As indicated in Section 4, evolutionary analyses revealed two types of R genes with contrasting sequence features. One comprises R genes either located in complex loci with a number of highly homologous genes or in single-gene loci with multiple divergent alleles in the population, which is indicative of diversifying selection. The other type comprises R genes in single-gene loci with simple, low-level
(including presence/absence) polymorphisms, which is indicative of balancing selection. A few recent studies indicated that there may be similar contrasting sequence patterns at Avr loci in pathogens that match those in the corresponding R loci in the hosts (Allen et al. 2004; Rehmany et al. 2005; Araki et al. 2006). An emerging theme is that these two patterns of R or Avr gene coevolution appear to be determined by the two modes of R-Avr recognition mechanisms described in the previous section.
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