Mechanisms of host plant induced plasticity of cuticular hydrocarbon phenotypes in mustard leaf beetles

• Applicant: Dr. Sven Geiselhardt
• Subject Area: Evolution, Anthropology ; Animal Physiology and Biochemistry
• Term: from 2017 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 392668240

Cuticular hydrocarbons (CHCs) play an essential role in species and mate recognition in insects. Among other biotic and abiotic factors, diet could affect the CHC phenotype of an insect and these changes may modify the mating behavior and lead to reproductive isolation between populations or closely related species with divergent host use. The two oligophagous mustard leaf beetle species Phaedon armoraciae and Phaedon cochleariae co-occur in the same habitat, but differ in host use. Under natural conditions (different host species), both species show behavioral isolation. However, if both species were reared on the same host plant species, behavioral isolation breaks down and both species mate randomly. Chemical analyses show that the CHC phenotype is strongly affected by host use. CHC phenotypes of beetles reared on the same host species are more similar than CHC phenotypes of beetles reared on different host species, irrespective of population or species identity of the beetles. Diet could have a significant effect on the gut microbiome. These changes in the gut microbiome could change the CHC phenotype as well as the mating behaviour. The aims of this research project are (A) to investigate whether host-specific assortative mating is adaptive in terms of performance and immunocompetence of the offspring, and (B) to elucidate the mechanisms behind the diet-inducted plasticity of CHC phenotypes in mustard beetles and the potential role of the gut microbiome. In a first step, we will rear both beetle species on different host plant species to illustrate the impact of divergent host use on the gut microbiome. In order to identify the plant-associated trigger for the change in CHC phenotypes, we will rear beetles (i) on a single host species of which we will manipulate the phyllosphere microbiomes differently, and (ii) on different artificial diets that differ in the supplementation with host-specific secondary metabolites. In a next step, we will follow the change of the gut microbiome after a host shift to associate the temporal change of the gut microbiome with the change in the CHC phenotype. Furthermore, we will use an RNA-seq approach to analyse the impact of divergent host use and gut microbiomes, respectively, on gene expression in beetles with a special focus on oenocytes, the site of CHC biosynthesis. Subsequently, the role of selected candidate genes in host-triggered plasticity of CHC phenotypes will be validated by RNA interference (RNAi). The results of our project will provide new fundamental knowledge about the regulatory mechanisms of the CHC biosynthesis and highlight the role of interspecific interactions and phenotypic plasticity in the divergence of communication systems in insects.

• DFG Programme: Research Grants