Genetic variation of an asexual invasive plant in resistance to biological control organisms

It is often assumed that introduced populations of asexual plants harbor little genetic variation, thereby restricting their potential for adaptive evolution and enhancing their vulnerability to biological control management.  In a common garden experiment we measured quantitative responses of the invasive apomictic weed Chondrilla juncea (Asteraceae) to two biological control organisms: the rust fungus pathogen Puccinia chondrillina and the gall mite Eriophyes chondrillae.  Nine plant clones (asexual family lines) were sampled randomly from two rust-resistant weed biotypes in Idaho (Post Falls and Banks) and three rust-susceptible populations in Washington, Oregon, and California.  There was significant variation in the quantitative severity of rust disease among the six rust-susceptible clones (means ranged 24% of maximum possible severity).  Rust-susceptible clones did not differ significantly in the severity of mite gall damage, but a single rust-resistant clone (Banks biotype) had 87% less mite gall damage than the average of rust-susceptible clones.  Rust disease caused a large reduction (85%) in mean rosette biomass by the end of the experiment, but otherwise the results do not provide evidence that natural enemies reduce fitness of C. juncea, although genetic differences in fecundity were detected among clones.  Growth traits also varied significantly among clones in rosette biomass and frequency of bolting, but not shoot biomass or trichome density.  Evidence of clonal differences in resistance within and among North American populations of C. juncea indicates that plant genetic variation could influence the biological control of this apomictic weed by contributing to adaptive host-parasite dynamics.