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Ascomycetes on bryophytes: systematics and biology

Frequency and geographic distribution

Some species, like the widespread Bryonectria phyllogena on Polytrichum juniperinum and P. piliferum, are truly rare. As their hosts are relatively well studied their rarity is not simply due to a lack of observation. They are only found accidentally. In Europe Frullania dilatata and Plagiochila asplenioides s. lat. and Radula flaccida in the Afro-American tropics are the most popular hepatic hosts, each with more than 10 recorded parasitic ascomycetes. No less than six hypocreaceous parasites occur on Frullania dilatata. A single collection of R. flaccida from Honduras yielded five ascomycete species. All eight parasites of R. flaccida recorded for the Neotropics and all six globally known perianthicolous species on epiphylls occur in Costa Rica.

Polytrichaceae harbour about 65 named ascomycetes. Twenty-one parasites occur on six species of the Austral-Asian genus Dawsonia, 15 alone on D. superba. Five specimens of D. grandis from New Guinea in the Botanische Staatssammlung München (M) yielded 13 parasites altogether and six per specimen on average. Polytrichum commune and P. formosum each have more than ten parasites. Six species out of a possible ten were observed on a single plant of Polytrichadelphus magellanicus from southern South America. An individual leaf can harbour up to five species. Nine species occur on P. aristatus in the northern Andes. Diversity tends to concentrate on a small scale. It is to be expected that the numbers indicated will considerably increase when further specimens and additional potential hosts have been screened on a wider scale.

Only a very small number of bryophytes, in particular hepatics and Polytrichaceae that are often infected, have been systematically screened. So far, hornworts have not been shown to act as hosts for ascomycetes. However, different fungal spores within the capsules of Anthoceros and related genera have sporadically been detected in European, North and South American and in Australian populations. These alien spores are intermingled with the host's meiospores. One spore type was described at the beginning of the 20th century as Tilletia abscondita, i.e. misidentified as belonging to a smut genus. Similar spores have been found in capsules of additional bryophytes, especially in Sphagnum. While it is clear that bryophytes are not attacked by smuts the systematic affiliation of these imperfect fungi has not been resolved. They most likely represent anamorphic states of ascomycetes.

Like all obligate parasites the geographical distribution of bryophilous fungi depends on that of their hosts. The few species actually studied in detail apparently co-occur together with their hosts, reflecting the host's distribution areas. There are examples of the following distribution types: European (Bryocentria brongniartii), holarctic (Lizonia sexangularis), neotropical (Ticonectria perianthii), tropical African-American (Epibryon deceptor), pantropical (E. filiforme), subantarctic (Potridiscus polymorphus), bipolar (Potriphila navicularis), almost cosmopolitan (Bryochiton perpusillus), and endemic (Dawsophila polycarpa, southern South America). In P. navicularis only pycnidia are present in the northern- and southernmost part of the distribution area whereas in climatically more favourable regions both morphs grow together, often on the same leaf. There is some evidence that some strictly host-specific bryophilous taxa can occupy just a restricted part of the host's distribution area. An example is Calonectria frullaniae on Frullania dilatata, which was found only in the western and southern part of the European host area.

Epibryon filiforme on Radula flaccida and epiphyllous Lejeuneaceae, based on 54 records (Original H. Hertel)

Bryochiton perpusillus on 21 species of Polytrichaceae, ased on 229 records (Original H. Hertel).