Diorygma megasporum was introduced by Klaus Kalb, Bettina Staiger and John A. Elix on the basis of a 19th-century collection from mixed montane forest near Yomah (Pegu Yoma, Myanmar). The epithetmegasporum reflects its unusually large, densely partitioned ascospores. Within Diorygma the taxon stands out by combining very broad lirellae with asci that hold only two to six spores—an arrangement otherwise rare in the genus—and by a chemistry dominated by stictic-series metabolites (stictic, O-acetylconstictic and constictic acids, with traces of norstictic and hypostictic acids). Its overall spore shape is reminiscent of D.hololeucum, but that species contains protocetraric acid and has far more robust, bulging lirellae, while other large-spored members of the genus differ in either hymenial height or iodine reactions.[2]
Description
The lichen forms a thin, matt crust 50–100micrometres (μm) thick that ranges from off-white through cream to pale grey-green. The surface is uneven, cracked along the elongated fruiting slits, and lacks a true outer skin ( pseudocortex ). Green algal cells reach almost to the surface, while a medulla 0–10μm below may hold tiny crystals.[2]
Fruiting bodies (lirellae) are plentiful, measuring 0.5–4 × 0.2–0.7mm. They lie immersed at first, but their pale, powder-dusted flanks often appear through fine cracks as the thallus matures. Margins are indistinct, only rarely rising into a thin rim separated from the crust by a slit. The disc itself opens narrowly, carrying a thick pale-grey to buff bloom ( pruina ). The supporting wall ( excipulum ) is inconspicuous and yellowish at the base, laterally pale yellow-brown and built from loosely interwoven, swollen hyphae.[2]
Inside, a clear hymenium 180–200μm tall stains bluish-violet in iodine along its upper and outer zones, while the top 30μm are brown owing to a well-developed, partly carbonised epithecium. Supporting threads (paraphyses) are only about 1μm wide, run close together and branch laterally into a delicate network. Each broadly club-shaped ( clavate ascus contains two to six colourless, thick-walled spores. These spores are conspicuously large, measuring 80–170 (more rarely up to 220)μm long by 21–55μm wide, and divided by both transverse and longitudinal walls into dozens of equally sized cubicles ( muriform ). Spore size can vary markedly within a single ascus; some are wrapped in a 2–3μm gelatinous sheath and display a faint violet internal staining reaction in iodine. No asexual pycnidia have been observed, and standard spot tests give negative results, consistent with the stictic-series chemistry detected by thin-layer chromatography.[2]
Habitat and distribution
The holotype grew on tree bark at about 2000m elevation in the eastern Himalaya, and additional specimens extend the range southward to the Sikkim Himalaya (Tonglo, 1800–2100m elevation) and westward to the moist evergreen forests of the Western Ghats (Amboli, roughly 1050m). Across this latitudinal span the species occupies humid montane or submontane rainforest where frequent mists, moderate light and constant air movement create a damp but aerated microclimate on smooth bark. At the time of its original publication, D.megasporum was regarded as an Indian endemic restricted to well-watered mid-elevation forests in both the eastern and western mountain chains.[2] It was later identified from China, indicating a broader pantropical distribution. It is one of ten Diorygma species that have been recorded in China.[3]
1 2 3 4 5 Kalb, Klaus; Staiger, Bettina; Elix, John A. (2004). "A monograph of the lichen genus Diorygma – a first attempt". Symbolae Botanicae Upsalienses. 34 (1): 133–181 [160].
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