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Ascomycetes on bryophytes: systematics and biology

Why study bryophilous ascomycetes?

Bryophytes do not live in splendid isolation. They interact in manifold ways with other organisms, including fungi. Bryophilous parasites enrich the bryosphere, the mycobiota and the complexity of ecosystems where mosses and hepatics occur. As hosts and parasites are anatomically completely different both partners of the symbioses are easy to distinguish even at the cellular level – in contrast to fungi on lichens. In many cases, hyphal growth can be traced exactly. Furthermore, bryophytes do not possess elaborate structures and complex tissues. For example, almost all their leaves only consist of a one-celled thick layer.

Bryophilous fungi are excellent taxonomists and can provide clues as to the systematic relationships between their hosts. Epibryon pogonati-urnigeri clearly distinguishes between its principal host Pogonatum urnigerum and the similar but non-host Polytrichastrum alpinum (syn. Pogonatum alpinum). Plagiochila asplenioides s. lat. and Pedinophyllum interruptum (Plagiochilaceae) have often been confused, but never by their strictly specific parasites E. plagiochilae and E. pedinophylli, respectively.

Necrotrophic species influence the size, dynamics and fertility of bryophyte populations. The parasites can destroy whole mats or cushions or, more selectively, certain organs or tissues of individual shoots. Lizonia species colonizing the antheridial cups of Polytrichaceae prevent spermatozoid production. Perianthicolous species inhibit normal sporophyte development and spore maturation. Bryorella acrogena destroys the growing apices of single shoots in pleurocarpous mosses. Dead and decomposed patches can act as gaps that enable other mosses and plants to establish.

Finally, bryophilous species contribute to decomposition processes of mosses and hepatics. Even the strong stereids in leaves of Polytrichaceae are withered by the attack of species like Aphanotria paradoxa or Rogellia nectrioidea.

The occurrence of parasitic ascomycetes in colonies of mosses or hepatics is a frequent and universal phenomenon. Where bryophytes exist, their parasites are present, no matter whether the hosts are found in soil crusts in deserts or in bogs, in the phyllosphere in lowland rain forests, as understory vegetation in temperate forest plantations, in late snow areas above the timberline or in high latitudes where vascular plants are sparse or absent. However, in spite of their number, frequency, global distribution, and uniqueness, bryophilous fungi must be regarded as cross-kingdom orphans, severely neglected by both bryologists and mycologists. Their lack of economic importance, the minute, inconspicuous and sporadically distributed fruit-bodies hidden between the host leaves, the distinctness of bryology and mycology as disciplines each with their own long traditions, terminology, methods, and journals, and the erroneous conception that bryophytes are resistant to fungal attack, may explain the almost Linnean situation. Most parts of the world do not even have a simple inventory. More species are reported from the surroundings of München, Germany, than in the whole of North America. The identification of species is a prerequisite for all further biological research like cultivation and infection experiments, ultrastructural and physiological or ecological investigations. Paradoxically, bryomycology began at the end of the 18th century when Johan Hedwig (the father of bryology) introduced the genus Octospora thereby describing several conspicuous discomycetes on mosses. Detailed anatomical drawings illustrate his descriptions, which represent one of the first microscopic studies of ascomycetes. Now, more than 220 years on, it is time to realize that the fungi on mosses and hepatics present one of the most rewarding and exciting novel fields of alpha taxonomy in recent times.