Genetic differences between two sympatric morphs of Arctic charr confirmed by microsatellite DNA J. I. W ESTGAARD *, A. K LEMETSEN AND R. K NUDSEN Department of Aquatic BioSciences, The Norwegian College of Fishery Science, University of Tromsø, N-9037 Tromsø, Norway (Received 27 November 2003, Accepted 9 July 2004) Significant genetic differences (F ST ¼ 0032) were found between littoral and profundal morphs of Arctic charr Salvelinus alpinus from Fjellfrøsvatn, northern Norway, using microsatellite DNA analysis. The morphs had strong reproductive isolation in time and space; the segregation of a separate profundal morph is rare in postglacial lakes. # 2004 The Fisheries Society of the British Isles Key words: microsatellite DNA; profundal charr morph; reproductive isolation. Postglacial lakes with few fish species provide good examples of the importance of resource polymorphism as a diversifying force in vertebrates (Skulason & Smith, 1995; Smith & Skulason, 1996). Ecological speciation by divergent selection between populations that exploit contrasting lacustrine resource habi- tats has repeatedly been suggested for postglacial fishes (Schluter, 1996, 2001). Most sympatric fish morphs of postglacial lakes, including Arctic charr Salvelinus alpinus (L.), segregate along the benthic (littoral)-pelagic axis (Schluter, 1996; Smith & Skulason, 1996). Arctic charr are also found in deep water of some lakes (Klemetsen et al., 2003), and a few cases indicate the presence of a separate profundal morph (Brenner, 1980; Hesthagen et al., 1995; Klemetsen et al., 1997; Alekseyev et al., 2000). These cases are interesting because the profundal zone have poor food resources and a physical environment that differs very much from the epipelagic and littoral habitats (Schindler & Scheuerell, 2002), with special physical conditions (dark, cool and flat), no vegetation and strong resource limitation (low density and low diversity of invertebrate prey). The intensity of divergent natural selection may therefore be strong with a profundal niche occupation. The profundal morph of the postglacial Lake Fjellfrøsvatn, northern Norway (69  04 0 N; 19  19 0 E), is confined to the profundal habitat for its entire life cycle (Klemetsen et al., 1997). The sympatric littoral and profundal morphs of the lake differ in their spawning time by 5 months (spawning in September and March) and are strongly segregated in ecology, size and life history. *Author to whom correspondence should be addressed. Tel.: þ4777646796; fax: þ4777646020; email: jonivarw@nfh.uit.no Journal of Fish Biology (2004) 65, 1185–1191 doi:10.1111/j.1095-8649.2004.00524.x,availableonlineathttp://www.blackwell-synergy.com 1185 # 2004TheFisheriesSocietyoftheBritishIsles