Some of these aphids’ most significant
natural enemies are parasitic wasps. These tiny insects lay their eggs in the bodies of aphids, and then the larvae develop inside the body and eat their way out from the inside, killing the aphid in the process. “Their lifestyle has inspired that of the aliens from the Ridley Scott Alien films!” jokes Vorburger. The interesting part of Vorburger’s study
is how a third organism plays a part in this host-parasite interaction. Many of the
aphids have microbial symbionts living inside of them, a type of bacteria that the aphid’s body does not reject because it provides them with protection from the wasps. This mechanism, which is known as defensive symbiosis, is made all the more interesting as it turns out that the bacteria
are maternally transmitted,
meaning that the aphids pass down the bacteria from generation to generation, making the bacteria a heritable part of the aphid lineage.
The genetic dynamics of coevolution as observed in one of our models. ©2012 Kwiatkowski et al., in ‘On Genetic Specificity in Symbiont-Mediated Host-Parasite Coevolution’ PLoS Comput Biol 8(8): e1002633. doi:10.1371/journal.pcbi.1002633
“These bacteria provide very strong
protection to the aphids from the wasps by producing a toxin that kills the eggs of the wasp. Their presence is not without cost though; if no parasitoid wasps are present, then the aphids with the bacteria have a shortened life span compared to those without,” explains Vorburger. The process of coevolution between
hosts and parasites and how such systems evolve is quite well understood. Theoretically, there is a whole suite of mathematical models
describing this
process that are able to make fairly accurate predictions. However, these systems are all based on the assumption that the interactions are occurring directly between the host and the parasite. In the example of
the aphids and the
wasps, it would appear that this is not the case. The microbial symbionts inside the aphids mean that many of the factors important to coevolution models
are
totally changed. “As I mentioned before, the variance is
changed, so aphids with the bacteria are way more resistant to the wasps than those without, which means that there is a lot more material for selection to act upon,” explains Vorburger. “Specificity is also altered; the aphids’ own natural defenses are relatively ineffective and very general, so they work equally well against different parasitoids, whereas the defense provided by the bacteria acts very specifically against certain lineages of parasitoids, adding another complication to the model. And of course there is also
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