Anthropogenic Parasite Evolution

During the last decades we have gained insight into the way humans, with their activities, modify selection on wild plants and animals - and how this will affect their evolutionary trajectories. Parasites (including pathogens) are organisms present in all branches of the tree of life, and that owe their success to the fact that they live and feed on other organisms. We all know them, because they cause infectious diseases; what is perhaps less known is that they represent the majority of all living species and are essential to ecosystem functioning, stability, and diversity. Yet despite their importance we still understand poorly how their life history, behaviour, and other characteristics are affected by human activities. This question has mostly been addressed theoretically, as it is notoriously difficult to track parasites and thus study their ecology and evolution in the wild. Human activities change the environment for parasites, both indirectly by changing the living conditions of their plant and animal hosts, and directly via drugs and vaccines that target them. One major human activity that combines these two types of effects is modern agriculture. Humans and their domestic animals vastly dominate the biosphere, and for parasites this represents a huge and global ecological change, with largely unknown long-term consequences. To understand the evolutionary impact of humans on parasites, we must first understand the mechanisms of adaptation in parasites, and in particular how environmental changes translate into selection on parasite traits. The aim of this project is to gain insight into these questions, using a highly relevant model species, the salmon louse. We have developed a mark-recapture approach that has allowed us to successfully monitor populations of this parasite as they were evolving under various scenarios. As we are reaching the end of our data collection spanning 10 generations of parasites, we are now able to start characterising selection on a range of parasite phenotypic traits, account for tradeoffs between traits and document their evolution in contrasting environments. This will significantly advance knowledge on how host-parasite interactions are affected by the changes imposed by humans on populations. One key feature of current global change is that plant and animal populations currently decline or increase rapidly. The knowledge generated by this project should therefore reinforce the knowledge base necessary to tackle the current biodiversity crisis.

The human ecological footprint on the biosphere is steadily increasing. For example, more than 90% of the global terrestrial mammal biomass now consists of humans and their livestock. We therefore need more focus on how such vast-scale anthropogenic changes are shaping the evolution of other organisms. Our understanding of contemporary evolution in non-parasitic species has significantly advanced in the last decade, but one piece of this puzzle is paradoxically missing: despite the fact that parasites are seen as fascinating examples of adaptation, we still have a poor understanding of their evolution, and in particular how they will respond to anthropogenic changes. This is a crucial question because in addition to causing disease, parasites play a pivotal role in fundamental ecological and evolutionary processes, such as stabilizing host populations, affecting species richness and as drivers of genetic diversity and speciation. This project offers to unravel the link between environmental change and parasite evolution, i.e. to understand the selective forces acting on parasites, their adaptive responses - and how these are being affected by human practices. Combining phenotypic, genetic and fitness measures in the ectoparasitic salmon louse (Lepeophtheirus salmonis) on Atlantic salmon (Salmo salar), we will measure the selective effects of host density on parasite traits, the evolutionary potential of those traits and their response to selection. We will also investigate whether and how the evolution of life history, virulence and drug resistance - which are traits with epidemiological relevance - trade off against each other. Overall this project will, by importing modern evolutionary biology with its powerful sets of tools into parasitology, provide an integrated view of contemporary adaptation in a parasite experiencing rapid environmental change worldwide.