Dictyonema
Dictyonema is a genus of mainly tropical basidiolichens in the family Hygrophoraceae. Unlike most lichens, which contain fungi related to yeasts and molds, Dictyonema species contain fungi more closely related to mushrooms. The genus includes about 40 recognized species found mainly in tropical regions, ranging from lowland forests to high mountain elevations up to 4,300 meters in the Andes. These lichens come in various forms, from crusty patches to leaf-like structures to thread-like mats, and most grow on soil, rocks, moss, or rotting logs. One species from the Amazon rainforest has traditional use by indigenous peoples as a hallucinogenic substance in shamanic rituals.
The ''Dictyonema'' symbiosis
Most lichens are a symbiosis between an ascomycete fungus and a photosynthetic green alga. However, a small percentage of lichens are cyanolichens and contain a photosynthetic cyanobacterium instead of green algae, and an even smaller number are basidiolichens and contain a basidiomycete fungus instead of an ascomycete. This makes Dictyonema more closely related to mushroom-forming fungi than it is to most other lichens.Taxonomy
The genus Dictyonema was circumscribed in 1822 by Carl Agardh and Carl Kunth after examining a novel fungus that was sent to them from Brazil. The genus was redefined in 1978 when Erast Parmasto assessed 40 different species of basidiolichens that were previously divided into 3 families and 8 genera, and reduced them to 5 species in the single genus Dictyonema. This resulted in a rather diverse group of lichens that grew in size to more than 20 species. There was, however, some debate over whether or not all of these species should be included in the same genus.Species
, Species Fungorum accept 38 species in Dictyonema.- Dictyonema aeruginosulum
- Dictyonema album – Africa
- Dictyonema andinum
- Dictyonema applanatum – Bolivia
- Dictyonema barbatum – Galápagos Islands
- Dictyonema caespitosum
- Dictyonema coppinsii – Europe
- Dictyonema darwinianum – Galápagos Islands
- Dictyonema diducens
- Dictyonema discocarpum – Bolivia
- Dictyonema duidense
- Dictyonema giganteum – Colombia
- Dictyonema gomezianum
- Dictyonema guadalupense
- Dictyonema hapteriferum – Bolivia
- Dictyonema hernandezii
- Dictyonema huaorani
- Dictyonema irpicinum
- Dictyonema irrigatum
- Dictyonema krogiae – Africa
- Dictyonema laurae
- Dictyonema lawreyi
- Dictyonema ligulatum
- Dictyonema metallicum
- Dictyonema moorei
- Dictyonema obscuratum
- Dictyonema pectinatum – Galápagos Islands
- Dictyonema phyllophilum
- Dictyonema ramificans – Galápagos Islands
- Dictyonema reticulatum
- Dictyonema scabridum
- Dictyonema subinvolutum
- Dictyonema subobscuratum – Galápagos Islands
- Dictyonema subsericeum
- Dictyonema thelephora
- Dictyonema tricolor – Africa
- Dictyonema umbricola
- Dictyonema yunnanum – China
Morphology and ecology
Evolutionary relationships and lichenization
The Dictyonema fungus is a basidiomycete, so it developed lichenization independently from the ascomycete lichens. Within the basidiomycetes, Dictyonema is closely related to three other genera of basidiolichens that are also in the family Hygrophoraceae: Lichenomphalia, Acantholichen, and Cyphellostereum. The molecular data indicates that lichenization has evolved independently at least twice, and perhaps three times, within these four genera, which suggests that for some reason the fungi in Hygrophoraceae are predisposed to evolve into lichens. The majority of the other, non-lichenized fungi in this family are saprotrophic or ectomycorrhizal, although numerous species, such as Arrhenia, grow on mosses and derive nutrition from them. It is not yet understood why these fungi are more inclined to become lichens.Multilocus phylogenetics analyses split Dictyonema sensu lato into five strongly supported genera—the basal Cyphellostereum, a paraphyletic Dictyonema sensu stricto, and the successively derived Acantholichen, Corella and Cora. These same trees place Cyphellostereum as the earliest-diverging lineage, underpinning all later morphological innovations within the clade. Ancestral-state reconstructions show a stepwise elaboration of the lichen body: from simple filamentous crusts with separate cyphelloid basidiocarps, through filamentous mats whose stereoide-corticioid basidiocarps are partly embedded, to microsquamulose and finally foliose or macrosquamulose thalli in which corticioid basidiocarps are fully supported by the lichen tissue.
This morphological gradient is tightly correlated with phylogeny: thalli become progressively more complex the further a species lies from the root of the tree. The authors interpret the pattern as evidence that basidiolichen thalli in this group evolved by gradual incorporation of ancestral reproductive tissue into the vegetative symbiosis, rather than sprouting de novo from undifferentiated mycelium. All five genera partner with the same Rhizonema cyanobacterium and, except for Cyphellostereum, share a distinctive "jigsaw-puzzle" haustorial sheath—suggesting a single origin of the cyanobacterial symbiosis followed by internal diversification. Extant non-lichenised relatives in the agaricoid genus Arrhenia provide a living model for the ancestral condition, making the Dictyonema complex a prime example of how lichenizationhas arisen relatively recently within Basidiomycota.