Abstract
BackgroundCoevolution between pairs of antagonistic species is generally considered an endless "arms race" between attack and defense traits to counteract the adaptive responses of the other species.Presentation of the hypothesisWhen more than two species are involved, diffuse coevolution of hosts and parasitoids could be asymmetric because consumers can choose their prey whereas preys do not choose their predator. This asymmetry may lead to differences in the rate of evolution of the antagonistic species in response to selection. The more long-standing the coevolution of a given pair of antagonistic populations, the higher should be the fitness advantage for the consumer. Therefore, the main prediction of the hypothesis is that the consumer trophic level is more likely to win the coevolution race.Testing the hypothesisWe propose testing the asymmetry hypothesis by focusing on the tritrophic system plant/aphid/aphid parasitoid. The analysis of the genetic variability in the virulence of several parasitoid populations and in the defenses of several aphid species or several clones of the same aphid species could be compared. Moreover, the analysis of the neutral population genetic structure of the parasitoid as a function of the aphid host, the plant host and geographic isolation may complement the detection of differences between host and parasitoid trophic specialization.Implications of the hypothesisGenetic structures induced by the arms race between antagonistic species may be disturbed by asymmetry in coevolution, producing neither rare genotype advantages nor coevolutionary hotspots. Thus this hypothesis profoundly changes our understanding of coevolution and may have important implications in terms of pest management.
Highlights
Coevolution between pairs of antagonistic species is generally considered an endless "arms race" between attack and defense traits to counteract the adaptive responses of the other species.Presentation of the hypothesis: When more than two species are involved, diffuse coevolution of hosts and parasitoids could be asymmetric because consumers can choose their prey whereas preys do not choose their predator
Testing the hypothesis: We propose testing the asymmetry hypothesis by focusing on the tritrophic system plant/aphid/aphid parasitoid
Many studies have been devoted to the hypothesis of an endless "arms race" between antagonistic species, in which each species develops escalating attack and defense traits to counteract the adaptive responses of the other species
Summary
Coevolution is the result of reciprocal selective pressures exerted by interacting species. The main prediction of AH is that the consumer trophic level is more likely to win the coevolution race [17] This may account for the paradox described by Holt & Hochberg [18] – the lack of clear published examples of an increase in host resistance after biological control using parasitoids, despite the potentially strong selective pressure associated with parasitism. Another key point deals with the supposed difference of speed in evolutionary responses between generalist and specialist traits It may depend on other important biological factors like the discrepancy between population sizes, generation times and the reproductive mode (e.g. a sexual parasitoid versus an asexual aphid) or the genetic make up (e.g. an haplo-diploid parasitoid versus a diploid aphid) of the considered species. Both authors have been involved in the elaboration of the hypothesis and the evaluation of its consequences, and in the drafting of the paper
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