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Ascomycetes on bryophytes: systematics and biology

Parasitic behaviour

Most species are biotrophic parasites, which live in equilibrium with their hosts and do not cause any visible damage to the host. These highly adapted fungi continue to be accidentally collected together with their hosts and deposited in bryological herbaria, unknown to the collector, unknown to the curator, and often unknown to science. Octosporaceous fungi attack the living, subterranean rhizoids of acrocarpous mosses. The apothecia sometimes occur on bare soil at a certain distance from the host plants, imitating terrestrial saprotrophic discomycetes. Necrotrophic parasites often cause conspicuous whitish, yellowish or brownish discolorations that ultimately decompose. An infection of Acrospermum adeanum causes irregular discoloured patches. Expanding infections of Nectria muscivora show concentric zonation with an outer whitish zone where superficial hyphae attack healthy green mosses, and inner discoloured parts where perithecia develop. The centre of the infection is often completely decomposed and drops out. Large fairy-ring–like structures are reported in polar regions. The rings may be concentrically arranged or they can converge and build irregular systems. It is relatively easy to find necrotrophic parasites in the field. Discoloured patches of moribund bryophytes always merit closer investigation although fruit-bodies will not be present in sterile infections. Non-fungal causes can also mimic the symptoms of fungal infections. It is unclear whether obligate saprotrophs occur on gametophytes. However, several species, including Epibryon diaphanum, prefer more basal, older parts of the plants showing signs of incipient decomposition. It occurs on Hylocomium splendens and other feather mosses, whose shoots consist of a living apical part that continues to grow, while the dying and dead sections remain attached to the lower parts.

Several parasites infect the perianths of epiphyllous hepatics where they prevent normal sporophyte development, while the gametophytic shoots remain unaltered. Hyhae appear to accumulate at the junction between the two generations and form a large haustorium-like structure that appears to exploit the nutrients meant for the developing embryo. These fungi can behave biotrophically toward the gametophyte and at the same time necrotrophically toward the sporophyte, indicating a highly evolved host-pathogen relationship.

Some species provoke hypertrophic reactions in the infected host tissue. Mniaecia jungermanniae induces the formation of giant or abnormal perichaetia in foliose hepatics. Perianthicolous species sometimes deform the perianths, especially when heavily infected. Octosporaceous fungi cause different types of complex galls on rhizoids of acrocarpous mosses.

Fungi differentiate between the sporophytic generations of hepatics and of mosses as potential substrates. Sporophytes of hepatics remain within the protective perianth until they reach maturity. Subsequently, the seta elongates and lifts the capsule out of the perianth. Spore dispersal begins and shortly afterwards the sporophyte collapses. Such weak structures are too ephemeral to be colonized. However, eight species attack the developing embryo while it is still enclosed by the perianth. In mosses, many ascomycetes and anamorphous species grow on the long-lived exposed setae and capsules. Although fruit-bodies have often been observed on green tissue indicating an early infection, most sporophyte-inhabiting fungi grow as saprotrophs on dead setae and capsules. Embryo-attacking fungi are largely absent in mosses with two notable exceptions. In one, the spores of an (as yet) unnamed fungus of uncertain systematic position supplant the sporophytic tissue while it is still enclosed in the calyptra of a moss. The much better known heterobasidiomycete Eocronartium muscicola infects numerous temperate moss genera by replacing the developing sporophytes with its own structures. The fundamental differences between hepatics (Marchantiophyta) and mosses (Bryophyta) are not only expressed by sporophyte development, but also indirectly by the mode of parasitic attack that afflicts them.