Bitunicate

Classification

• A bitunicate ascus is enclosed in a double wall. This consists of a thin, brittle outer shell and a thick elastic inner wall. When the spores are mature, the shell splits open so that the inner wall can take up water. As a consequence this begins to extend with its spores until it protrudes above the rest of the ascocarp so that the spores can escape into free air without being obstructed by the bulk of the fruiting body. Bitunicate asci occur only in pseudothecia and are found only in the classes Dothideomycetes and Chaetothyriomycetes (which were formerly united in the old class Loculoascomycetes). Examples: Venturia inaequalis (apple scab) and Guignardia aesculi (Brown Leaf Mold of Horse Chestnut).
• Prototunicate asci are mostly spherical in shape and have no mechanism for forcible dispersal. The mature ascus wall dissolves allowing the spores to escape, or it is broken open by other influences, such as animals. Asci of this type can be found both in perithecia and in cleistothecia, for instance with Dutch elm disease (Ophiostoma). This is something of a catch-all term for cases which do not fit into the other three ascus types, and they probably belong to several independent groups which evolved separately from unitunicate asci.