Epithelantha

Ten accepted species

POWO accepts ten species in the genus, E. bokei, E. cryptica, E. greggii, E. ilariae, E. micromeris, E. pachyrhiza, E. polycephala, E. potosina, E. pulchra and E. spinosior. The genus's natural range includes Arizona, New Mexico, Texas and northeastern Mexico. Older North American floras focused on the two species occurring in the United States. [2-WWW]

Epithelantha potosina (formerly E. greggii subsp. potosina) and E. pulchra (formerly E. pachyrhiza subsp. pulchra) received combinations at species rank in 2019. E. spinosior now includes two accepted subspecies, subsp. spinosior and subsp. huastecana. Identifications from older publications retain information about the material distinguished by their authors. [2-WWW]

Stem covered with spines

The tubercles bear areoles at their tips that produce very dense, usually white or grey spines. Woolly areoles near the apex of a mature stem produce small flowers. In North American descriptions, stems are usually spherical or short-cylindrical, a few centimetres in diameter. The distinction between radial and central spines can be unclear. The naked red fruits are elongated and dry quickly after ripening. [Parfitt and Zimmerman 2003, vol. 4, pp. 239–240, genus description]

Habitat and evolutionary history

Aquino and colleagues compared 101 geographically separate Epithelantha sites with maps of climate, soil and topography at a resolution of about 1 km². The genus was common in foothill zones and on typical kastanozems. Individual species differed in soil type, landform and climatic seasonality. The spatial data describe the environment of the sites at an accuracy corresponding to the maps used. [Aquino et al. 2021, p. 4521–4525, ryc. 3–4]

A tree based on four regions of plastid DNA divided the genus into two major lineages. One included E. bokei, E. cryptica, E. ilariae, E. micromeris, E. potosina and E. spinosior, the other E. greggii, E. pachyrhiza, E. polycephala and E. pulchra. Among the environmental traits examined, soil pH, isothermality, temperature seasonality and annual rainfall showed a phylogenetic signal. Environmental similarity partly coincided with close relatedness. [Aquino et al. 2021, p. 4522–4526, ryc. 2, 4]

E. micromeris and E. cryptica remained very similar in the plastid markers used. They differed in morphology, range and the climate of their sites. E. micromeris also occupied a wider range of soils and landforms. The authors interpreted these differences as evidence for the ecological maintenance of species distinctness. Gene flow and local selection were not measured in a direct experiment. [Aquino et al. 2021, pp. 4525–4527]

Stem crystals and seed chemistry

Prismatic crystals have been documented in the stem cortex of E. micromeris. A separate chemical study of E. micromeris and E. greggii seeds detected no lignin using thioacidolysis and NMR spectra. The results concern different organs and processes, the deposition of mineral compounds in stem tissues and the structure of seed cell-wall polymers. These traits need to be examined separately in the other species of the genus. [De la Rosa-Tilapa et al. 2022, De la Rosa-Tilapa et al., p. 8, fig. 6I. Chen et al., pp. 203–206] [Chen et al. 2013, De la Rosa-Tilapa et al., p. 8, fig. 6I. Chen et al., pp. 203–206]

Areoles, roots and flowers in the species revision

An areole is a small zone on a tubercle where spines develop. In the 2019 revision, spine counts and areole dimensions were compared in the middle part of the stems of adult plants. This choice reduced the influence of the age of fresh areoles near the apex. Epithelantha bokei had 60–80 spines with persistent small projections on their surface. The other species had fewer spines, and their projections did not persist in the same way. E. micromeris was distinguished by very narrow areoles and flowers 6–7 mm long, and E. ilariae by flowers 17–18 mm long that opened in the evening. [Aquino et al. 2019, p. 601, 606, 608, 612, 615; ryc. 5]

Thickened roots occurred in E. pachyrhiza, E. polycephala, E. potosina and E. pulchra. In E. greggii they were ordinary and slender. E. spinosior combined slender roots with a section of stem developing underground. The root and the buried part of the stem are two different organs, identified separately in the key. The study compared their structure, without measuring root contraction or the drawing of the plant into the substrate. [Aquino et al. 2019, pp. 606, 608, 610–612, 615]

Ten species from different types of data

Aquino and colleagues collected morphological data from 195 adult plants at 39 sites in Coahuila, Nuevo León and San Luis Potosí. The DNA tree included 39 Epithelantha samples and 23 outgroup samples, analysing four regions of the plastid genome. Most named groups showed consistent morphological and molecular distinctness. E. micromeris and E. cryptica were separated by adding morphology to their very similar plastid sequences. E. potosina and E. pulchra received combinations at species rank at that time. [Aquino et al. 2019, p. 600–609, 611–612]

The seed surface also provided diagnostic traits. Scanning electron microscopy revealed uneven sculpturing spanning many seed-coat cells and expansions at the edge of the hilum region, the area of the scar left by seed attachment, in the material of E. pachyrhiza and E. pulchra examined. Such sculpturing was not found in most other groups. These are structural traits, separate from the chemical findings on lignin in seeds. [Aquino et al. 2019, p. 601–602, 608, 610–611, 615; ryc. 5M–O]

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