Refardt, Dominik, Dr.

My research interests - evolution of parasitism

Living at the expense of others is a clever life style which has been adopted by a large part of the living world. At the same time it is puzzling, because the parasite harms the resource it relies on, its host. How does it manage to bite the hand that feeds it and get away with it?

Evolutionarily speaking, parasites constantly adapt to counter-adaptations of their hosts, creating a never ending race between two antagonists. This provides the launchpad for the evolution of sex and recombination, it helps to understand what parasite virulence is and why it can't be avoided and last but not least it underlies the evolution of generally fascinating phenotypes such as highly reduced genomes and complicated life cycles.

Since I entered the biological sciences, I studied causes and consequences of parasitism, first with unicellular parasites of waterfleas, now with acellular parasites of bacteria. If you share my interests, contact me. I am interested in all sorts of interactions, be it a collaboration among peers or the supervision of a masters thesis.

Ecology and Evolution of Temperate Bacteriophages

Bacteriophages are viral predators of bacteria. While some of them invariably kill their host after infection (lysis), temperate phages can decide to remain benign: they insert themselves into the genome of their host and remain there quietly as a so called prophage (lysogeny). Such a prophage can be triggered to excise itself again and kill the host, typically via the bacterial SOS response (prophage induction).

Thus, we have here a very simple organism with a dual transmission strategy. Either aggressive (lysis) or benign (lysogeny). How the phage behaves depends on two decisions. One is made after infection (kill now or sit and wait?), the second is made during lysogeny (leave now or stay longer?), and together they define the phage life cycle. How phage behavior works mechanistically is understood in great detail thanks to decades of experimental investigation.

I want to place this knowledge in an ecological and evolutionary framework. What are the advantages of having those two life styles and under what conditions should a phage choose either lysis or lysogeny? Now that the importance of phages in many ecosystems is recognized, we must understand what selective forces shape them and how they respond.

To this end, I perform experiments that address the adaptive value of these traits. For example by experimentally evolving them under controlled conditions to test whether their evolution contributes to an improvement in phage fitness. Using this approach, I was able to show that prophage induction can rapidly be adjusted if environmental conditions change. Moreover, different phage strains vary in their responses to induction, which suggests that this trait is under selection in the wild.

Another possibility to better understand how phages interact with the environment is by direct comparison of different phages. To this end, I conducted competition experiments between different phages, which allowed me to characterize the consequences of a key ecological mechanism in great detail. Phages compete strongly, and there is evidence that both exploitation and interference competition are operating.

What's next? Together with Louis Du Plessis, a gifted PhD student of our group, I am in the process of comparing the genomes of various phages. Together with the experimental data, this will allow us to better understand how they have been and are still being shaped by natural selection. And then there are a number of further experiments, all in different stages of completeness...

The Role of Temperate Phages in a Tri-trophic System

I am extremely excited to collaborate with Olivia Roth on this project. We will investigate how temperate phages interact with their bacterial hosts (Vibrio sp.) who are themselves pathogens of fish (pipefish, Syngnathus typhle). Much is known about the ability of temperate phages to manipulate the phenotype of their bacterial host (lysogenic conversion), but only rarely has this been place in an ecological and evolutionary context. This will be addressed here.

Evolution of Microsporidian Parasites

Microsporidia a highly specialized unicellular parasites. Possibly because of their parasitic life style (they are intracellular parasites), they have small and compacted genomes and highly reduced mitochondria. In addition to that, they evolve at a high rate. Taken together, they have a rather "protist-like" appearance, which has tricked taxonomists for a long time into believing that they are very ancient eukaryotes.

Now it is fairly certain that they have a fungal origin. Their ancestor was probably a chytrid or zygomycete which adopted a parasitic life-style that subsequently gave rise to thousands of highly specialized species that gained the status of a phylum. Who the ancestor is and where exactly in the fungal tree it is located is not known.

We might have identified a species that could be the missing link between fungi and microsporidia. Together with people from Dieter Ebert's group (where I did my PhD) and with some help of Tim James (University of Michigan) I am currently tightening this up. Stay tuned.

Prospective students

I am currently looking for new students to join my lab. If you are looking for a place to do your masters, I would be delighted if you contacted me to discuss mutual interests and the possible opportunities. I mainly work with phages nowadays, but it doesn't take a lot to spark my interest for microsporidia again.

Publications

Click here to find an up to date list

Links

The Phage Cookbook - my collection of (useful) notes on handling phages

Zotero - close to perfect reference management for free

MyClimate - imperative when flying to a congress

www.soet.ch - completely irrelevant