Krogia Microphylla

''Krogia'' is a genus of corticolous lichens in the family Ramalinaceae. It occurs in tropical humid forests and rainforests. The genus was circumscribed by Norwegian lichenologist Einar Timdal in 2002, with '' Krogia coralloides'' assigned as the type species. These tiny lichens form distinctive coral-like cushions on tree bark, with their scale-like lobes () divided into finger-like branches that give them a miniature branching appearance. The genus is quite rare, with only six known species found scattered across mountain forests from the Caribbean to Southeast Asia and the Pacific islands.

Taxonomy

The Norwegian lichenologist Einar Timdal circumscribed ''Krogia'' in 2002 on the basis of the Mauritian type species ''Krogia coralloides''; the genus name honours his long-time colleague Hildur Krog. Timdal placed it close to '' Phyllopsora'' but pointed out several decisive anatomical differences: the ascus wall shows only a faint amyloid reaction, the thread-like ascospores are barely 1 micrometre (μm) thick and often coil inside the ascus, the thallus lacks a true medulla yet possesses a distinct lower , and the chemistry is dominated by boninic acid rather than the depsidones common in ''Phyllopsora''. Because these characters do not fit comfortably into the Bacidiaceae concept then in use, Timdal left the familial placement open, although subsequent workers have treated ''Krogia'' within the Ramalinaceae (''sensu lato'') alongside ''Phyllopsora''.

The genus remained monospecific for several years, but concerted collecting in tropical cloud forests has since uncovered additional taxa. '' Krogia antillarum'' was described from the Greater and Lesser Antilles in 2009, differing from the type in squamule shape and secondary chemistry; '' K. microphylla'' followed in 2011 from a single Dominican cloud-forest site, its epithet referring to its minute lobes. A phylogeny-guided revision in 2018 added three Paleotropical species—'' K. borneensis'' (Borneo) plus '' K. isidiata'' and '' K. macrophylla'' (both New Caledonia)—raising the current total to six. All are corticolous inhabitants of humid, montane forests and appear to be rare, each known from only a handful of localities.

Molecular data place the six ''Krogia'' species in a well-supported clade that is sister to ''Phyllopsora'' ''sensu stricto'' within the Ramalinaceae, confirming that the unique ascus and chemical profile evolved within that family rather than indicating a separate lineage. The genus can therefore be regarded as a diminutive tropical branch of the otherwise mainly epiphytic ''Phyllopsora'' complex.

Description

''Krogia'' forms minute, coral-like cushions composed of numerous overlapping scale-lobes. Each is at most about 1 mm wide but is divided into forked, finger-like branches barely 0.1 mm across that stand slightly upright and interlock, giving the thallus a finely branched appearance. The surface is grey-green and smooth, yet it is often flecked with tiny crimson patches that instantaneously turn violet-purple when touched with potassium hydroxide solution (the standard K test). No powdery reproductive granules (soredia) or wart-like outgrowths (isidia) are produced. The underside is pale brown to whitish, showing a thin lower "skin" (), while a distinct (margin of fungal strands) is absent.

A thin, gel-like upper cortex—essentially a protective outer layer of tightly cemented fungal threads about 15–30 μm thick—covers the upper surface; unlike many lichens it bears neither crystalline deposits nor traces of embedded algal cells. Just beneath this lies a narrow green , roughly 30–50 μm deep, whose cells are only about 10 μm in diameter and are sprinkled with microscopic crystals that dissolve in the K test. ''Krogia'' lacks a true medulla (the loose inner fungal layer common in many other lichens), but it does have a second, pale lower cortex up to 30 μm thick where the fungal filaments are longer-celled and loosely bound. Chemical analyses show the thallus to contain boninic acid—an uncommon orcinol meta-depside in this group—together with a closely related compound; under ultraviolet light the thallus therefore fluoresces white.

The lichen reproduces with small, pale brown to medium-brown disc-like apothecia (fruiting bodies) that sit directly on the squamules and seldom exceed 1 mm in diameter. Their margin is inconspicuous and bears minute projecting fibrils at the base. Internally, the hymenium (spore-bearing tissue) is low, only about 40 μm high, and stains weakly with iodine, while the surrounding tissues are densely gelatinised. Each narrowly club-shaped ascus produces eight exceedingly slender, curved ascospores, 22–30 μm long and roughly 1 μm wide, that often coil in a loose spiral and may show one to three faint partition lines. Asexual spores (pycnidia) have not been observed in this genus. These distinctive thread-like spores, coupled with the absence of a medulla and the presence of a lower cortex, separate ''Krogia'' from its superficially similar relative ''Phyllopsora''.

Species

, Species Fungorum (in the Catalogue of Life) accept six species of ''Krogia'':
*'' Krogia antillarum'' – West Indies
*'' Krogia borneensis'' – Borneo
*'' Krogia coralloides'' – Mauritius
*'' Krogia isidiata'' – New Caledonia
*'' Krogia macrophylla'' – New Caledonia
*'' Krogia microphylla'' – Dominican Republic

References

{{Taxonbar, from=Q6438766

Ramalinaceae
Lecanorales genera
Lichen genera
Taxa described in 2002
Taxa named by Einar Timdal