Ascus apical apparatus and ascospore characters in Xylariaceae

Abstract

Members of Xylariaceae (Ascomycota) are recognized and classified mainly on the morphological features of their sexual state. In a number of genera high morphological variation of stromatal characters has made confident recognition of generic and specific boundaries difficult. There are, however, a range of microscopical characteristics which can in most cases make distinctions, especially at generic level, even in the absence of molecular data. These include details of the apical apparatus in the ascus (e.g. disc-shaped, inverted hat-shaped, rhomboid, composed of rings, amyloid, non-amyloid); position and length of the germ slit; and presence and type of ascospore wall ornamentation as seen by scanning electron microscopy (SEM). Unfortunately many of the classical studies on xylariaceous genera omitted these features and were undertaken long before the development of scanning electron microscopy. More recent studies have, however, demonstrated their value as diagnostic characters in the family. Camillea is for example, instantly recognizable by its rhomboid or diamond shaped apical apparatus, and the distinctive inverted hat or urniform type is usually prominent in Xylaria, Rosellinia, Kretzschmaria, and Nemania. At least six categories of apical apparatus based on shape and size can be recognized. Ascospore ornamentation as seen by SEM has been exceptionally useful and provided the basis for separating Camillea from Biscogniauxia and other xylariaceous genera.