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One of the fundamental questions in evolutionary biology is: why is sex the dominant mode of reproduction when it is so much costlier than reproducing asexually? One potential hypothesis is that the recombination associated with sexual reproduction generates genetic diversity and allows mothers to produce offspring that are resistant to rapidly and continuously evolving parasite populations. The Red Queen hypothesis predicts that hosts and parasites should continually change in response to one another but this requires variation on which fluctuating selection can act. Additionally, sex can also provide genetic benefits through intense sexual selection, where mothers mate with the fittest males. However, when inbreeding is common, the benefits of sex compared to asex could decline or even disappear, as recombination would generate less genetic diversity. This could be problematic, for example, when habitat fragmentation isolates populations and increases levels of inbreeding. Despite many years of theory, there are few conclusive experimental tests of these predictions.

The NERC-funded Stirling Outdoor Disease Experiment (SODE) provides an opportunity to test the relationships between host sex, sexual selection, inbreeding and host-parasite coevolution. SODE is an established long-term project where twenty replicate pond populations of the facultatively sexual freshwater crustacean Daphnia magna and its sterilising bacterial parasite Pasteuria ramosa have been maintained over multiple years. Each pond was initiated with the same suite of Daphnia genotypes and the same parasite population. Over the past four years, the populations have diverged, and the host populations have experienced different levels of inbreeding.

The student will conduct a series of integrated field, mesocosm (SODE) and laboratory investigations to dissect the implications of host sexual reproduction for (co)evolutionary change in both host and parasite populations.