Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2002. Lichen Flora of the Greater Sonoran Desert Region. Vol 1.
Thallus: foliose, adnate to loosely adnate, 3-15 cm in diam., lobate lobes: subirregular, elongate, slightly imbricate, plane, separate, 8-15 mm wide; apices: rotund, ciliate; cilia: sparse, 0.3-2.0 mm long upper surface: gray with some blackened areas, smooth, dull to shiny, sometimes white pruinose, usually white maculate soredia: granular, common, in linear, marginal to submarginal, soralia; isidia and pustulae: absent medulla: white with continuous algal layer lower surface: black with brown to mottled white naked zone peripherally, centrally rhizinate; rhizines: scattered, simple Apothecia: very rare, substipitate, up to 5 mm in diam.; margin: sparsely sorediate; disc: brown, imperforate ascospores: ellipsoid, 22-30 x 13-16 µm Pycnidia: not seen Spot tests: upper cortex K+ yellow, C-, KC-, P-; medulla K+ yellow turning orange, C-, KC-, P+ orange Secondary metabolites: upper cortex with atranorin and chloroatranorin; medulla with norstictic acid (major) and connorstictic acid (minor). Substrate and ecology: usually on trees in open habitats, rarely on rocks World distribution: eastern USA and NW Mexico Sonoran distribution: southern, coastal regions of California and Baja California, Baja California Sur, Sonora and Sinaloa. Notes: Culberson (1973) proposed a new taxonomy based on the assumption that chemical evolution had occurred prior to morphological differentiation. The result is that P. hypoleucinum, P. hypotropum and P. louisianae are all morphological nearly identical. The species have not yet been investigated from a molecular perspective.
Type: USA [at the time of the collection still part of MEXICO]. Texas, Comancharies, Berlandier, J.L. 2093 & Trécul, A. (P – holotype, H – isotype, fide Hale 1965).
Description.Thallus corticolous; uppersurface white to whitish gray, dull to shiny, epruinose, densely and very finely maculate, the maculae effigurate, typically not cracked; abundantly sorediate; soralia initially marginal, labriform to crescent-shaped, often merging and becoming subcapitate, rarely ± marginal and then ± convex-capitate; soredia farinose, surface often discolored by a ± grayish tinge, pale inside; lobes moderate-sized, 2–7 mm wide, rotund, axils ± rounded, margins sparsely to abundantly ciliate; cilia short and slender, 0.3–1.5 mm long, black, mostly simple, very rarely branched; lowersurface often blackened throughout or becoming dark brown towards the margin or, especially below the soredia, often mottled white, typically with a distinct, 1–2 mm wide, erhizinate marginal zone; rhizines long, slender, black, mostly simple, rarely sparsely branched; medulla white. Apothecia and pycnidia not observed among the Galapagos specimens.
Chemistry. Cortex with atranorin [P+ yellow, K+ yellow, KC–, C–, UV–]; medulla with norstictic, connorstictic and stictic acid [P+ yellow, K+ yellow, soon turning orange (crystals), KC–, C–, UV–].
Ecology and distribution. North America (Culberson 1973; Egan et al 2016); in South America only known from the Galapagos, first reported by Weber (1986) and subsequently by Elix & McCarthy (1998). A corticolous species found only on a few islands; all specimens collected in the dry zone, all on Burseragraveolens.
Notes. Abundantly maculate specimens of P. hypotropum may resemble those of P. reticulatum or P. clavuliferum, although its maculae are effigurate and generally much less conspicuous, forming a finer pattern that typically does not break into distinct cracks. The species further differs by its medullary chemistry (with stictic and norstictic instead of salazinic acid). Unlike P. clavuliferum, the soralia of P. hypotropum do not develop at the tip of elongate, laciniate lobes (clavulae); they are instead initially lining the lobe margins and resemble more closely the linear marginal soralia of P. reticulatum. However, as observed in P. clavuliferum, the surface below soredia of P. hypotropum is typically ‘mottled’, i.e., large patches of white, unpigmented areas are splotched among the otherwise uniformly deep brown to soon-blackened lower side.