Evolution and morphology
Multilocus phylogenetic trees recover Cyphellostereum as a monophyletic, basal branch that is sister to all other genera in the Dictyonema clade. Occupying this early-diverging spot, the genus retains the simplest thallus: a bluish, crust-like weave of cyanobacterial filaments (trichomes) sheathed by the fungus, with no layered cortex or medulla that would distinguish an upper or lower surface. Its reproductive structures are tiny, cup-shaped cyphelloid fruiting bodies (basidiocarps) that sit on the thallus rather than being swallowed by it; the hyphae that build these cups differ markedly from the slender hyphae that merely clothe the photobiont threads. The fungal sheath is made of irregular, cylindrical cells full of air-gaps and it entirely lacks the tubular haustoria and "jigsaw-puzzle" cell walls that later genera use to tap nutrients from the cyanobacteria. The cyanobacterial partner ( Rhizonema ) forms unusually narrow cells (roughly 5–8 μm wide), and specimens often harbour extra cyanobacteria or even stray green algae, signalling a looser, more opportunistic symbiosis than is typical for the group. [3] Comparative studies also show that the Rhizonema photobiont in Cyphellostereum develops trichomes with narrower, higher, roughly quadrate cells that are more distinctly blue than the broader, low-celled forms typical of Dictyonema . [4]
Internal transcribed spacer (ITS) DNA in Cyphellostereum shows the highest length variation and alignment ambiguity within the clade, consistent with an ancient, slowly diverging lineage. Because it preserves this primitive, loosely integrated architecture while its descendants evolve ever more complex, leaf-like thalli, Cyphellostereum offers a living snapshot of the earliest steps toward basidiomycete lichenisation. [3]
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