Synarthonia bicolor
Synarthonia bicolor is a species of corticolous crustose lichen of uncertain familial classification in the order Arthoniales. This tropical lichen forms thin, whitish-grey crusts on tree bark and is distinguished by its fruiting bodies that start as single structures but later cluster together into compound formations surrounded by white margins. It is chemically unique within its genus due to the presence of lichexanthone, a lichen product that causes the thallus to glow bright yellow under ultraviolet light. The species has a disjunct distribution, being known from Costa Rica where it was originally described and from mangrove forests in the Sundarbans region of India.
Taxonomy
Synarthonia bicolor was described by Johannes Müller Argoviensis in 1891 from material collected by Henri François Pittier near San José, Costa Rica. The species belongs to a genus characterised by ascomata that start out solitary and later aggregate into clusters embedded in a slightly raised to immersed with a thin white margin; this developmental pattern is well documented in S. bicolor. Within Synarthonia, S. bicolor is set apart chemically and microscopically: its thallus contains lichexanthone and the hymenium reacts iodine-positive red.Description
The thallus forms a thin, closely adherent crust on bark, white to whitish grey, with a narrow dark brown border line; calcium oxalate crystals are absent. Young fruiting bodies start out single with a well-developed white rim, but they soon begin clustering together: around the edges of a colony they form small groups of 1–3 fruiting bodies, while in the center they merge into larger compound structures that can contain many fruiting bodies fused together. are usually, only rarely thinly pruinose.Internal anatomy is typical for the genus: the is hyaline to pale brownish; the is brownish to greyish with some gelatinous material and crystals; the hymenium is clear and iodine-positive directly red ; are richly branched with slightly thickened brownish tips; and the is hyaline to pale yellowish and also I+.
Ascospores are hyaline, becoming pale brownish when mature, macrocephalic, and 4–5-septate, measuring about 20–24 × 7–8.5 μm. In terms of spot test reactions, thallus is K−, C−, and P−, but glows bright yellow under long-wave ultraviolet owing to lichexanthone; within the genus, S. bicolor is the only species with this UV-positive yellow thallus linked to lichexanthone.