Molecular Phylogenetics and Evolution 39 (2006) 209–222 www.elsevier.com/locate/ympev 1055-7903/$ - see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.ympev.2006.01.019 A mtDNA-based phylogeny of the brown algal genus Fucus (Heterokontophyta; Phaeophyta) James A. Coyer ¤ , Galice Hoarau, Marie-Pierre Oudot-Le Secq, Wytze T. Stam, Jeanine L. Olsen Department of Marine Biology, Centre for Ecological and Evolutionary Studies, University of Groningen, 9750 AA Haren, The Netherlands Received 27 June 2005; revised 5 December 2005; accepted 12 January 2006 Available online 21 February 2006 Abstract Species of Fucus are among the dominant seaweeds along Northern Hemisphere shores, but taxonomic designations often are con- founded by signiWcant intraspeciWc morphological variability. We analyzed intra- and inter-speciWc phylogenetic relationships within the genus (275 individuals representing 16 taxa) using two regions of the mitochondrion: a variable intergenic spacer and a conserved portion of the 23S subunit. Bayesian ML and MP analyses veriWed a shallow phylogeny with two major lineages (previously reported) and resolved some intra-lineage relationships. SigniWcant species-level paraphyly/polyphyly was observed within lineages 1A and 2. Despite higher species richness in the North Atlantic, a North PaciWc origin of the genus is supported by a gradient of decreasing haplotype and nucleotide diversities in F. distichus from the North PaciWc to the East Atlantic.  2006 Elsevier Inc. All rights reserved. Keywords: Fucus; mtDNA; Phylogeny; Center of origin; Hybridization; Trans-Arctic connectivity; Species-level paraphyly/polyphyly 1. Introduction A widely occurring group of marine organisms that con- tinues to challenge notions of species identity is the brown algal genus Fucus. Species are found in habitats ranging from the rocky intertidal to brackish salt marshes through- out the North PaciWc and North Atlantic Oceans (Bergs- tröm et al., 2005; Lein, 1984b; Lüning, 1990; Wynne and Magne, 1991). Most reproduce sexually, although two are hermaphroditic with variable levels of selWng (Coleman and Brawley, 2005b; Engel et al., 2005; Coyer et al., unpub. data) and another two can reproduce asexually by vegeta- tive propagation (Bergström et al., 2005; Malm et al., 2001; Tatarenkov et al., 2005). Nearly all species display signiW- cant morphological plasticity with over 125 subspecies, formae, varieties, ecotypes, and ecads described for just the eight most commonly recognized species (http:// www.algaebase.org). Causes of intraspeciWc morphological variation have been attributed to: (1) direct responses to single or combined abiotic factors (e.g., high temperatures, wave stress, pollution, and salinity) (Jordan and Vadas, 1972; Kalvas and Kautsky, 1993, 1998; Knight and Parke, 1950; McLachlan et al., 1971; Powell, 1963; Ruuskanen and Bäck, 1999; Sideman and Mathieson, 1983a, 1985; Tracy et al., 1995); (2) localized mosaics of phenotypes linked to genotypes adapted to speciWc environmental conditions (Kalvas and Kautsky, 1998; Knight and Parke, 1950; McLachlan et al., 1971; Munda and Kremer, 1997; Scott et al., 2001; Sideman and Mathieson, 1985), or (3) self-fer- tilization (in the hermaphroditic species), hybridization, and introgression (Burrows and Lodge, 1953; Coyer et al., 2002b, 2006; Engel et al., 2005; Munda and Kremer, 1997). Phylogenetic studies have utilized nuclear rDNA-SSU and LSU sequences to survey fucoid genera at the family level (Lee et al., 1998; Rousseau et al., 1997) and the more variable nrDNA-ITS (ITS) sequences to explore relation- ships at the genus level (Leclerc et al., 1998). In the most comprehensive evolutionary study of the Fucaceae to date, * Corresponding author. Fax: +31 50 363 2261. E-mail address: j.a.coyer@rug.nl (J.A. Coyer).