Combining Markers

In general terms the use of different markers can give rise to a hierarchic system, with particular techniques giving more, or less, resolution than others [see Brans et al. (1991)], and so it may be possible to select an appropriate marker for the situation under study. An example of the way in which a broadly hierarchic arrangement of markers can be used for the study of fungal plant pathogens is detailed later with G. boninense. This approach will however not always generate consistent results, and one example of this is the group of fungal plant pathogens known as the "Ascochyta-complex" that occurs on beans, peas, and other legumes. In this case there are a number of distinct species currently assigned to either the genus Ascochyta or Phoma (see Table 2). Most of these species can be defined individually from their ITS sequences, and some can in turn be subdivided on the basis of their mtDNA RFLPs. When a single part of the mitochondrial genome is considered there is less variability, and the species can be arranged in three groups. The groupings obtained from RFLPs derived from the b-tubulin gene are less consistent and group some species together, while also showing subspecific groups in others (Fatehi 2000).

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