Olfactory genomics of bark- and ambrosia beetles: Evolution and function of chemoreceptors

Insect behaviours, such as host and mate selection, are often mediated by chemical cues. The chemical cues
are detected by large and rapidly evolving families of chemoreceptors which include odorant receptors (ORs),
gustatory receptors (GRs) and ionotropic receptors (IRs). In this thesis, I used a model system of beetle
(Coleoptera) species to study the functional evolution of insect ORs, their ligand binding mechanism, and the
diversification of the three chemoreceptor gene families in relation to differences in species ecology. Specifically,
I targeted beetles in the Curculionidae family that are pests on conifer trees, including Eurasian spruce bark
beetle Ips typographus and mountain pine beetle Dendroctonus ponderosae, striped ambrosia beetle
Trypodendron lineatum and the large pine weevil Hylobius abietis.
In paper I, orthologous ORs from I. typographus, D. ponderosae, H. abietis were functionally characterised.
This study revealed conserved responses across all species, with one set of orthologues responding to 2-
phenylethanol while the other set of orthologues responded to green leaf volatiles (GLVs) which serves as a
cue to avoid non-host angiosperms. Paper II focuses on the functional characterisation of two paralogous I.
typographus ORs (ItypORs) which responded to I. typographus pheromone compounds with different specificity.
The phylogenetic position of these ORs suggested multiple origins of pheromone receptors in bark beetles. This
study also revealed conserved amino acid residues in the binding pockets of the two ORs, and site-directed
mutagenesis confirmed direct involvement of two amino acids in the ligand binding. In paper III, I recorded
neuronal responses of the ambrosia beetle T. lineatum, which has a different ecology compared to bark beetles,
specifically in terms of fungal symbiosis. Using single sensillum recordings, thirteen olfactory sensory neuron
(OSN) classes were characterised. I found several OSNs responding specifically to volatiles produced by the
nutritional fungal mutualist Phialophoropsis ferruginea, indicating the importance fungal odors in this
association. In paper IV, I annotated the chemoreceptor gene families in the T. lineatum genome for
evolutionary comparisons with such receptors in bark beetles. This study revealed a comparatively small
chemoreceptor repertoire, which could relate to the specialized ecology of T. lineatum. Also, I found that T.
lineatum has lost several sugar receptors, and has comparatively few bitter taste GRs.
In conclusion, the overall work included in this thesis revealed that: 1) orthologous and paralogous receptors
may respond to the same or similar odors in various curculionid species; 2) pheromone receptors in bark beetles
have multiple evolutionary origins; 3) several OSNs in the ambrosia beetle are tuned to odors from the obligate
fungal mutualist, and; 4) the ambrosia beetle’s reduced chemoreceptor gene repertoire correlates with its
specialised diet.