6 Wood Discoloration

2.4 Identification 45
cleotide units that are distributed throughout the genomes of most Eukaryotes
(Powell et al. 1996). The variability of the number of repeat units at a particular
locus and the conservation of the sequences flanking the repeat make
microsatellites valuable genetic markers. They provide information for identification
and on genetic diversity and relationships among genotypes. For
example, DNA fingerprinting with multilocus microsatellite probes suggested
thatCapeTownisolatesofArmillaria mellea s.s. were introduced from Europe
more than 300 years ago (Coetzee et al. 2001).
Amplified fragment length polymorphism (AFLP)
AFLP is a powerful tool for DNA fingerprinting and is based on (1) total genomic
restriction, (2) ligation of primer adapters, and (3) unselective followed
by selective PCR amplification of anonymous DNA fragments from the entire
genome (Vos et al. 1995). AFLP markers are recognized as more reproducible
compared to RAPD analyses and inter-simple sequence repeats (ISSRs), and are
also able to give a higher resolution. AFLP analysis by Kauserud et al. (2004a)
of European isolates of Serpula lacrymans belonging to five somatic incompatibility
groups indicated that the species in Europe is genetically extremely
homogenous by observing that only five out of 308 scored AFLP fragments
were polymorphic. In contrast, S. himantioides as the closest relative to S.
lacrymans possessed 31.3% polymorphic fragments.
2.4.2.3
Further Molecular Methods
DNA-Arrays
DNA-arrays (DNA-chips, microarrays) are tools in medical, pharmaceutical,
and biological diagnosis of pathogens (genotyping, pathotyping) (Beier et al.
2002; Wiehlmann et al. 2004). Basis is the increasing availability of sequence
information of various viruses and bacteria. One chip can carry up to 10,000
different DNA probes (e.g., oligonucleotides), which are raster-like bound on
its surface. Nucleic acid molecules of the sample hybridize specifically with the
corresponding DNA probe, and the hybridized chip areas are detected colorimetrically.
Compared to PCR techniques, the sensitivity of the chip technology
is lower than with species-specific PCR, and the chip techniques need experienced
staff and expensive laboratory equipment. The great miniaturization and
automation, however, allow the analyses of a great number of samples in a short
time. Specific oligonucleotides to be used for arrays are already commercially
available for several pathogenic bacteria and yeasts. A possible future use for
wood fungi using specific oligonucleotides from rDNA sequences (Table 2.8)
could be a new technique for fungal diagnosis.
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