Molecular approaches to population structure
Fungal species are dynamic entities. Their populations
fragment by geographical isolation or the develop-
ment of somatic incompatibility barriers, then the
fragments diverge by genetic drift or in response to local
selection pressure. This is particularly true for the
clonal mitosporic fungi, because strains of different
vegetative compatibility groups (VCGs) are isolated
permanently from one another. Many sexual species
also have VCGs but the mating-type genes override the
somatic incompatibility genes so that strains of differ-
ent VCGs can mate.
In general, fungi have too few morphological features
for identification of population subunits, so biochem-
ical and molecular tools must be used for this. One
approach is to compare the electrophoretic banding
patterns of proteins on gels, using either total protein
extracts or different forms (isozymes) of particular
enzymes such as pectic enzymes, visualized on the gels
by color reactions with the enzyme substrate. These
zymograms(e.g. Fig. 9.8) reflect random mutations in
the DNA encoding the enzyme, although not at the
enzyme active site which is highly conserved. About
30% of the amino acid changes resulting from muta-
tions will affect the net charge on the protein and thus
alter its electrophoretic mobility. Since these changes
are random, they tend to accumulate over time and thus
reflect the history of a population. As one practical
example, MacNish et al. (1993) used a combination of
pectic zymogram and VCG typing to identify different
subgroups of the soil-borne fungus Rhizoctonia solani
that causes bare patch (stunting) disease of wheat in
Australia (Chapter 14). All fungal isolates from within
each disease patch were of identical VCG +zymogram
group, but different patches often represented differ-
ent VCG +zymogram groups.
Figure 9.9 shows how this approach can be used
to understand the population biology of the fungus.FUNGAL GENETICS, MOLECULAR GENETICS, AND GENOMICS 169
Fig. 9.9Mapped positions of patches of stunted wheat plants caused by Rhizoctoniain a field trial site in Australia.
Two adjacent patches were caused by different pectic zymogram groups (ZG1.1 and ZG1.5) which never invaded the
territory of the other. The patches expanded and contracted in successive years. The central dot is a fixed reference
point. (After MacNish et al. 1993.)
Fig. 9.8Five distinctive zymogram groups (ZGs) of Rhizoctoniastrains that cause bare patch disease of wheat in Australia
(similar to crater disease, shown in Fig. 14.3). Protein extracts were run on acrylamide gels containing pectin then stained
to develop the bands of pectic enzymes. (Courtesy of M. Sweetingham; from MacNish & Sweetingham 1993.)