When facing multiple enemies : the impact of host development time and exposure to multiple parasites in shaping coevolutionary adaptations in host and parasites
Interactions between hosts and parasites represent a very important ecological relationship, occurring across ecosystems. Because of the tight nature of these interactions, hosts and parasites enter into coevolutionary dynamics which can be, to a certain extent, replicated and studied under laboratory conditions. In experimental coevolution of host and parasites, specific antagonistic selective pressures are employed to study population dynamics, genetics of adaptations, disease dynamics and species diversification. Though some studies have focused on host s interaction with multiple strains of the same parasite, a facet of host-parasite coevolution that has received little attention is the involvement of multiple species of parasites.In nature, host interaction with multiple parasites either simultaneously or sequentially is more a norm than an exception and has been reviewed in chapter 1 Introduction. Therefore, in this thesis, I aimed to assess a multicellular host s interaction with multiple parasites in an experimental evolutionary context. For the first time, adaptations in both host and parasites in a simultaneous multiple parasite exposure has been explored using a tripartite (one host-two parasite system) experimental coevolution approach. Also for the first time, the consequence of sequential multiple parasite exposure has been investigated in a previously parasite coevolved host.Tripartite experimental coevolution was performed using the red flour beetle Tribolium castaneum, and its two natural parasites Beauveria bassiana and Bacillus thuringiensis, for an equivalent of ten host generations, with all the treatments replicated in two host development regimes, FAST (generation time of 21 days) and NORMAL (generation time of 28 days). Host and parasites were sampled throughout the experiment, and host immune response (internal and external immune proxies) was measured. I observed that host development regime not only influenced host immune response but also led to adaptations in the parasites. Beetles from the coevolution experiment were resistant to non-evolved B. thuringiensis and varied in their immune profile based on their development time. Neither parasites showed any significant trend regarding virulence but displayed very distinct protective adaptations as a consequence of coevolutionary interactions. B. thuringiensis isolated from the environments of FAST regime unanimously formed biofilms, reported for the first time for this strain. B. bassiana isolates from the NORMAL regime exhibited resistance to the beetle s external defensive secretions.I also investigated emergence of cross-resistance as a consequence of sequential multiple parasite exposure. For this, I used T. castaneum, previously coevolved beetles with the fungus B. bassiana, and exposed it to the parasites, B. thuringiensis and P. entomophila. Using survival and gene expression experiments I was able to show evidence that the positive cross-resistance displayed by the B. bassiana coevolved beetles towards B. thuringiensis can be attributed to a similarity in the route and mechanism of infection of both parasites.With my thesis, I have been able to highlight the different host and parasite responses to simultaneous and sequential interactions of multiple parasites and one host. While simultaneous exposure led to evolutionally adaptation in host immune response and parasites protective features, the host exhibits an advantage in sequential exposure, when the routes of infection are similar between the parasites. Finally, these results draw the attention to the need for host-parasite evolution experiments involving multiple multicellular hosts and natural parasites, with longer evolutionary timescales, for better understanding of a ubiquitous natural interaction.
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