Enigmotrema

''Enigmotrema'' is a fungal genus in the family Graphidaceae. It contains only one known species, ''Enigmotrema rubrum'', a lichen which was discovered in Costa Rica. Both the genus and species were first described in 2012 by the lichenologist Robert Lücking. The genus name combines the Greek words for "mystery" and "hole", referring to the unusual appearance of its reproductive structures. ''Enigmotrema'' is characterised by its olive-coloured body (thallus) and distinctive dark red fruiting bodies (apothecia) that undergo a unique opening process as they mature. The genus is closely related to '' Cruentotrema'' but differs in its genetic makeup and the development of its fruiting bodies. Currently, ''Enigmotrema'' is only known from a single location in a montane rainforest in Costa Rica, where it was found growing on tree bark.

Taxonomy

The genus ''Enigmotrema'' was circumscribed by the lichenologist Robert Lücking in 2012. The name ''Enigmotrema'' combines the Greek words ''enigma'' (meaning mystery or puzzle) and ''trema'' (meaning hole or pore), referring to the unusual appearance of its fruiting bodies.

''Enigmotrema'' is closely related to the genus '' Cruentotrema'' but differs in several key aspects. Genetic analysis of the mitochondrial small subunit ribosomal DNA (mtSSU) revealed unique characteristics that place ''Enigmotrema'' outside the ''Cruentotrema''-''Dyplolabia'' clade, a group of closely related lichen genera. Later molecular analysis suggests that the species is nested with ''Fissurina'', and that it is closely related to the '' Fissurina pseudostromatica'' clade.

Description

''Enigmotrema'' lichens possess a distinctive appearance that sets them apart from their close relatives. The thallus of the lichen has an olive-green to olive-yellow colouration. Its surface is smooth to slightly uneven, protected by a dense outer layer, the . This cortex is composed of tightly interwoven fungal threads, or hyphae, forming a protective barrier for the lichen.

A characteristic feature of ''Enigmotrema'' is its apothecia. These fruiting bodies undergo a unique development process that gives the genus its enigmatic character. Initially, the apothecia appear as small, closed, dark red structures reminiscent of tiny flasks. As they mature, these structures, measuring between 0.7 and 1.2 millimetres in diameter, begin a transformation. First, the outer layer of the apothecium splits open, revealing a dark red inner layer. This is followed by the remaining covering splitting into triangular teeth-like structures that curl inwards. This process gradually exposes the brown-black surface, known as the , where spores are produced. The end result is a complex, open structure that bears little resemblance to its initial closed form.

Inside these apothecia, ''Enigmotrema'' produces colourless, ellipsoid spores called . These spores are relatively small, measuring 10–12 micrometres (μm) in length and 5–6 μm in width. They possess three cross-walls, or septa, and have a distinctive internal structure characterised by thick walls and diamond-shaped chambers (. This internal arrangement is similar to that found in some other lichen genera, particularly ''Astrothelium''.

From a chemical perspective, ''Enigmotrema'' is notable for containing a dark red pigment called isohypocrellin in the inner layer (medulla) of its apothecia. This pigment contributes to the striking red colour of the fruiting bodies. Standard analytical techniques did not detect any other significant chemical substances in the lichen's composition.

Habitat and distribution

''Enigmotrema rubrum'', was discovered growing on tree bark in Costa Rica. The specimen was collected from the Las Cruces Biological Station at the Las Cruces Biological Station, located in the Talamanca mountain range. This area is characterised as a montane rainforest zone at an elevation of about 1,200 metres above sea level.

The lichen was found growing on the lower trunk of a tree in a semi-exposed location. While the immediate surroundings were described as "anthropogenic vegetation" (vegetation influenced by human activity), it was within the context of a protected botanical garden.

References

{{Taxonbar, from1=Q107289178 , from2=Q107656257

Graphidaceae
Taxa described in 2012
Graphidales genera
Lichen genera
Taxa named by Robert Lücking