The potential for evolution of resistance to leaf rust (Coleosporium asterum) in a clonal perennial herb (Euthamia graminifolia)

Abstract

Heritable genetic traits that increase disease resistance will be favored by natural selection if greater resistance is associated with increased host reproduction. This will lead to an increase in mean resistance level in future host generations (i.e. evolution of resistance). Leaf rust resistance in Euthamia varies quantitatively (thereby reducing infection intensity rather than preventing infection), and can effect both sexual and vegetative reproduction in this clonal perennial host. In a theoretical chapter, I argue that vegetative reproduction (clonal growth) will play a major role in microevolution because of the importance of genotypic selection in clonal populations. It leads directly to the spread of a trait in the ramet population, which is then very likely to impact future genet populations, especially when compounded over time. In empirical potted and experimental field studies, heritable genotypic effects were responsible for 10-40% of infection intensity variation in 12 host genotypes across a range of environments. Although genotype by environment interactions in resistance level were common, two genotypes were consistently more resistant than the others. In a related experiment, I compared resistance levels of these same 12 genotypes to vegetative reproduction in the presence and absence of disease (Fig. 3-14). More resistant genotypes had higher rhizome biomass in the presence of disease, suggesting that genotypic selection will favor more resistant genotypes under high infection conditions. Moreover, less resistant genotypes had higher rhizome mass in the absence of disease, demonstrating that there is also a cost of resistance in this system. Coleosporium infection also had a major impact on sexual reproduction, although rhizome mass was more strongly affected than viable seed number (58% vs. 45% reduction, respectively). This finding shows that disease will have a detrimental effect on colonization of new sites by Euthamia. However, extremely low probability of seed dispersal to a suitable site and subsequent survival to adulthood (demonstrated in Chap. 4) suggest that decreased seed production will have limited longterm effects. Decreased vegetative reproduction due to disease, on the other hand, will have persistent effects on future reproduction of Euthamia genets, indicating the potential for evolution of resistance in this system.