Body size and shape evolution in host races of the tick Ixodes uriae MURIEL DIETRICH 1 *, LORENZA BEATI 2 , ERIC ELGUERO 1 , THIERRY BOULINIER 3 and KAREN D. MCCOY 1 1 Maladies Infectieuses et Vecteurs, Ecologie, Génétique, Evolution et Contrôle, UMR UM1 UM2 CNRS 5290 – UR IRD 224, Centre IRD, 911 Avenue Agropolis, BP 64501, 34394 Montpellier, France 2 Institute of Arthropodology and Parasitology, Georgia Southern University, Georgia Avenue, Building 204, PO Box 8056, Statesboro, GA 30460, USA 3 Centre d’Ecologie Fonctionnelle et Evolutive, UMR CNRS 5175, Montpellier, France Received 18 June 2012; revised 31 August 2012; accepted for publication 31 August 2012 The tick Ixodes uriae is a common ectoparasite of seabirds, and is widely distributed across the circumpolar regions of both hemispheres. Previous work demonstrated the existence of genetically distinct host races of this ectopara- site, occurring across its current range. The objective of the present study was to examine whether these host races have evolved measurable morphological differences. We measured a set of morphological variables on 255 non-engorged ticks (nymphs and adults) collected from three sympatrically occurring host species in the North Atlantic. Genotyping at eight microsatellite markers enabled us to analyse the relationship between patterns of morphological and neutral genetic variation. Multivariate analyses showed that most morphological variation was associated with size differences among tick individuals. Body size differed among races, but only in adult life stages. A linear discriminant analysis based on shape variation revealed three distinct morphological clusters correspond- ing to the three tick host races. These results, along with correlated patterns of host-related genetic variation, suggest that differences among host-related groups are not simply the result of phenotypic plasticity or drift, but rather reflect host-associated adaptations. Experimental work and observations across the range of I. uriae will now be required to test the genetic basis and adaptive nature of morphological differences. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 108, 323–334 ADDITIONAL KEYWORDS: adaptation – bird – morphometry – parasite – sympatric speciation. INTRODUCTION Molecular genetic studies have revealed an increas- ing number of cryptic parasite lineages that are genetically divergent, but considered to be morpho- logically indistinguishable (Bickford et al., 2007). A key mechanism driving the evolution of such cryptic diversity is the adaptation of parasites to different host habitats or resources (De Meeûs, Michalakis & Renaud, 1998). However, parasite adaptation often involves morphological changes (Poulin, 2007), thus begging the question of whether these lineages are indeed cryptic. Direct evidence for an association between morphological variation and the diversifica- tion process is provided by a number of studies on phytophagous insects (e.g. Schmidt, Walter & Moore, 2000; Pappers et al., 2002; Svensson, Althoff & Pellmyr, 2005), but remains scarce among other groups of parasites (e.g. Huyse & Volckaert, 2002; Klimov, Bochkov & OConnor, 2006; Malenke, Johnson & Clayton, 2009). Understanding the fre- quency and nature of the morphological changes that accompany host-associated divergence are essential if we want to better comprehend the factors that favour parasite diversification. With the increasing popularity of molecular methods, morphological approaches have been over- shadowed by population genetics studies, thereby *Corresponding author. Current address: CRVOI – GIP CYROI, 2 rue Maxime Rivière, BP 80005, 97491 Sainte Clotilde cedex, France. E-mail: muriel.dietrich@gmail.com Biological Journal of the Linnean Society, 2013, 108, 323–334. With 3 figures © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 108, 323–334 323 Downloaded from https://academic.oup.com/biolinnean/article-abstract/108/2/323/2452764 by guest on 27 May 2020