Introduction to Fungi, Third Edition

VEGETATIVE STRUCTURES
227
Fig 8.1 (a) Hypha of Hormonema dematioides.The positions of
the first septa are indicated by arrows. (b) Candida parapsilosis.
Pseudohypha budding off cells which continue to bud in a
yeast-like manner.
Candida albicans is the cause of diseases such as
thrush in mammals, including man.
The mycelial septa of ascomycetes are usually
incomplete, developing as transverse centripetal
flange-like ingrowths from the cylindrical wall of
a hypha, which fail to meet at the centre so that
in most ascomycete septa there is one central
pore permitting cytoplasmic continuity and
streaming between adjacent segments of mycelium
(Buller, 1933; Gull, 1978). This means that
organelles such as mitochondria and nuclei
are free to travel from cell to cell; the large
nuclei are constricted as they pass through the
pore (Fig. 8.2). Individual cells may be uni- or
multinucleate and the cytoplasmic continuity
between the cells means that the mycelium
of an ascomycete is effectively coenocytic.
Proteinaceous organelles termed Woronin
bodies (Buller, 1933) may be closely grouped
near the central pore (Fig. 8.3). Woronin bodies
are globose structures or ‘hexagonal’ (polyhedral)
crystals made up essentially of one protein
(Tenney et al., 2000), and surrounded by a unit
membrane. They measure 150 500 nm in width
and are sufficiently large to block the septal
pore. They rapidly do so near regions where
a hypha is physically damaged. Usually one
Woronin body blocks one pore. The blockage in
the septal pore is consolidated by deposition of
further material (for references see Markham &
Collinge, 1987; Markham, 1994; Momany et al.,
2002). Woronin bodies are formed near the
hyphal apex and are transported to more distal
regions of the hypha as septa develop. Woronin
bodies have been recorded from ascomycetes and
their related conidial fungi, but there are no
reliable reports from other fungal phyla.
The mycelium of many ascomycetes is
homokaryotic (Gr. homoios ¼ like, resembling;
karyon ¼ a nut, meaning nucleus), i.e. all nuclei
in a given mycelium are genetically identical.
Heterokaryotic mycelia also occur and generally
arise through anastomosis, i.e. the cytoplasmic
fusion of vegetative hyphae. Following anastomosis
between homokaryons of differing genotypes,
nuclei, other organelles and plasmids may
be transferred between one mycelium and
another so that a given mycelium or even a
single cell may contain nuclei of different kinds.
However, the ability to form heterokaryons is
under genetic control and a degree of genetic
similarity between homokaryons is necessary
for it to occur. Failure to establish a heterokaryon
is a phenomenon known as heterokaryon
incompatibility or vegetative incompatibility
(Caten & Jinks, 1966; see pp. 320 and 594).