Introduction to Fungi, Third Edition

228 ASCOMYCOTA (ASCOMYCETES)
Fig 8.2 Nucleus of Botrytiscinerea passing through a septal
pore. (a) View of the entire hyphal diameter. (b) Close up.Note
the constricted appearance of the nucleus.
Fig 8.3 Transmission electron micrograph of a transverse
septum in the hypha of Emericella nidulans showing
five Woronin bodies near the central septal pore.
Scale bar ¼ 0.25 mm. Reprinted from Momany et al. (2002),
Mycologia,withpermission.ßThe Mycological Society of
America.
Some ascospores, e.g. those of Neurospora tetrasperma,
are heterokaryotic, and multinucleate
conidia can also be heterokaryotic. Following
mycelial anastomosis between homokaryons,
nuclear division succeeded by migration may
result in the rapid spread of an introduced
nucleus into a mycelium, thus transforming a
homokaryon into a heterokaryon. An ascomycete
mycelium may thus consist of a mosaic of cells,
some of which are homokaryotic and others
heterokaryotic. Because the different types of
nuclei do not always divide at the same rate,
the ratio of nuclear types in a heterokaryotic
mycelium may change with time and respond to
changes in external conditions such as nutrient
availability. This gives the mycelium a degree of
genetic flexibility which sometimes manifests
itself as the formation of sectors in a mycelium
in agar culture (Fig. 8.4). Another important
source of genetic variability which may arise
within a heterokaryon is the parasexual cycle, a
process in which genetic recombination is
brought about in the absence of meiosis. This is
discussed more fully below.
8.3 Life cycles of ascomycetes
8.3.1 Sexual life cycles
Sexual life cycles in the strict sense, i.e. involving
nuclear fusion and meiosis, occur only in those
ascomycetes which possess asci, because it is
within the young ascus that these events occur.