Abstract:
As the outermost organ of insects, the cuticle has diverse functions conferring
strong adaptability to various environments: 1) as a solid exoskeleton, it offers
support and muscle attachment sites needed for locomotion; 2) as a barrier, it
not only prevents inner water evaporation under extreme conditions, but also
protects against penetration of xenobiotics and pathogenic microorganisms; 3)
it may also serve as a vector to transport attached pollens, microorganisms and
even diseases, thereby contributing to ecological communication.
To fulfill its ecological impact, the insect cuticle is composed of mainly three
components: the polysaccharide chitin, proteins and lipids. Thickness mainly
depends on the co-assembly of chitin and cuticular proteins; while the barrier
function counts on the presence and composition of cuticular lipids.
In this study, two insect species (Drosophila melanogaster and Locusta
migratoria) were chosen to address different aspects of the function of the
cuticle in legs. Using RNA interference (RNAi), I studied the function of cuticular
lipids in D. melanogaster with respect to their importance as xenobiotics barrier
and as a possible substrate for the cuticle microbiome. When silencing key
genes (desat1, fasn2, cyp4g1) of the lipid synthesis pathway, unsaturated,
methyl branched and total CHC production were reduced, respectively.
Xenobiotics penetration (insecticides and dye) and surface microorganism
analyses showed that especially unsaturated CHCs are important for both
aspects. Moreover, data presented here indicate that the tarsal cuticle actively
contribute to the interaction of the insect with its proximal environment and its
modification at least partly through the surface lipids.
The laminate structure of cuticle in locust tibia has been reported. It is formed
following a circadian rhythm. Indeed, two different chitin orientation modes
during day and night occur. The main difference between day and night
organization is possibly the involvement of specific proteins. Here, two proteins,
LmObst-B and LmObst-C, were identified by proteomics in “night” cuticlesamples. Antibody staining was performed to study the localization of Obst- B
and Obst-C during the day. I show that Obst- B and Obst-C activity does not
follow the circadian rhythm of their expression but rather of the subcellular
localization from the cytoplasm (night) to the membrane (day). Functional
studies were conducted by RNAi; after co-injection of dsLmObst-B and
dsLmObst-C, locomotion (walking and jumping) deficiency was observed. In
addition, mechanical analyses indicate a weaker cuticle with a lower Young's
modulus.
In summary, the functions of different components of the insect cuticle were
described in this dissertation, focusing on the legs. This work contributes, hence,
to the advancement of our understanding of the function of the insect leg cuticle.
locomotion