Corinne Hertäg is a PhD student at ETH Zürich. She was recently shortlisted for Functional Ecology’s Haldane Prize for Early Career Researchers.
In this Insight, she talks about her shortlisted paper, Defensive symbionts mediate species coexistence in phytophagous insects
What is the background behind your paper?
How two species can coexist on the same resource has been a long-standing question in ecology. One mechanism is natural enemies like shared predators or parasitoids. Natural enemies can reduce competition between two species by having a more negative impact on the stronger competitor. This favors the weaker competitor and thus coexistence. However, natural enemies can also intensify competition if one of the competitors supports very high densities of an enemy that also attacks the second competitor.
Natural enemies are not the only factors influencing competition. An often-neglected factor that could influence species competition and coexistence is microbial endosymbionts. Endosymbionts live inside the body of another organism in a very close and mainly mutualistic relationship with their host. Such endosymbionts are very common in many animals, but particularly in insects where some endosymbionts have evolved the ability to defend their hosts against natural enemies. Since this protection also comes at a cost to the host, defensive endosymbionts are good candidates to influence species competition in absence and presence of natural enemies.
What’s your paper about?
Our paper’s aim was to investigate the influence of defensive symbionts on the competition between two aphid species living on the same host plant and sharing the same natural enemy.
Many aphids are naturally infected with defensive endosymbionts and they transmit them vertically to all of their offspring. Under summer-like conditions, aphids reproduce asexually, which allows us to keep them as monoclonal lines in the laboratory. These aphid lines can be artificially infected with new endosymbionts from other aphid lines, which gives us the opportunity to produce two aphid lines from different species that carry exactly the same protective endosymbiont. Both aphid species are also attacked by the same parasitoid wasp. Parasitoid wasps lay their eggs inside aphid hosts where their larva develop and finally kill their hosts and turn them into so-called mummies. If an aphid is protected by an endosymbiont, the wasp egg will not develop and the aphid stays alive. Nevertheless, parasitoid wasps are one of the main aphid enemies and they are able to eradicate whole aphid colonies.
To investigate how the defensive endosymbionts change competition between the aphids, we conducted a competition experiment with replicated communities of the two, either symbiont infected or uninfected, aphid species. We let them compete in the absence and presence of a shared parasitoid wasp.
Based on the known and predicted costs and benefits of defensive symbionts, we had two different hypotheses of what could happen in the communities. Since carrying defensive symbionts is costly, our first hypothesis was that the symbiont infected aphids would be outcompeted by the uninfected ones when no enemies are present. This was indeed the case for one of the aphid species.
Our second hypothesis predicted that the infected and thus protected aphids would outcompete the unprotected ones in the presence of the parasitoid wasp. Surprisingly, this was not the case. The only case in which the aphids actually benefitted from the defensive symbionts was when both aphid species were protected, which led to an effect similar to herd immunity and kept wasp numbers very low.
Where you surprised by any of your results?
I was indeed surprised to observe this intriguing form of apparent competition that led to the collapse of both aphid species, when only one of them was protected by the endosymbiont. The uninfected aphid species supported wasp densities that were high enough to suppress the protected aphids, even if they couldn’t parasitize them. Since we only found very low numbers of mummified protected aphids, the wasps must have disturbed them to the point of starvation or they literally stabbed them to death.
It turned out that our simple insect communities were sufficient to demonstrate that endosymbionts can have strong direct and indirect effects on species interactions.
How is your paper new or different from other work in this area?
Although the idea that defensive symbionts can influence species coexistence and community structures is not new, there is little empirical evidence supporting this claim. Many studies documented the costs and benefits of defensive symbionts in aphids but this was mostly done by studying pairwise interactions. We wanted to go beyond this point and investigate the influence of defensive symbionts in simple multispecies communities, in our case, three interacting insect species.
What does your work contribute to the field?
From an ecologists’ point of view, it is particularly interesting to see that defensive endosymbiotic bacteria can indeed change the community composition of phytophagous insects. The effects we observed were unexpected and even more exciting than our initial hypotheses, which highlights the importance of doing such competition experiments.
Since parasitoid wasps are frequently used for biological pest control, these findings could also be important in the context of crop protection. In the last few years, concerns have risen that symbiont-conferred resistance could make biological control with parasitoid wasps less effective. However, our experiment shows that high wasp densities can also control symbiont-protected pest insects.
What are the key messages of your article?
Knowledge about the presence and influences of defensive symbionts is important to understand the structure of food webs because defensive symbionts can drastically change predation- and competition-related effects. Endosymbionts are best studied in arthropods but there is no reason why they couldn’t also play important roles in other animals.
About the Author
How did you get involved in ecology?
Since I was a child, I have always been fascinated by the overwhelming diversity of living things around me. When I came to university and started to learn about the complex ways of how all these organisms interact and form whole ecosystems, there was no doubt that I had to become an ecologist.
What are you currently working on?
I am currently doing a PhD at ETH in Zürich, during which I am studying an intriguing phenomenon of chemical mimicry. I am working with parasitoid wasps that chemically mimic their aphid hosts. This mimicry allows the wasps to oviposit into ant-protected aphid species without being attacked by honeydew collecting ants. I continued working with my previous study system but I’m currently focusing on the chemical aspects of species interactions and potential species diversification.
What is the best thing about being an ecologist?
There are many great things about being an ecologist. I really enjoy having a job that links being outdoors and working with organisms that most people don’t even notice (or don’t like) with working in different laboratories and at my computer. And last but not least, I also like to communicate my findings and see the wonder on other peoples’ faces when they realise how cool and important “my” tiny insects actually are.
What do you do in your spare time?
I like playing the trumpet, drawing (also very useful when you can’t get pretty photographs of the tiny organisms you are working with), reading, being outdoors, and recently I have discovered the pleasures of lindy hop dancing.