Host-induced phenotypic diversification in Phytophthora ramorum

Forestry, agriculture, and native ecosystems face ever-increasing threats by invasive species. Not all introduced species are, however, invasive. In order to establish and persist in a non-native land, introduced species have to adapt to different environments, unfamiliar food, and predators. There are a number of examples where invasive species evolved quickly in non-native regions (Brasier 1995, Davis 2009), such as leg size of cane toad (Phillips et al. 2006) and leaf size of St John's wart (Maron et al. 2004). Large phenotypic variation has been observed in clonal lineages of the generalist plant pathogen Phytophthora cinnamomi (H�berli et al. 2001). Rapid phenotypic changes in invasive animals and plants are believed to result from selection of existing genetic variation; in a non-native location, under different selective pressure, different traits from their native range may be selected. Contribution of de-novo mutations for adaptation is probably not sufficient to explain rapid phenotypic changes observed in invasive species. De-novo DNA mutations are rare: spontaneous mutation rate estimated for a plant is only one base substitution per genome per generation (Becker et al. 2011) and nonneutral mutation rate in eukaryotes is approximately 1/300 per genome per generation (Drake 1999). Hence, it is unclear how an invasive clonal pathogen, which is devoid of genetic variation, will adapt to new hosts and a new environment. It may be that, in response to environmental stress, invasive clones may generate variation on which selection acts.