Assessing targets of selection and coevolution in a symbiont-mediated aphid-parasitoid antagonism

Melissa Margaret Carpenter

doi:10.17918/00010781

Association with heritable microbial symbionts is common insects. These symbionts can perform a variety of functions, including ones which benefit the fitness of their host. While some of these symbionts are obligate, being required by their host for survival, others are facultative. They are only found in some members of a population, can be lost by the host, and frequently are able to transfer to new hosts via horizontal transmission. Beneficial facultative symbionts can also serve as a source of additional variation for their hosts. Hosts can leverage symbionts to respond to selection pressures, thus leading to host-level selection acting on the symbionts. This could allow for rapid adaptation of insects in the field. In cases where the selective pressure is being applied by another organism, particularly a pathogen or parasite, it also creates the potential for antagonistic coevolution between a pathogen/parasite and the protective symbiont. In collaboration with other members of the Russell lab, I looked for evidence of host-level selection acting on symbionts in the pea aphid model symbiont system. I found that while in some cases significant shifts in symbiont frequency were due to hitchhiking with coinfecting symbiont, there was strong evidence of selection for symbionts by environmental pressures they were predicted to protect against. Building on the theme of symbiont-mediated host adaptation, I also sought to determine if defensive symbiont Hamiltonella defensa, its prophage APSE, mediate coevolution between pea aphids and the parasitoid wasp Aphidius ervi, introduced to United States as a biocontrol agent. I found a significant amount of strain diversity of H. defensa-APSE in the field. In some cases strain composition varied across years in a manner resembling negative frequency dependent selection, potentially supporting the idea that antagonistic coevolution is occurring in this system. I also found that H. defensa-APSE strains mediate genotype x genotype interactions between A. ervi and pea aphid hosts. Such specificity is expected under allele matching/trench warfare-like coevolution, but the trade-offs expected under these models are not yet evident from my experiments. Finally, I found evidence that A. ervi populations may be evolving higher virulence toward aphids with regionally common symbiont strains in those fields. In summary, while it is not yet clear whether parasitoids drive the observed symbiont dynamics, shifts in symbionts have clearly driven changes in A. ervi, revealing how biocontrol agents adapt to changing defenses of their targeted hosts.

Assessing targets of selection and coevolution in a symbiont-mediated aphid-parasitoid antagonism

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Assessing targets of selection and coevolution in a symbiont-mediated aphid-parasitoid antagonism

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