Flora of North America species comparison

Species ca. 22 (21 in the flora).

Both homoploid hybridization and allopolyploidy have played important roles in the diversification of sect. Trachyphytum (J. M. Brokaw 2009; Brokaw and L. Hufford 2010, 2010b). H. J. Thompson and H. Lewis (1953) and J. E. Zavortink (1966) reported diploid, tetraploid, hexaploid, and octoploid species in the section. The small desert and grassland annuals of this section are often difficult to distinguish due to the simplicity of their shoot systems, inter-population variability, and morphological intermediacy of allopolyploids. In contrast, homoploid hybridization appears to be rare among existing populations. Evidence of homoploid hybridization has been found primarily in molecular studies (Brokaw and Hufford 2010) and has been observed in natural populations only among hexaploid species (Zavortink).

Seed characteristics have been some of the most reliable characters for discrimination of the common, widespread species but often require an advanced developmental stage for observation. However, the two major clades in sect. Trachyphytum, “Affines” [containing Mentzelia affinis, M. dispersa, and M. micrantha (J. M. Brokaw and L. Hufford 2010)] and “Trachyphyta” [containing all other diploids and most polyploids (Brokaw and Hufford 2010, 2010b)], can be distinguished relatively early in seed development. Even when immature, plants of the “Affines” clade have a single triangular ovule in the cross-sectional view of the top of the ovary. With the exception of the narrowly distributed M. packardiae, immature plants of the “Trachyphyta” clade have two or more ovules at the top of the ovary. Further, these two groups differ in the size of their seed coat cells, which can be distinguished early in seed development: cells of the “Affines” clade are small and barely discernible at 10× magnification, whereas those of the “Trachyphyta” clade are large and clearly visible at 10× magnification, resulting in a tessellate to tuberculate appearance of the seed coat.

The only species in sect. Trachyphytum not found in the flora area is the South American Mentzelia bartonioides (C. Presl) Urban & Gilg.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Genera 20, species ca. 350 (4 genera, 94 species in the flora).

Loasaceae are in Cornales and are the sister family of Hydrangeaceae. The two families likely diverged in western North America or Mexico in the Late Cretaceous to Paleocene, 92–58 million years before present (J. J. Schenk and L. Hufford 2010). Most extant species of Loasaceae in western North America date to the Oligocene or more recent times, with the Miocene and later periods being especially rich phases for speciation (Schenk and Hufford; J. Grissom and Hufford, unpubl.). In Loasaceae, Eucnide and Schismocarpus S. F. Blake, a genus restricted to southern Mexico, are the earliest diverging lineages, and the cliff-dwelling habits of these two genera may be plesiomorphic for the family (Hufford et al. 2003). The species-rich genus Mentzelia is well supported as the sister of a clade that consists of the taxonomically depauperate genera Cevallia, Fuertesia Urban, Gronovia Linnaeus, and Petalonyx. Phylogenetic studies support the monophyly of subfamilies Gronovioideae Fenzl and Loasoideae Gilg as conceived by I. Urban and E. Gilg (1900), but their early concept of Mentzelioideae as including both Eucnide and Mentzelia (and also Schismocarpus according to S. F. Blake 1918b; Gilg 1925b) is paraphyletic (Hufford et al.). To enable classification based on monophyly, circumscription of Mentzelioideae should be restricted to Mentzelia. The Mentzelia plus Gronovioideae clade is sister to the more taxon rich subfam. Loasoideae, which is especially prevalent in Andean South America (Hufford et al.).

(Discussion copyrighted by Flora of North America; reprinted with permission.)