Frequency-dependent selection and host defenses

Background/Question/Methods Finding mechanisms maintaining genetic variation in natural populations remains as a central aim in evolutionary biology. For the case of host defenses, recent empirical evidence reveals that individual hosts present intermediate levels of both resistance and tolerance; that is, hosts usually express a mixed pattern of defense allocation. It has been suggested that natural enemies have the potential to maintain genetic polymorphisms by creating frequency-dependent selection on their hosts. However, the frequency-dependent advantage of particular genotypes could be diluted if natural enemies reach high densities rapidly. The objectives of the present study were to evaluate if (1) host defenses, resistance and tolerance, are under frequency-dependent selection; (2) oviposition preference is one of the mechanism through which herbivores create frequency-dependent selection on their hosts, and (3) herbivore population growth rate covariates with the frequency of resistant and tolerant hosts. In order to answer these questions, the frequency of resistant and tolerant genotypes within experimental plots was manipulated. Three frequency treatments were used (0.1, 0.5 and 0.9 of resistant hosts in plots where the alternative host was tolerant) with 12 replicates each. Oviposition preferences were assessed weekly as the total number of eggs per plant. At the end of the experiment, plant fitness (total number of seeds) and herbivore growth rate (difference between final and fixed initial number of adults) were estimated.

Results/Conclusions Preliminary results suggest no evidence of frequency-dependent selection acting on host resistance. In contrast, host tolerance was under positive frequency-dependent selection as predicted by previous theoretical studies. Although the herbivores preferred to oviposit their eggs on tolerant genotypes at the beginning of the experiment, their preference changed as the number of eggs and larvae on tolerant hosts increased suggesting density- and frequency-dependent regulation of oviposition preferences. Finally, herbivore growth rate was almost three times higher in those experimental plots where the frequency of tolerant hosts was high supporting the expectation that tolerance would promote an increment in the population size of the herbivore. This result suggests that when the density of natural enemies increases even rare genotypes would present a considerable amount of damage that could reduce its fitness.