bitunicate ascus (noun,ascus term, pl. bitunicate asci) – an ascus with two functional layers; one of several main categories of asci, characterized by an ascus dehiscence where two functional layers can be distinguished: the exo– and the endoascus. During dehiscence the both functional layers typically rupture at the tip. The flexible endoascus expands then shoots out of the rigid exoascus, actively discharging spores from the shrivelling, empty ascus. Early studies believed that bitunicate asci were one of two fundamentally different types (see unitunicate). Ultrastructural examination later revealed that several more ascus wall layers can be distinguished. During dehiscence some of these layers are more closely adherent than others, but the exo- and endoascus are not generally formed by the same layers. Thus, the exoascus of one ascus may not necessarily be ultrastructurally identical to that of another type. In early classification systems ascomycetes with bitunicate asci were generally believed to have an ascolocular ontogeny and were consequently referred to as Loculoascomycetes = Ascoloculares = Bitunicatae. Related terms: fissitunicate, semi-fissitunicate. Kind of:ascus dehiscence type.