Within and among population differences in cuticular hydrocarbons in the seabird tick Ixodes uriae

The hydrophobic layer of the arthropod cuticle acts to maintain water balance, but can also serve to transmit chemical signals via cuticular hydrocarbons (CHC), essential mediators of arthropod behavior. CHC signatures typically vary qualitatively among species, but also quantitatively among populations within a species, and have been used as taxonomic tools to differentiate species or populations in a variety of taxa. Most work in this area to date has focused on insects, with little known for other arthropod groups such as ticks. The worldwide distribution and extensive host-range of the seabird tick Ixodes uriae make it a good model to study the factors influencing CHC composition. Genetically differentiated host-races of I. uriae have evolved across the distribution of this species but the factors promoting sympatric population divergence are still unknown. To test for a potential role of host-associated CHC in population isolation, we collected I. uriae specimens from two of its seabird hosts, the Atlantic puffin ( Fratercula arctica ) and the common guillemot ( Uria aalge ) in different colonies in Iceland. Using gas-chromatography and mass-spectrometry, we detected a complex cuticular mixture of 22 hydrocarbons, including n-alkanes, methyl-alkanes and alkenes ranging from 17 to 33 carbons in length. We found that each population had a distinct CHC profile. The host group explained the greatest amount of population divergence, with long-chain hydrocarbons being more abundant in puffin tick populations compared to guillemot tick populations. Future work will now be required to test whether the different CHC signals reinforce assortative mating, thereby playing a role in generating I. uriae population divergence patterns, and to evaluate diverse hypotheses on the origin of distinct population signatures.