Gallaicolichen

Gallaicolichen is a fungal genus in the division Ascomycota. The relationship of this taxon to other taxa within the division is unknown (incertae sedis), and it has not yet been placed with certainty into any class, order, or family. This is a monotypic genus, containing the single species Gallaicolichen pacificus, a foliicolous (leaf-dwelling) lichen.

Taxonomy
The type specimen of Gallaicolichen pacificus was originally collected from Kolukola Park in Hamakua, Hawaii, where it was found growing on Syzygium cumini. The genus name honours the New Zealander botanist and lichenologist David Galloway. Both the genus and its species were proposed in 2007 by Emmanuël Sérusiaux and Robert Lücking.

Description
Gallaicolichen pacificus is characterised by its leaf-dwelling, thallus, which appears as pale greenish yellow to pale yellowish grey rounded patches, ranging from 0.5 to 1.7 mm in diameter. The edges of these patches are slightly lobed, and the surface is typically smooth and somewhat shiny, becoming more intricate when examined closely. The thallus is relatively thin, measuring up to 20 μm, and comprises an interlaced network of fungal filaments (hyphae) and large, organized algal cells that form the lichen's photosynthetic component, known as the. Within the mature regions of the thallus, sizeable oxalate crystals are found, which diminish in number towards the edges.

Occasionally, a layer known as the cortex is present, which may or may not be fully developed, made of a single layer of blocky or multi-sided cells that sometimes have a slightly brownish tinge. Surrounding the thallus is the prothallus, a typically colourless, algae-free membranous layer that may sometimes appear bluish or brownish. The photobiont of this species is a type of alga from the genus Phycopeltis, belonging to the family Trentepohliaceae. The algal cells are greenish-brown, measuring roughly 8 to 11 by 4 to 5 μm, and are neatly arranged in radial rows forming plates.

Structures reminiscent of, termed by the authors, are consistently present, with each thallus patch containing 1 to 8 of these structures. These peltidiangia begin to develop at the centre of the patch and are almost perfectly circular, featuring a relatively thick, upright margin that is white and composed of raised hyphae that are not fused together. This margin is typically enveloped by the thallus.

The reproductive particles of Gallaicolichen pacificus, referred to as, are numerous and fill the cavity of the peltidiangia. These propagules are disc-shaped, around 25 to 55 μm in diameter and 10 to 15 μm thick, with a regular arrangement. They consist of branching cells from the photobiont Phycopeltis, which are linked to a central, root-like structure and curled inward on the other side, surrounding a small, orderly layer of fungal cells. Typically, one to two ends of fungal filaments are found between the branches of the algal cells and are visibly apparent on the outer surface of the propagules. The ends of the algal branches are slightly enlarged and have a shiny appearance. Typical lichen reproductive structures, ascomata, and conidiomata, have not been observed to occur in this species.

Habitat and distribution
In addition to the type locality in Hawaii, Gallaicolichen pacificus has also been recorded from Queensland, Australia, New Caledonia, Vanuatu, and Japan.