Introduction to Fungi, Third Edition

230 ASCOMYCOTA (ASCOMYCETES)
wall as seen by electron microscopy, the finestructure
of nuclear division, and also close
resemblances of conidial structure and development.
Some genera contain species which reproduce
by asexual means only, whilst closely
similar forms have sexual as well as asexual
reproduction. Examples include Aspergillus and
Penicillium, which are anamorphs of several
genera of Ascomycota (Trichocomaceae; see
pp. 308 313) and Fusarium which is the
anamorph of Gibberella and Nectria (members of
the Hypocreales; see p. 343). It is presumed that
fungi which reproduce only by conidia have lost
the capacity to form ascocarps in the course of
evolution.
8.3.3 Parasexual reproduction
This is a process in which genetic recombination
can occur through nuclear fusion and crossingover
of chromosomes during mitosis. Meiosis
does not occur, and instead haploidization takes
place by the successive loss of chromosomes
during mitotic divisions. It is believed that the
necessary cytological steps take place in a regular
sequence which Pontecorvo (1956) has termed
the parasexual cycle. The essential steps include
(i) nuclear fusion between genetically distinct
haploid nuclei in a heterokaryon to form
diploid nuclei; (ii) multiplication of the diploid
nuclei along with the original haploid nuclei;
(iii) the development of a diploid homokaryon;
(iv) genetic recombination by crossing-over
during mitosis in some of the diploid nuclei;
and (v) haploidization of some of the diploid
nuclei by progressive loss of chromosomes
(aneuploidy) during mitosis.
This process was discovered in Emericella
(Aspergillus) nidulans, which can reproduce sexually
by forming asci and asexually by forming
conidia (see Fig. 11.17). By changing the nutrient
content of the medium on which the fungus is
grown, the development of asci and therefore of
normal sexual reproduction can be prevented.
Genetic mapping based on gene recombination
following conventional sexual reproduction
has been compared with mapping based on
parasexual recombination and has yielded identical
results.
Parasexual recombination is known to occur
not only in Ascomycota but also in Oomycota
and Basidiomycota. It makes possible genetic
recombination in organisms not known to reproduce
by sexual means and helps us to understand
why purely asexual fungi such as many
species of Aspergillus and Penicillium have achieved
success and have continued to flourish in the
course of evolution. However, because parasexual
reproduction is comparatively rare in nature,
it is probably only a partial substitute for sexual
reproduction, so that purely asexual species are
more prone to accumulating deleterious mutations
(Geiser et al., 1996).
8.4 Conidia of ascomycetes
The asexual spores or conidia of ascomycetes
are remarkably diverse in form, structure and
modes of dispersal, but their development or
conidiogenesis occurs in a limited number of
ways (see below). The cell from which a conidium
develops is the conidiogenous cell and usually
one or more such cells are borne on a stalk, the
conidiophore. Conidiophores which are narrow
and not differentiated from the vegetative mycelium
are said to be micronematous (Gr. nema ¼ a
thread) whilst those that are clearly differentiated
are macronematous. Conidiophores
frequently arise singly as in Eurotium repens
(Fig. 11.16), Emericella nidulans (Fig. 11.17) and in
many species of Penicillium (Fig. 11.18).
However, in certain fungi the conidiophores
may aggregate to form a conidioma. Descriptive
terms have been given to different types
of conidioma. Conidiophores aggregated into
parallel bundles (fascicles) are termed coremia
(Gr. korema ¼ a brush) or synnemata (Gr. prefix
syn ¼ together). Examples are Penicillium
claviforme (Fig. 11.19) and Cephalotrichum
(Doratomyces) stemonitis (Fig. 12.39). Seifert (1985)
has distinguished several types of synnema, some
of which are simple, some compound, some
made up of parallel conidiophores, and others