Frequent Gain and Loss of Resistance against Tobacco Mosaic Virus in Nicotiana Species

Resistance (R) genes represent one of the most divergent gene families in plants. Novel resistance function might arise through point mutations or sequence exchanges between paralogues (Kuang et al., 2004; Luo et al., 2011, 2012). Sequence exchanges between homologues may generate a large number of distinct genes. In fact, some R gene families are extensive chimeras, contributing considerable resistance diversity for a species. In contrast, other R gene families are highly conserved between different genotypes.