Metabolic traits of westslope cutthroat trout, introduced rainbow trout and their hybrids in an ecotonal hybrid zone along an elevation gradient JOSEPH B. RASMUSSEN 1 *, MICHAEL D. ROBINSON 1 , ALICE HONTELA 1 and DANIEL D. HEATH 2 1 Department of Biological Sciences, University of Lethbridge, 4401 University Dr W Lethbridge, AB, T1K 3M4, Canada 2 Department of Biological Sciences, Great Lakes Institute for Environmental Research, University of Windsor, 401 Sunset Windsor Ontario, N9B3P4, Canada Received 31 March 2011; revised 7 July 2011; accepted for publication 8 July 2011 In the Upper Oldman River, Alberta, introduced non-native hatchery rainbow trout (Oncorhynchus mykiss) hybridize with native westslope cutthroat trout (O. clarkii), resulting in a hybrid swarm. Rainbow trout dominate at low elevations (< 1250 m) in the river mainstem, cutthroat in high-elevation tributaries (> 1400 m), and hybrids are numerically dominant in the mid-elevation range. We hypothesized that metabolism of rainbow trout would exceed that of cutthroat trout, and that the elevation gradient in genetic makeup would be mirrored by a gradient in metabolic traits, with intermediate traits in the hybrid-dominated ecotone. Metabolic traits were measured and regressed against the genetic makeup of individuals and elevation. Rainbow trout had higher oxygen consumption rates (OCRs), higher white muscle lactate dehydrogenase (LDH), and citrate synthase (CS) activity, and higher plasma acetylcholinesterase (AchE) than cutthroat trout. Hybrids had intermediate OCRs and AchE, but LDH activity as high as rainbow trout. While hybrid zones are usually modelled as a balance between cross species mating and selection against hybrids, ecotonal hybrid zones, where hybrids proliferate in intermediate habitats and have traits that appear well suited to ecotonal conditions, have been proposed for some plants and animals, and may have important implications for resource management and conservation. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105, 56–72. ADDITIONAL KEYWORDS: acetylcholinesterase – citrate synthase – ecotone – elevation – hybrid zone – introgression – lactate dehydrogenase – oxygen consumption. INTRODUCTION Hybrid zones pose a paradox for taxonomy, evolution- ary biology and ecology in that they appear to chal- lenge the ‘separateness’ of species, and the efficacy of isolating mechanisms. Endangered or important resource species often hybridize with others, posing vexing management problems (Allendorf et al., 2004). Although hybridization can threaten species, intro- gression can also be a bridge for the exchange of functional traits among species (Rieseberg & Wendel, 1993; Grant & Grant, 1996). Hybridization occurs widely among salmonids, but little is known about the ecological aspects of such introgression and the func- tional traits involved (Williams et al., 2008). Hybrid zones are usually modelled as zones of tension between interspecific mating and factors that limit hybrid fitness (Fig. 1A), either through genomic incompatibility (Barton & Hewitt, 1985), eco- logical selection (Hatfield & Schluter, 1999), or both (Sperling & Spence, 1991; Springer & Heath, 2007). Alternatively, contact between populations that are segregated by habitat can produce an ecotonal hybrid zone, with fertile and successful hybrids exhibiting *Corresponding author. E-mail: joseph.rasmussen@uleth.ca Archived data: data deposited at Dryad: doi:10.5061/ dryad.rr388. Biological Journal of the Linnean Society, 2012, 105, 56–72. With 7 figures © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105, 56–72 56 Downloaded from https://academic.oup.com/biolinnean/article-abstract/105/1/56/2452490 by guest on 11 June 2020