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Ascomycetes on bryophytes: systematics and biology

Adaptations to bryophily

The bryophilous habitat has evoked surprising adaptations, for example, the formation of tiny ascocarps that often measure less than 100 µm in diameter. Very small ascomata (30 µm diam.) with just a single mature ascus occur in Bryochiton perpusillus and Epibryon pogonati-urnigeri and others. Pyrenocarpous species predominate. Even on drying the apothecia may move their margins over the hymenium to protect it. The modified apothecia of the hepaticolous pezizalean genus Octosporella are ovoid with an apical ostiole-like opening. They imitate nectriaceous perithecia. Many species have gelatinous ascocarp walls or a gelatinous hymenium that frequently reacts with Lugols iodine solution resulting in a reddish or bluish colour after pre-treatment with KOH. Globose or pyriform ascocarps and mucilage may be relevant in water retention. This is in agreement with the selection of microsites on the host plants that exhibit relatively low loss of moisture. At the same time, effective spore liberation must not to be impeded. Phototropic reactions of the whole ascomata, ostioles, or ascus tips favour directional spore discharge into the open air.

Several species, e.g. Epibryon deceptor or E. diaphanum, exhibit a remarkable tendency towards reduced fertility. Scattered ascomata contain few asci with large spores, and anamorphs are unknown. Growing at the same rate as their hosts, these fungi apparently occupy stable niches in which the production of a high number of diaspores does not confer a selective advantage. However, the necrotrophic Bryocentria merospora, like E. deceptor a hyperepiphyllous species, has a very high number of diaspores per ascoma (approximately 24,000). It is unclear whether there is a correlation between having a high reproductive potential and the constraints involved in establishing a new infection after having killed the host.