The evolution of a complex trait: cuticular hydrocarbons in ants evolve independent from phylogenetic constraints.

Cuticular hydrocarbons (CHCs) are ubiquitous and highly diverse in insects, serving as communication signal and waterproofing agent. Despite their vital function, the causes, mechanisms and constraints on CHC diversification are still poorly understood. Here, we investigated phylogenetic constraints on the evolution of CHC profiles, using a global data set of the species-rich and chemically diverse ant genus Crematogaster. We decomposed CHC profiles into quantitative (relative abundances, chain length) and qualitative traits (presence/absence of CHC classes). A species-level phylogeny was estimated using newly generated and previously published sequences from five nuclear markers. Moreover, we reconstructed a phylogeny for the chemically diverse Crematogaster levior species group using cytochrome oxidase I. Phylogenetic signal was measured for these traits on genus and clade level and within the chemically diverse C. levior group. For most quantitative CHC traits, phylogenetic signal was low and did not differ from random expectation. This was true on the level of genus, clade and species group, indicating that CHC traits are evolutionary labile. In contrast, the presence or absence of alkenes and alkadienes was highly conserved within the C. levior group. Hence, the presence or absence of biosynthetic pathways may be phylogenetically constrained, especially at lower taxonomic levels. Our study shows that CHC composition can evolve rapidly, allowing insects to quickly adapt their chemical profiles to external selection pressures, whereas the presence of biosynthetic pathways appears more constrained. However, our results stress the importance to consider the taxonomic level when investigating phylogenetic constraints.