How do cuticular hydrocarbons evolve? Physiological constraints and climatic and biotic selection pressures act on a complex functional trait

Abstract

Cuticular hydrocarbons (CHCs) cover the cuticles of virtually all insects, serving as a waterproofing agent and as a communication signal. The causes for the high CHC variation between species, and the factors influencing CHC profiles, are scarcely understood. Here, we compare CHC profiles of ant species from seven biogeographic regions, searching for physiological constraints and for climatic and biotic selection pressures. Molecule length constrained CHC composition: long-chain profiles contained fewer linear alkanes, but more hydrocarbons with disruptive features in the molecule. This is probably owing to selection on the physiology to build a semi-fluid cuticular layer, which is necessary for waterproofing and communication. CHC composition also depended on the precipitation in the ants' habitats. Species from wet climates had more alkenes and fewer dimethyl alkanes than those from drier habitats, which can be explained by different waterproofing capacities of these compounds. By contrast, temperature did not affect CHC composition. Mutualistically associated (parabiotic) species possessed profiles highly distinct from non-associated species. Our study is, to our knowledge, the first to show systematic impacts of physiological, climatic and biotic factors on quantitative CHC composition across a global, multi-species dataset. We demonstrate how they jointly shape CHC profiles, and advance our understanding of the evolution of this complex functional trait in insects.

Keywords: adaptation; climatic niche; cuticular hydrocarbons; selection pressure; viscosity; water loss rate.

Figure 1.

Effects of median chain length of the total CHC profile on the percentage of different substance classes. Black circles, Camponotus s.l.; grey circles, Crematogaster. The lines represent model regressions with confidence intervals. For better visibility, jitter was added to the chain length values (x-axis). ***p < 0.001; **p<0.01; *p < 0.05; n.s., non-significant.

Figure 2.

Effects of annual precipitation at the collection site on the percentage of different substance classes and on median chain length (non-parabiotic species only). Black circles, Camponotus s.l.; grey circles, Crematogaster. The lines represent model regressions with confidence intervals. For better visibility, jitter was added to the precipitation values (x-axis) and, in the lowermost plot, to the chain length data (y-axis). ***p < 0.001; **p < 0.01; *p < 0.05; n.s., non-significant.

Figure 3.

Effects of the parabiotic lifestyle (a) on the abundance of different CHC classes and (b) on median chain length (tropical species only). The plots show model estimates (mean ± s.e.). The effects are shown separately for Camponotus s.l. and Crematogaster if they differed between the two taxa, but pooled if the interaction was non-significant. ***p < 0.001; **p < 0.01; *p< 0.05; n.s., non-significant.