Journal of Fish Diseases 2002, 25, 575–582
Ribosomal DNA sequences indicate isolated populations of Ichthyophonus hoferi in geographic sympatry in the north-eastern Pacific Ocean
C D Criscione1, V Watral2, C M Whipps2, M S Blouin1, S R M Jones3 and M L Kent2
1 Department of Zoology, Oregon State University, Corvallis, OR, USA 2 Department of Microbiology, Center of Salmon Disease Research, Oregon State University, Corvallis, OR, USA 3 Department of Fisheries and Oceans, Pacific Biological Station, Nanaimo, British Columbia, Canada
Abstract Introduction Infections of Ichthyophonus hoferi, a cosmopolitan Ichthyophonus hoferi (Plehn & Mulsow) is a parasite of marine fish, have recently been reported cosmopolitan parasite that infects most organs and in rockfish, Sebastes spp., from the north-eastern tissues of many marine fish (McVicar 1999). The Pacific. Because I. hoferi also infects Pacific herring, disease caused by I. hoferi is of economic signifi- Clupea pallasi Valenciennes, and salmonids in this cance because epizootics have resulted in mass region, we wanted to determine if Ichthyophonus mortality of commercial fish species such as parasites from rockfishes, Pacific herring and chi- Atlantic herring, Clupea harengus L. (Patterson nook salmon, Oncorhynchus tshawytscha (Walbaum), 1996; Rahimian & Thulin 1996; Mellergaard & were the same. Small subunit ribosomal deoxyri- Spanggaard 1997). Although systemic infections are bonucleic acid sequence data revealed two haplo- lethal, variation in pathogenic effects has been types that were fixed among host species in observed between isolates of I. hoferi and among geographic sympatry, one from rockfish and the host species (McVicar & McLay 1985; Kocan, other from both Pacific herring and salmon. These Hershberger, Mehl, Elder, Bradley, Wildermuth & isolated populations of Ichthyophonus could be part Stick 1999). For example, experimental infections of the same species that are ecologically separated with a Pacific isolate of I. hoferi resulted in 80% because of host behaviours, or they could be distinct mortality of Pacific herring, C. pallasi Valenciennes, species that are host specific. Dietary patterns of the whereas no mortality resulted from an Atlantic hosts indicate that ecological separation among isolate (Kocan et al. 1999). Such variation, along hosts is possible, but the presence of distinct species with the broad range of hosts from which I. hoferi may better explain the observed Ichthyophonus has been reported (McVicar 1999), raises the haplotype association with host species. question of whether cryptic species (genetically distinct, but morphologically indistinguishable) Keywords: Clupea pallasi, Ichthyophonus hoferi, occur among populations of I. hoferi. Oncorhynchus tshawytscha, Pacific Ocean, Sebastes Molecular data provide a means to identify spp., small subunit 18S rDNA. cryptic species, as was suggested by the descrip- tion of I. irregularis by Rand, White, Cannone, Gutell, Murphy & Ragan (2000). Otherwise, little attention has been given to the use of this approach to understand speciation among popula- tions of I. hoferi. Use of deoxyribonucleic acid Correspondence C D Criscione, Department of Zoology, (DNA) sequence data has primarily focused on Oregon State University, 3029 Cordley Hall, Corvallis, OR 97331, the phylogenetic relationship of I. hoferi to other USA. eukaryotes (Ragan, Goggins, Cawthorn, Cerenius, (e-mail: [email protected])