Comparative population genetic study of an important marine parasite from New Zealand flat oysters

The comparative genetic structure of hosts and parasites can reveal constraints acting on parasite dispersal among host populations and the evolution of local adaptation. We sampled New Zealand flat oysters Ostrea chilensis from 12 sites throughout New Zealand to (1) determine the distribution and prevalence of the haplosporidian parasites Bonamia exitiosa and B. ostreae, and (2) test for congruent patterns of host and parasite genetic structure. B. exitiosa was detected at three sites: Hauraki Gulf (5% prevalence), Marlborough Sounds (30%), and Foveaux Strait (7%), whereas B. ostreae was only detected in the Marlborough Sounds (37%). Using nuclear internal transcribed spacer (ITS) rDNA sequences of B. exitiosa, as well as mitochondrial cytochrome c oxidase subunit 1 gene (CO1) sequences of O. chilensis from the same sites plus other key O. chilensis growing areas (Tasman Bay and Chatham Islands), we compared the genetic structure of host and parasite. B. exitiosa displayed genetic structure across all three sites which were reflected in populations of O. chilensis except for gene flow between Tasman Bay-Marlborough Sounds-Chatham Islands. The observed patterns reflect the host specificity of Bonamia parasites and the limited dispersal capability of oysters. O. chilensis may experience long distance dispersal which is likely influenced by oceanographic factors. Nonetheless, a failure to detect Bonamia parasites among genetically connected O. chilensis populations suggests natural long distance co-dispersal of Bonamia parasites with O. chilensis is unlikely. Instead, the dispersal of Bonamia parasites is likely influenced by anthropogenic factors