A keystone gene underlies the persistence of an experimental food web

Ecosystems – the basics
In an ecosystem, species interact directly or indirectly with each other, and thus establish a complex food web. We know that some species have a disproportionally large effect on the ecosystem, and are therefore known as keystone species. At the DNA level, previous studies indicate that genotypic variations affect diversity and composition in ecological systems. But, at the same time, we understand very little about the impact of individual genes.

Findings from the Barbour et al. study
Barbour et al. looked at the role of genotypic variation within an experimental food web in the lab. They imitated a part of a naturally-occurring food web consisting of a plant model organism, Arabidopsis thaliana, two aphid herbivore species (Brevicoryne brassicae and Lipaphis erysimi) and a wasp whose larvae develop in aphids (a so-called parasitoid, species Diaeretiella rapae). The interactions among these species are i,a. mediated through plant metabolites called aliphatic glucosinolates.

In their study, Barbour et al. looked at whether genotypic variation in the genes coding the abovementioned metabolites affected the composition of a food web. In the lab, the researchers constructed ecosystems containing plant individuals with different alleles. In the beginning, these always consisted of different variants of Arabidopsis, the two aphid species and the parasitoid. The researchers then investigated how stable the food web was over time and what role the different genotypes of Arabidopsis played in this.

Their findings showed that a higher plant allelic diversity reduced the extinction rate of the insects. In this experimental setup, ecosystems with a higher genetic diversity are more robust. A special function could be observed for one gene of the investigated metabolites (APO2): if a mutation in the APO2 gene leads to a loss of function of the enzyme, the average extinction rate of the insects declines on average by about 29%. The reason: The plants with this allele grew faster, allowing the aphids to grow as well. This enabled the coexistence of the aphids and the wasps, which stabilised the ecosystem. In ecosystems where the other alleles were (pre)dominant, extinction rates of aphids and parasitoids were higher.

Significance of these findings
Because of its huge impact on the different populations within the food web as well as on the stability of the investigated ecosystem, Barbour et al. called the AOP2 gene a ‘keystone’ gene – following the concept of keystone species. However, to confirm the relevance of plant keystone genes in food webs in field, more research is needed, especially studies which include more complex experimental food webs and field studies.

However, the study shows two interesting aspects: First, it supports the observation that higher genetic diversity contributes to greater stability within an ecosystem. High genetic diversity is thus important for the stability of ecosystems and species conservation. Even the loss of a single key gene in one species could cause changes in the composition and extinction probabilities of other species in the food web. Conversely, this shows that even small, precise genetic changes, e.g. through new molecular methods, can have far-reaching consequences for entire food webs and ecosystems. Therefore, the results are important for the discussion on risk assessment and the possible use of new genetic engineering methods that can be used to modify cultivated or wild species.