Seabird tick diversification and cuticular hydrocarbons
Within and among population differences in cuticular hydrocarbons in the seabird tick Ixodes uriae
Ticks are notorious vectors of diseases in humans and other vertebrates. Much effort has been expended to understand tick diversity and ecology with the aim of managing their populations to alleviate the misery they bring. Further, the fundamental question of whether ticks are usually host generalists or host specialists has been debated at length and is important both for understanding the mechanisms of their diversification as well as for focusing control of ticks .
One elegant resolution of this question is to consider most tick species to be global generalists but local specialists . This is well illustrated in a series of studies of the seabird tick, Ixodes uriae, which is comprised of host-specific races that show genetic , morphological  and host performance  differences associated with the seabirds they feed on. Such a pattern has clear ramifications for sympatric speciation; however, the factors that potentially act to drive these differences have remained elusive.
Dupraz et al.  have now made intriguing and important steps toward bridging the gap between demonstrating local patterns of tick host association and understanding the physiological mechanisms that may facilitate such divergences. They collected I. uriae ticks from the nests of two seabirds – Atlantic puffins and common guillemots – on the north side of Iceland. Four populations of ticks were sampled, with one island providing both puffin ticks and guillemot ticks, to give two tick populations from each of the two seabird host species. They then washed the ticks in solvent and analyzed the dissolved cuticular hydrocarbons (CHCs) using GC mass spectrometry, revealing 22 different hydrocarbon compounds common to most of these samples. CHCs are known to be important across arthropods for a variety of functions ranging from reducing water loss to facilitating communication and recognition between individuals with species.
Dupraz et al.  found three hydrocarbons that distinguished puffin ticks most consistently from guillemot ticks. A cross-validation test for host type also assigned 75% of the tick pools to the seabird host of origin. However, with these limited sample sizes, statistical analysis revealed no significant difference in CHC profiles between the host types, although a tendency was evident. Nonetheless, this study revealed a number of potentially diagnostic CHCs for tick host type, as well as some that may be more diagnostic of locations. This provides a fascinating and actionable foundation for further work using additional sites and host types, as well as an entry point into discerning the mechanisms at play in producing the diversity, complexity and adaptability that make ticks such medical menaces.
 McCoy, K.D., Léger, E., Dietrich, M., 2013. Host specialization in ticks and transmission of tick-borne diseases: a review. Front. Cell. Infect. Microbiol. 3. https://doi.org/10.3389/fcimb.2013.00057
 McCoy, K.D., Chapuis, E., Tirard, C., Boulinier, T., Michalakis, Y., Bohec, C.L., Maho, Y.L., Gauthier-Clerc, M., 2005. Recurrent evolution of host-specialized races in a globally distributed parasite. Proc. R. Soc. B Biol. Sci. 272, 2389–2395. https://doi.org/10.1098/rspb.2005.3230
 Dietrich, M., Beati, L., Elguero, E., Boulinier, T., McCoy, K.D., 2013. Body size and shape evolution in host races of the tick Ixodes uriae. Biol. J. Linn. Soc. 108, 323–334. https://doi.org/10.1111/j.1095-8312.2012.02021.x
 Dietrich, M., Lobato, E., Boulinier, T., McCoy, K.D., 2014. An experimental test of host specialization in a ubiquitous polar ectoparasite: a role for adaptation? J. Anim. Ecol. 83, 576–587. https://doi.org/10.1111/1365-2656.12170
 Dupraz, M., Leroy, C., Thórarinsson, T. L., d’Ettorre, P. and McCoy, K. D. (2022) Within and among population differences in cuticular hydrocarbons in the seabird tick Ixodes uriae. bioRxiv, 2022.01.21.477272, ver. 5 peer-reviewed and recommended by Peer Community in Zoology. https://doi.org/10.1101/2022.01.21.477272
Felix Sperling (2022) Seabird tick diversification and cuticular hydrocarbons. Peer Community in Zoology, 100014. https://doi.org/10.24072/pci.zool.100014
Evaluation round #1
DOI or URL of the preprint: https://doi.org/10.1101/2022.01.21.477272
Author's Reply, 30 Jun 2022
Decision by Felix Sperling, 27 Mar 2022
Dear Marlène Dupraz and coauthors,
Two reviews of your manuscript have now been received, and I agree with both of them that this is an interesting and generally well written study. However, they raise some key issues and edits that should be addressed before publication can be considered. In particular, Reviewer 1 recommends the addition of further information on the methods as well as clarification of several items and explicit consideration of factors such as mating status and microclimate. Reviewer 2 suggests that the authors tie in hydrocarbon synthesis pathways, and include more explicit discussion of how environmental factors may alter the tick hydrocarbon profile. All points raised by the reviewers should be addressed, whether in a revised version of the ms or in rebuttal.
In addition, from my own review of the ms: 1) although this manuscript does a good job of considering the role CHCs may play in reproduction, the discussion seems a bit biased toward the hypothesized involvement of these hydrocarbons in host race formation. Other functions should be considered more fully. For example, Yoder and Domingus (2003) demonstrated that long chain hydrocarbons secreted by Dermacentor variabilis ticks act as a defence against ant predation. Also, 2) this statement needs to be clarified: “For example, aging favors the production of longer hydrocarbon chains and decreased attractiveness in Drosophila melanogaster (Kuo et al., 2012).” It is not fully clear what kind of attractiveness is intended here.
Consequently, this paper still requires revision before it could be recommended. However when the methods and other items listed in the reviews have been clarified, and the discussion has been expanded to include the factors outlined above, this paper should be a valuable addition to the literature on the role of tick cuticular hydrocarbons.
- Felix Sperling