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RESEARCH PRODUCT

Evolutionary ecology of immune priming in the mealworm beetle, Tenebrio molitor

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Many organisms can improve their immune response as a function of their immunological experience, a phenomenon called immune priming. While the mechanisms through which immune priming is achieved remain unknown, individuals that survived to a given parasite are better protected against subsequent exposures. This immune priming can cross generations (trans-generational immune priming – TGIP), preparing offspring for prevailing parasite environment. Both individual and trans-generational immune priming might be adaptive and may have evolved from repeated challenges by the same pathogens during the host lifetime or across generation. While protection could be cross-reactive, a certain level of specificity may exist in response to the range of pathogens from which immue priming may have evolved. Thus, immune priming and TGIP should be more efficient and less costly with respect to pathogens exposing the host to the greatest probability of re-infection. Moreover, it is now known that insect immune response is genetically variable. To understand the evolution of TGIP and its impact on life history evolution, we need to explore its quantitative genetics. During my thesis, I found that the expression of individual immune priming and TGIP in the mealworm beetle, Tenebrio molitor, is dependent of a range of pathogens that might have been a major selective pressure on the immune system of this insect species. This was done through the characterisation of costs and benefits of the expression of immune priming in response to challenges with a large range of bacterial pathogens. This work also highlighted potential mechanisms through which these immune phenomena could be achieved.In a first chapter of this thesis, we examined the survival of individuals to infection with different bacteria according to their own immunological experience or that of their mother with these bacteria. We found that priming response to Gram-positive bacteria was particularly more efficient and less costly than priming response to Gram-negative bacteria. This study also shows that, contrary to what is currently believed, the cellular component of the T. molitor immune system does not necessarly play a major role in providing immune protection through individual immune priming or TGIP.In a second chapter, we have stimulated the immune system of adult females with two Gram-positive bacteria to study maternal transfer of immunity to the eggs. We found that the process throght which eggs are protected is dependent on the bacterial pathogen used to immune challenge the mother. Indeed, depending of the bacterial pathogen that immune challenged the mother, antibacterial activity in the eggs are either transfeered by the mother or produced by the egg itself, Furthermore, whatever the mechanism through which egg protection was achieved, primed eggs exhibited enhanced hatching rate and the resulting larvae even showed improved early survival to food privation.In a third chapter, we used inbred lines of T. molitor to study the quantitative genetics of TGIP. The aim of this work was to test whether TGIP could be heritable and whether its expression is genetically associated to other fintness traits of mothers and offspring. Unfortunately, due to a low number of inbred lines available and a low number of samples within some of these lines, it was impossible to conclude about the genetic basis associated to TGIP.In a fourth chapter, we produced a review on TGIP. This allowed us to highlight the main characteristics and mechanisms curently identified, and the ecology and the evolution of the phenomenon.Costs and benefits associated to immune priming and TGIP suggest that Gram-positive bacteria might have been a major selective pressure at the origin of these phenomena in T. molitor. Whether TGIP has genetic basis still required further research.