Molecular Ecology (2007) 16, 993–1005 doi: 10.1111/j.1365-294X.2006.03220.x © 2006 The Authors Journal compilation © 2006 Blackwell Publishing Ltd Blackwell Publishing Ltd Parasite phylogeographical congruence with salmon host evolutionarily significant units: implications for salmon conservation CHARLES D. CRISCIONE and MICHAEL S. BLOUIN Department of Zoology, Oregon State University, 3029 Cordley Hall, Corvallis, OR 97331, USA Abstract Comparative phylogeographical studies between parasites and their hosts or with bio- geographical regions are useful to predict parasite dispersal potential over a broad geographical range. We used both microsatellite markers and mtDNA sequence data from a trematode parasite, Plagioporus shawi, to test for congruence across two evolutionarily sig- nificant unit (ESU) boundaries of its salmonid hosts (Oncorhynchus spp.). We find congruent patterns with the nuclear loci of P. shawi and the ESU boundaries of its salmonid hosts. This pattern indicates that broad-scale phylogeographical patterns of a parasite can be predicted by the biogeographical history of their hosts. Furthermore, this pattern provides independent support for these ESU boundaries as biologically relevant barriers. The mtDNA shows some discordance with nuclear loci and a level of genetic differentiation greater than can be explained by genetic drift. Thus, the mtDNA cannot be used in isolation to infer the population history of P. shawi. The genetic differentiation at both the nuclear and mtDNA markers will be useful for salmon fisheries management by providing a tool to assign ocean-migrating salmonids back to their freshwater population of origin. Keywords: comparative phylogeography, conservation, gene flow barriers, Oncorhynchus spp., Plagioporus shawi, Trematoda Received 20 August 2006; revision accepted 30 October 2006 Introduction Comparative phylogeographical studies between parasites and their hosts or with biogeographical regions can be useful in several regards. (i) As with any comparative study among species, congruent phylogeographical patterns can identify historically and evolutionarily independent regions, and suggest shared biogeographical factors in shaping intraspecific population histories (Bermingham & Moritz 1998; Avise 2000). Furthermore, when species are of con- servation concern, the identification of distinct evolutionarily units is informative in management strategies (Bermingham & Moritz 1998). (ii) Comparative studies between hosts and their parasites can expand our understanding of parasite evolution and population history. For example, Nieberding et al. (2004) used cophylogeographical patterns between a parasite and its host to calibrate a molecular clock for the parasite. Because parasites are tightly bound to their hosts for survival, host movement or other behaviours may influence parasite genetic structure on local or regional scales (McCoy et al. 2003; Criscione & Blouin 2004). Thus, comparative host-parasite studies are useful to predict parasite dispersal potential and scale of gene flow over a broad geographical range. (iii) Recent studies indicate that parasite molecular data can potentially elucidate host population history or demography (Nieberding et al. 2004; McCoy et al. 2005; Whiteman & Parker 2005; Wirth et al. 2005; Biek et al. 2006), and can serve as useful markers to indicate host-source populations (Criscione et al. 2006). Despite the potential usefulness of parasite genetic data, phylogeographical studies on nonhuman parasites are relatively scarce (reviewed in Criscione et al. 2005; recent papers by McCoy et al. 2005; Nieberding et al. 2005). Correspondence: Charles D. Criscione, Department of Genetics, Southwest Foundation for Biomedical Research, PO Box 760549, San Antonio, TX 78245, USA. Fax: 210-258-9796; E-mail: ccriscio@darwin.sfbr.org