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International audience; Antimicrobial peptides (AMPs) play a key role in the external immunity of animals, offering an interesting model for studying the influence of the environment on the diversification and evolution of immune effectors., alvinellacin (ALV), arenicin (ARE) and polaricin (POL, a novel AMP identified here), characterized from three marine worms inhabiting contrasted habitats (‘hot’ vents, temperate and polar respectively), possess a well conserved BRICHOS domain in their precursor molecule despite a profound amino acid and structural diversification of the C-terminal part containing the core peptide. Data not only showed that ARE, ALV and POL display an optimal bactericidal activity against the bacteria typical of the habitat where each worm species lives but also that this killing efficacy is optimal under the thermochemical conditions encountered by their producers in their environment. Moreover, the correlation between species habitat and the cysteine contents of POL, ARE and ALV led us to investigate the importance of disulfide bridges in their biological efficacy as a function of abiotic pressures (pH and temperature). The construction of variants using non-proteinogenic residues instead of cysteines (α-aminobutyric acid variants) leading to AMPs devoid of disulfide bridges, provided evidence that the disulfide pattern of the three AMPs allows for a better bactericidal activity and suggests an adaptive way to sustain the fluctuations of the worm's environment. This work shows that the external immune effectors exemplified here by BRICHOS AMPs are evolving under strong diversifying environmental pressures to be structurally shaped and more efficient/specific under the ecological niche of their producer.

Host-parasite coevolution is expected to present a strong force in nature, with interactions between hosts and parasites leading to rapid reciprocal adaptations. In recent years several coevolution experiments have been carried out, testing predictions on the dynamics and outcomes of coevolutionary processes. One aspect of host and parasite ecology that has received little attention in host-parasite coevolution experiments is the ability of both parties to shape their environment. A number of parasites are able to produce transmissive stages that persist in the environment across host generations and many host species are able to manipulate their environment through secretion of antimicrobial compounds or hygienic behaviours. The main objective of my PhD thesis is to extend our knowledge of how these traits may affect the outcome of coevolution. In chapter 1 I review previous evolution experiments and discuss how transmission mechanisms may have been responsible for confounding results that have been seen in the past. The subsequent chapters relate to a host-parasite coevolution experiment where I allowed the red flour beetle, Tribolium castaneum, to coevolve separately with two parasites, the general entomopathogenic fungus, Beauveria bassiana, and its natural microsporidian parasite, Paranosema whitei. T. castaneum is able to secrete antimicrobial compounds into its environment and both parasites are able to produce persistent environmental stages. Within my experimental design I allowed for both the benefits of releasing transmissive stages into the environment for the parasites and secretion of antimicrobial compounds for the beetles to be realised. Coevolution resulted in both parasites increasing in virulence chapters 2 and 3. Fascinatingly, I was also able to show that in B. bassiana increased virulence was obtained via evolved resistance to the external immune compounds of the beetles chapter 3. Subtle reciprocal changes were also seen in hosts coevolved with B. bassiana chapter 4. This work highlights the importance of external immunity and transmissive parasite stages in coevolutionary interactions and the need for more studies where such effects are taken into account. Es wird angenommen, dass Wirt-Parasit-Koevolution eine starke Kraft in der Natur darstellt, mit der Interaktionen zwischen Wirt und Parasit zu schnellen reziproken Anpassungen führen. In den vergangenen Jahren wurden diverse Koevolutionsexperimente durchgeführt, um Vorhersagen über Dynamik und Folgen von Koevolutionsprozessen zu testen. Ein Aspekt der Wirt- und Parasit- Ökologie, dem in Wirts-Parasit Koevolutionsexperimenten bisher nur wenig Aufmerksamkeit geschenkt wude, ist die Fähigkeit beider Seiten, ihre Umgebung zu formen. Viele Parasiten sind in der Lage, transmissive Stadien zu bilden, die über mehrere Wirtsgenerationen in der Umwelt überleben können, und viele Wirtarten können ihre Umgebung durch Sekretion von antimikrobiellen Substanzen oder hygienischen Verhalten manipulieren. Das Hauptziel meiner Doktorarbeit ist es, unser Wissen darüber wie diese Eigenschaften das Ergebnis von Koevolution beeinflussen können, zu erweitern. In Kapitel 1 gebe ich eine Literaturübersicht über Koevolutionsexperimente und diskutiere, wie Transmissionsmechanismen die vergangenen Beobachtungen beeinflusst haben könnten. Die nachfolgenden Kapitel beziehen sich auf ein Wirts-Parasit-Koevolutionsexperiment, in dem ich den rotbraunen Reismehlkäfer, Tribulium castaneum, separat mit zwei seiner Parasiten koevolvieren ließ, dem entomopathogenen Pilz, Beauveria bassiana, und seinem natürlichen Mikrosporidium, Paranosema whitei. T. Castaneum kann antimikrobielle Substanzen in seine Umwelt abgeben und beide Parasiten sind imstande in der Umwelt persistente Stadien zu bilden. Durch mein experimentelles Design ermöglichte ich, dass die Vorteile der Parasiten, transmissive Stadien zu bilden, und die Vorteile des Wirts, antimikrobielle Substanzen in die Umwelt abzugeben, realisiert waren. Das Ergebnis dieser Koevoution war, dass beide Parasiten eine höher Virulenz entwickelten (Kapitel 2 und 3). Wir waren außerdem in der Lage zu zeigen, dass die höhere Virulenz in B. Bassiana auf eine erhöhte Resistenz gegenüber den externen Immunkomponenten der Käfer zurückzuführen war (Kapitel 3). Desweiteren konnten geringe Anpassungen des mit B. Bassiana coevolvierten Wirts (Kapitel 4) 9 nachgewiesen werden. Diese Arbeit zeigt die Wichtigkeit eines externen Immunsystems und transmissiver Stadien von Parasiten in coevolutionären Interaktionen, sowie die Notwendigkeit für weitere Studien, die diese Effekte betrachten.