Two other methods that have been used to characterize populations of plant pathogenic fungi are the analysis of overall repetitive DNA, and chromosome size and number. When total fungal DNA is digested with frequent cutting restriction enzymes, a large number of fragments of many different sizes are generated, and these appear in a gel as a "smear." Brighter staining bands can be seen in these smears where there are multiple copies of fragments of the same size.
These bands are, in part, the result of multiple copy DNA such as the rRNA genes and mitochondrial DNA (mtDNA). Differences in the patterns of these bands can be a quick, simple way for differentiating some closely related fungi, and this approach has been used for race designation in F. oxysporum sp. f. pisi [see Coddington et al. (1987)].
Unlike plants and animals, fungal chromosomes can be very variable, and in many fungal species different isolates may show differences in the number and size of their chromosomes. This has been investigated in some isolates of Colletotrichum and special forms of F. oxysporum. In Colletotrichum variation was found in the number and size of the smaller (type B) elements, and in F. oxysporum sp. f. cubense variation in chromosome sizes was found between different races and vegetative compatibility groups [see Masel et al. (1993); Ploetz (1990)].
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