Tunica
tunica (noun,ascus term, pl. tunicae; synonym: tunicle) – the functional layers of an ascus wall. Early classifications generally distinguished two types: unitunicate asci (with a single tunica, i. e., a wall that remains intact when ascospores are released), and bitunicate asci (two tunicae, i. e., a wall separating into two layers during ascospore discharge). Ultrastructural studies have demonstrated that the situation is much more complex and several ascus wall layers can be distinguished. The layers that form the exo– and endoascus, are structurally not generally composed by the same wall layers. Thus, the exoascus can be composed of the a- and b-layer, or the a-, b- and c-layer. Likewise, the endoascus may be composed of the c- and d-layer or of the d-layer alone. The terms unitunicate and bitunicate are therefore inappropriate to describe the ascus ultrastructure, instead they refer to functional ascus types. To emphasize that the ascus wall is not generally segregated into one or two layers, the terms exo– and endotunica should be avoided. These terms suggest that the exo- and exoascus are structural layers, whereas they are clearly functional, i. e., frequently composed of different ultrastructural layers. This is:part of ascus.
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