Chroodiscus Graphideus

''Chroodiscus'' is a genus of leaf-dwelling lichens in the family Graphidaceae. These lichens form thin, smooth crusts directly on living leaves and are characterized by distinctive star-shaped fruiting bodies that split into triangular segments, with centres ranging from grey to bright scarlet-red depending on their chemical composition. The genus is found throughout tropical regions worldwide, growing in the shaded understory of rainforests from lowland areas to mountain cloud forests, where they serve as sensitive indicators of undisturbed forest conditions.

Taxonomy

''Chroodiscus'' was first introduced by the Swiss lichenologist Johannes Müller Argoviensis in 1883 as a section of the genus ''Ocellularia''. In his original description, Müller characterized the section by its layer being phyllactideum (containing green algae arranged in a specific pattern) and its apothecia (fruiting bodies) being pale or variously coloured. He described the type species, ''Ocellularia argillacea'', as having a clay-coloured to pale or greenish-clay coloured thallus that was very thin, smooth, and interrupted by small globular swellings. The apothecia were measured at 0.2 to 0.5 millimetres wide and described as having broad, prominent margins. In 1890 he promoted the section to generic status.

Description

''Chroodiscus'' forms a thin, smooth to slightly glossy crust (thallus) that grows directly on living leaves (foliicolous) and lacks a true . Its ascomata are strikingly : minute discs that split into several triangular lobules which then curl back to give a star-shaped outline; the exposed centre ranges from grey to scarlet-red, according to pigment chemistry. A pale, borders each disc, while the clear hymenium contains lax, branched paraphyses and no . The asci are ''Thelotrema''-type (thin-walled, iodine-negative) and usually release eight hyaline ascospores that are transversely 3–7-septate, thin-walled and non-amyloid (about 12–30 × 4–7 Î¼m). Many species synthesise orange-red anthraquinones or stictic acid-derivatives that tint the discs; others are chemically inert.

Species differ mainly in disc colour, spore size and chemistry: for example, ''C. argillaceus'' has grey discs and stictic acid, whereas ''C. mirificus'' develops vivid red discs rich in anthraquinones. The combination of star-shaped discs, absence of periphysoids and thin-walled, non-amyloid spores separates ''Chroodiscus'' from superficially similar genera such as '' Chapsa'', '' Astrochapsa'' and '' Pseudochapsa'', whose discs lack the lobules or whose spores react differently with iodine.

Ecology

The genus is strictly pantropical: records span Central and South America, West and Central Africa, Madagascar, India, Southeast Asia and north-east Australia. All known species are obligately foliicolous, colonising the shaded, humid understory of evergreen rainforests from lowland plains up to the montane cloud forest belt. A phenotype-based phylogeny indicates a South American origin in the mid-Cretaceous (roughly 120–100 Ma) followed by stepwise dispersal through Africa, India and Southeast Asia, with most present-day taxa now restricted to a single continent. Because they depend on intact, moist canopy microclimates, ''Chroodiscus'' species are considered sensitive indicators of undisturbed tropical forest and tend to disappear after forest clearance or prolonged drought.

Species

, Species Fungorum (in the Catalogue of Life) accepts 19 species of ''Chroodiscus'':

*'' Chroodiscus africanus''
*''Chroodiscus anomalus''
*'' Chroodiscus argillaceus''
*'' Chroodiscus australiensis''
*'' Chroodiscus australis''
*'' Chroodiscus coccineus''
*'' Chroodiscus defectus''
*'' Chroodiscus graphideus''
*'' Chroodiscus homchantarae''
*'' Chroodiscus khaolungensis''
*'' Chroodiscus khaosokensis''
*'' Chroodiscus mirificus''
*'' Chroodiscus neotropicus''
*'' Chroodiscus parvisporus''
*'' Chroodiscus subhimalayanus'' – India
*'' Chroodiscus submuralis''
*'' Chroodiscus verrucosus''

References

{{Taxonbar, from=Q5114276

Graphidaceae
Taxa described in 1883
Graphidales genera
Taxa named by Johannes Müller Argoviensis
Lichen genera