Introduction to Fungi, Third Edition

9
Archiascomycetes
9.1 Introduction
Several independent phylogenetic analyses of
DNA sequence data (e.g. Berbee & Taylor, 1993;
Sjamsuridzal et al., 1997; Liu & Hall, 2004) have
grouped together a range of seemingly very
diverse genera of ascomycetes. This group is considered
to be the oldest of three broad evolutionary
lineages of Ascomycota and has thus been
named Archiascomycetes (Nishida & Sugiyama,
1994). The core of the Archiascomycetes consists
of the genera Taphrina and Protomyces, which
are facultative biotrophic plant pathogens, and
the saprotrophic fission yeast Schizosaccharomyces.
Also now included are the yeast-like Pneumocystis,
which causes pneumonia in immunocompromised
patients (see p. 259); the filamentous
fungus Neolecta, which parasitizes the roots
of higher plants (Redhead, 1977; Landvik
et al., 2003); and the anamorphic yeast Saitoella.
Yet other genera are included as possible
members because even though their appropriate
DNA sequences have not yet been obtained,
they are known to be related to confirmed
members. In total, the class Archiascomycetes
currently contains some 150 species in
10 genera.
Because of their diverse morphological
appearances and modes of life, it is difficult
to describe common characters typical of the
Archiascomycetes. With the exception of Neolecta,
which produces apothecia, ascocarps are lacking
and asci are produced individually by yeast
cells or by conversion of hyphal tips. There are
no differentiated ascogenous hyphae. Asexual
reproduction is usually by simple division of
vegetative yeast cells by budding or fission. Even
in Neolecta, the apothecia are highly unusual in
that they lack ascogenous hyphae and paraphyses,
and in that the ascospores are capable of
producing yeast-like conidia by budding while
still within the ascus or after discharge (Redhead,
1977). This phenomenon is also found in Taphrina
(see Fig. 9.2c).
The presence of Neolecta in the most basal
group of ascomycetes indicates that the capacity
to produce fruit bodies is probably an ancient
trait. The inclusion of both yeasts and mycelial
forms among the Archiascomycetes makes it
impossible to decide the chicken-and-egg question
as to which of these states is ancestral and
which is derived. It is significant that all recent
molecular studies have placed Taphrina within
the Archiascomycetes because this genus has
long been suspected to be close to the
origin of both the higher ascomycetes and the
basidiomycetes (Savile, 1968; Alexopoulos et al.,
1996). Further, the position of Pneumocystis
has been unclear until recently, oscillating
between Basidiomycota (Wakefield et al., 1993)
and Archiascomycetes (Sjamsuridzal et al., 1997;
Kurtzman & Sugiyama, 2001), and thereby
further supporting the suspected ancestral
status of the organisms included among the
Archiascomycetes.
Two genera Taphrina and Schizosaccharomyces
are of special significance to mycology
and will be discussed more fully below, together
with a brief account of Pneumocystis.