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Ascomycetes on bryophytes: systematics and biology

Microniches occupied

Both phases of the gametophyte, the protonema as well as the proper plant, are suitable for fungal colonization. Whereas just a few Octospora species and Bryonectria protonematis grow on the persistent protonemata of mosses, the vast majority of ascomycetes occur on thallose (in hepatics) or leafy plants (in hepatics and mosses). Fruit-bodies rarely form without recognizable preference for certain tissues or organs. Normally species-specific microsites on the hosts are targeted for ascoma formation, e.g. leaf axils, the ventral leaf border, or even (in hepatics) individual cells. Several species develop their proper ascomata on the protected ventral leaf side and perforate the leaf cells in the apical part in order to discharge the spores effectively into the open air (fig. 3, 23. The sex- and organ-specific perianth-inhabiting fungi occupy one of the most distinct and nutrient-rich microniches available in bryophytes.

Polytrichaceae, with their enormous structural complexity, offer a highly differentiated suite of microhabitats covering all parts of the gametophyte. As mentioned above, Octospora infects the rhizoids below the soil surface. Lizonia forms its ascomata where (in healthy plants) the male and female gametangia develop. More than 50 species occur on the leaves (subcuticular, immersed within the epidermis or nerve (figs 3, 4, 23), on or between the leaf lamellae (figs 7, 15, 16)). Furthermore, there is a longitudinal differentiation in infection site specialization. Some species prefer the proximal leaf parts, others the distal ones. The interlamellar species are often extremely reduced due to lack of space resulting in linear (seen from above) or plate-like ascomata (laterally seen) with a single row of asci and a reduced excipulum. Even the asci may be laterally compressed. These minute fungi are systematically unrelated and offer striking examples of convergent evolution. Spore production per ascoma is low but may be compensated for by the production of a large number of fruit-bodies on a single leaf. About 120 ascomata of Dawsophila polycarpa have been counted on a single, 10 mm long leaf of Polytrichadelphus magellanicus. The substantially larger leaves of Dawsonia superba may harbour up to 1,000 ascomata of Dawsophila pygmaea. Most species are more or less deeply immersed in the interlamellar spaces, but D. polycarpa is restricted to the wedge-shaped spaces between the lamellar end cells resulting in a triangular outline when observed in transverse sections (fig. 8, 28).