Ultrastructure of the ascus apical apparatus in Hymenoscyphus and other genera of the Hymenoscyphoideae (Leotiales, Ascomycotina)

The ultrastructure of the ascus apical apparatus is compared in 19 species of the Hymenoscyphoideae, currently placed in seven genera. The ascus wall consists of an outer layer of two strata, and an inner layer of also two, and in one species of three strata. At the apex only the inner layer increases in thickness. On the basis of the general morphology and PA-TCH-SP reactivity pattern of the apex five main groups are recognized. A further division into subgroups is also outlined. The most important diagnostic features used in the analysis are the relative development and the reactivity pattern of the apical thickening, the occurrence of an annular protrusion, the structure and the reactivity pattern of the annulus, and the apex maturation pattern. In addition to the electron micrographs diagrammatic schemes are given to illustrate the author’s interpretation. The species studied are thus arranged as follows: Group 1a. Hymenoscyphus caudatus, H. fructigenus, H. salicellus, H. salicinus, and Bisporella pallescens; 1b. H. imberbis and Phaeohelotium subcarneum; 1c. H. consobrinus, H. repandus, and Crocicreas pallidum; 1d. Discinella boudieri; Group 2a. H. herbarum; 2b. Pezizella gemmarum; 2c. Chlorociboria aeruginascens and Pezizella alniella; 2d. Crocicreas cyathoideum [var. cyathoideum]; Group 3. Bisporella sulfurina; Group 4. Cudoniella clavus var. grandis; Group 5. Cudoniella acicularis. Most fundamental are considered firstly the position of the annulus in the apical thickening, either partly (groups 1, 2, 4, 5) or fully (3) occupying the thickening, either associated (2, 4, 5) or not associated with an annular protrusion (1), and secondly the dehiscence mechanism, either an eversion of the annulus over an angle of about 90° (1, 2, 3?, 4) or a twostep mechanism (5) previously undescribed in Leotiales. The absence of an amyloid reaction in the apex, which is a diagnostic feature in Cudoniella acicularis and C. clavus var. grandis, is based on two fundamentally different structures in these species. The apex in the last mentioned fungus closely resembles that in Ombrophila violacea, while the apex in C. acicularis is unique in general morphology and dehiscence mechanism. The ultrastructural data of the apical apparatus are found to correlate with characters of excipulum anatomy, especially in the genera Hymenoscyphus and Bisporella. Their importance in segregating more natural genera from large ill-defined genera like Hymenoscyphus or Pezizella is discussed.