Assessment of correlational selection on tolerance and resistance traits in a host plant-parasitic plant interaction

Resistance and tolerance are considered to be different plant strategies ag ainst disease. While resistance traits prevent hosts becoming parasitized o r reduce the extent of parasitism, tolerance traits reduce the fitness-impa ct of parasitism on infected hosts. Theoretical considerations predict that in some circumstances mutual redundancy will give hosts with either high r esistance or high tolerance a fitness advantage over hosts that exhibit bot h of these traits together. However, empirical evidence has provided mixed results. In this paper, I describe the pattern of phenotypic selection impo sed by the holoparasitic mistletoe Tristerix aphyllus upon resistance (spin e length) and tolerance (branching) traits in the cactus Echinopsis chilens is. Results indicate that branching was an efficient compensatory mechanism , reducing 75.5% of the fitness-impact attributable to parasitism. Even tho ugh both traits showed a negative correlation, as expected from the presenc e of allocation costs between strategies, no correlational selection coeffi cient was significant indicating that selection did not favor alternative c ombinations of traits. Consequently, I did not find evidence for selection promoting mutually exclusive defense strategies against the mistletoe, whic h suggests that tolerance and resistance traits may coexist stably in popul ations of E. chilensis.