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

The comparative genetic structure of host and parasite is important for understanding parasitic dispersal among host populations as well as the evolution of local host–parasite adaptations (Keeney et al. 2009). Dispersal among populations maintains gene flow, which is largely determined by the dispersal capabilities of a species and the presence or absence of barriers to dispersal, e.g., salinity gradients (Haskin and Ford 1982). The lifecycle of a parasite is intimately linked with its host; therefore, gene flow among parasite populations is often largely dependent on the host, and for parasites with complex lifecycles, population genetic structure is determined by the vagility of the most mobile host (Feis et al. 2015). Host and parasite population structure is often congruent (Criscione and Blouin 2007); however, different processes can contribute to different dispersal routes for parasites that are reflected in their population structure (Blasco-Costa et al. 2012). Indeed, a recent metaanalysis of comparative studies of host–parasite genetic structure has indicated that host-limited dispersal is not always the rule (Mazé-Guilmo et al. 2016). The Haplosporidian genus Bonamia contains three described species that are small (typically 1–3 μm), uninucleate, and infect oyster haemocytes causing a disease known as bonamiosis (Arzul and Carnegie 2015). Due to the severity of disease that these parasites can cause Bonamia Abstract 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