sampled taxa of Hunt’s and Lu ?thy’s series Ancistracanthae and subgenus Cochemiea, respectively, thus clearly demonstrating paraphyly within Mammillaria. Furthermore, within the core group of series Ancistracanthae sensu Hunt and subgenus Cochemiea sensu Lu ?thy, our phylogeny places Ortegocactus macdougallii and Neolloydiaconoidea. Discovered by MacDougall in the early 1950s and described by Alexander (1961), Ortegocactus macdougallii has been contentious in its placement in relation to other members of tribe Cacteae. Bravo-Hollis and Sanchez-Mejorada (1991) sank this genus into Neobesseya, members of which are now commonly accepted as species of Escobaria (Hunt, 1992, 1999; Barthlott and Hunt, 1993). Hunt and Taylor (1986, 1990) suggested that Ortegocactus may be referable to the genus Mammillaria, although an official transfer to Mammillaria was not made. Barthlott and Hunt (1993) also commented on the similarities of Ortegocactus and Mammillaria, going so far as to suggest that Ortegocactus is reminiscent of M. schumannii. Butterworth et al. (2002) also suggested that Ortegocactus shared a greater affinity with members of Mammillaria than with Escobaria or Coryphantha. The data presented in this paper do indeed show that O. macdougallii is embedded within members of Mammillaria, its closest Mammillaria relatives including M. schumannii. However, at present the transfer of Ortegocactus to Mammillaria would be inappropriate because of the polyphyletic nature of Mammillaria as seen in our analyses.
Translation: Ortegocactus appears to share the same lineage of many Mammillaria, so much so that its DNA (and the model we reconstruct from this DNA of Ortegocactus evolution) would justify calling it a Mammillaria... EXCEPT that other genera also have this same problem, too—Neolloydia, Cochemiea, some Coryphantha and Escobaria. They all seem to be closely embedded within the evolutionary tree of what we otherwise call Mammillaria. So it would be hasty to rename Ortegocactus until we figure out what to do with ALL these naming problems.