Mentzelia pectinata Kellogg var. chrysopetala Keil & Brokaw, var. nov., is a yellow-petaled race of the otherwise orange-petaled M. pectinata, a species endemic to central California. The yellow-and orange-petaled varieties are largely allopatric, with var. chrysopetala nearly restricted to San Luis Obispo County, whereas var. pectinata extends eastward into the dry hills and mountains around the southern end of the San Joaquin Valley, including eastern portions of San Luis Obispo County. Petals of var. chrysopetala are golden yellow, usually with orange bases, unlike the petals of var. pectinata, which are coppery orange with red-orange bases. We designate a neotype for M. pectinata because original material, which presumably was in CAS, apparently is no longer extant; the holotype was probably destroyed in the 1906 San Francisco earthquake and fire. Mentzelia pectinata Kellogg is an annual species endemic to the southern part of California's San Joaquin Valley and the surrounding hills and mountains of the South Coast Range, Transverse Range, and foothills of the southern Sierra Nevada. In spring growing seasons with abundant rainfall, these plants often form large populations, and the mass displays of their flowers form conspicuous splashes of color across the landscape. The species exhibits two strikingly different flower color forms. Plants from a large part of its range have coppery-orange petals with red-orange bases. Plants from the southern Diablo Range through central San Luis Obispo County and south-central Monterey County have golden yellow petals, usually with orange bases. Taxonomic treatments of these plants have varied. Kellogg (1863) originally described Mentzelia pectinata with "flowers of a shining golden color, with a lustrous metallic hue. shading from a deep, vivid orange to a burnt carmine center." Jepson (1925, 1936) treated M. pectinata as part of a broadly defined M. gracilenta Torr. & A.Gray, describing the petals of the species as "yellow with orange base." Munz (1959) recognized M. pectinata and described the petals as "orange above, coppery-red toward base." Hoover (1970) treated the orange-flowered plants as M. pectinata and the yellow-flowered plants as M. gracilenta. He noted that that flower color appears to be the only reliable differentiation between the two kinds of plants, but that they are distinct ecologically, geographically, and apparently genetically. Brokaw (2016) distinguished diploid M. pectinata from tetraploid M. gracilenta using several characters, including (1) orange petals (in most collections of M. pectinata, absent in M. gracilenta), (2) bracts with white bases (in most collections of M. gracilenta, absent in M. pectinata), and (3) elevation/habitat (M. pectinata occurs at lower elevations than M. gracilenta where the ranges overlap in San Luis Obispo County).
Keil, David J. and Brokaw, Joshua M., "Mentzelia pectinata var. chrysopetala (Loasaceae), a yellow-petaled race from west-central California" (2020). Biology. 5.
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