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Evolutionary divergence in Philonema (Nematoda; Philometridae) parasites of B.C. salmonids Clease, Derek Fraser


Philonema (Nematoda; Philometridae) from anadromous hosts, sockeye salmon (Oncorhynchus nerka), and non-anadromous hosts, rainbow trout (Oncorhynchus mykiss) , were studied in order to determine if hosts with different life histories were infected with the same or different species of Philonema. Worms from the two host species were morphologically indistinguishable. However, electrophoretic banding patterns produced by restriction enzyme digestion of DNA extracted from Philonema demonstrated the presence of two genetic types corresponding to the two host species. This supports the idea that at least two species of Philonema are endemic in British Columbia. Philonema oncorhynchi Kuitunen-Ekbaum, 1933 is a parasite of sockeye salmon which undergo a long ocean migration before returning to freshwater to spawn, while P. agubernaculum Simon and Simon, 1936 is a parasite of rainbow trout (and other salmonids) which live in lakes. Kokanee (O. nerka kennerlyi), a non-anadromous offshoot of sockeye, were infected with the same worm as sockeye probably because the two hosts have similar life histories. Steelhead smolts, anadromous O. mykiss, contained worms identified as P. agubernaculum. This likely represented an accidental infection because steelhead do not usually contact Philonema. Philonema were examined from various localities in B.C. Philonema agubernaculum showed population divergence corresponding to the different geographic localities from which it was collected. This likely reflects the isolation of these parasite populations in unconnected watersheds. Philonema oncorhynchi showed polymorphisms spread throughout many of the populations. The lack of population divergence probably results from gene flow between parasite populations brought about by wandering hosts.

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