Ultrastructure of the ascus apical apparatus in species of Cenangium, Encoelia, Claussenomyces and Ascocoryne

Abstract

The ascus apical apparatus and dehiscence mechanism were studied with TEM in Cenangium ferruginosum and Encoelia fimbriata, and Claussenomyces atrovirens and Ascocoryne cylichnium. The asci, which are non-reactive with iodine in all species except the last mentioned, varied in general morphology of the apical apparatus and dehiscence mechanism. The apical apparatus in C. ferruginosum was extremely small, with an annulus fully occupying the apical thickening. It played no particular role in dehiscence, but the tractus and apical funnel demonstrated in this species were associated with it. The ascus opened by an irregular slit. The apical apparatus in E. fimbriata was characterized by a well-developed apical thickening, which lacked an annulus, surrounding a broad, cylindrical structure. This structure became externally eroded and stretched during maturation of the ascus, and was finally torn in an almost circular fashion, apparently functioning as a lid. It is argued that this structure is probably homologous to the central cylinder in other Leotiales which form an annulus in their apical thickening. The similarity to the operculum characteristic of the Pezizales seems to be merely a case of convergence in structure and function. The apical apparatus was different from several other species of Encoelia, including the type species. The ascus in Cl. atrovirens is similar in general morphology and reactivity pattern of the annulus to that in Leotia lubrica. The ascus in A. cylichnium lacked an annular protrusion, and resembled more closely that in Hymenoscyphoideae than Sclerotiniaceae.