Introduction to Fungi, Third Edition

252 ARCHIASCOMYCETES
swollen tips which have been termed chlamydospores
by Martin (1940) but are, in fact, ascus
initials or ascogenous cells (see Fig. 9.2). The
interface between the parasitic fungus and the
host takes the form of contact between their
walls. No specialized haustoria have been found
in T. deformans (Syrop, 1975), although they have
been reported in some other species. Cytological
studies (Martin, 1940; Kramer, 1961; Syrop &
Beckett, 1976) have revealed that the segments of
the mycelium and the young ascogenous cells
are mostly binucleate. If the cells are multinucleate,
then the nuclei are at least arranged in
pairs (Syrop & Beckett, 1976). In the ascogenous
cell, the two nuclei fuse and the diploid nucleus
divides mitotically. The upper of the two daughter
nuclei then undergoes meiosis followed by a
mitosis so that eight nuclei result, which form
the nuclei of the eight ascospores. The lower
daughter nucleus remains in the lower part of
the ascogenous cell and is often separated from
the upper nucleus by a cross wall. During these
nuclear divisions, the wall of the ascogenous
cell has stretched to form an ascus. Delimitation
of the ascospores occurs at the eight-nucleate
stage. The individual nuclei become enclosed
by double-delimiting membranes which do not
arise from the nuclear envelope as in most
ascomycetes, but by invagination of the plasmalemma
of the developing ascus (Syrop & Beckett,
1972). Within the ascus, the ascospores may
bud so that ripe asci may contain numerous
yeast cells (see Fig. 9.2c). These yeast cells can be
regarded as the anamorphic state of Taphrina,
and they have been named Lalaria (Moore, 1990;
Inácio et al., 2004). The asci form a palisade-like
layer above the epidermis, and it is their presence
which gives the infected leaf its waxy
bloom.
The ascospores and yeast cells are projected
from the ascus which often opens by a characteristic
slit (Fig. 9.2c). Yarwood (1941) has
shown that there is a diurnal cycle of ascus
development and discharge in T. deformans.
Nuclear fusion takes place during the afternoon
or evening; nuclear divisions are complete by
Fig 9.2 Taphrina deformans. (a) T.S. peach
leaf showing intercellular mycelium and
subcuticular ascogenous cells. (b) T.S. peach
leaf showing ascogenous cells and asci,
containing eight ascospores. (c) T.S. leaf
showing a dehisced ascus, an eight-spored
ascus and an ascus in which the ascospores
are budding. Ascospores budding outside
theascusarealsoshown.(d j) Cytology of
ascus formation (after Martin,1940). (d,e).
Fusion of nuclei in ascogenous cell.
(f) Elongating ascogenous cell containing
two nuclei formed by mitosis from the
fusion nucleus.The upper nucleus has begun
to divide meiotically. (g) Uninucleate ascus
with uninucleate basal cell. (h,i) Four- and
eight-nucleate asci. (j) Binucleate germ tube
in germinating ascospore.