Phylogeographic perspective on the distribution and dispersal of a marine pathogen, the oyster parasite Bonamia exitiosa

The significance of infectious disease has intensified as our marine ecosystems are increasingly altered, with molluscan taxa being among the affected. One of the important pathogens to emerge in recent years, the oyster parasite Bonamia exitiosa,has a broad geographic distribution and has been found to infect a number of oyster species. In order to better understand how B. exitiosa achieved this wide distribution, a gene genealogy was constructed using internal transcribed spacer region ribosomal DNA sequencing data from across the host species range.The analysis revealed population structure in the form of 4 well-defined groups of sequences: 3corresponding to geographic regions (temperate Atlantic and Pacific waters of the Southern Hemisphere, California, and the western Atlantic along the coast of the Americas) and the fourth geographically cosmopolitan. Inclusion of B. exitiosa sequences from New Zealand, Australia, and Argentina in the Southern Hemisphere group may reflect natural dispersal of the parasite via raft-ing with oyster hosts, whereas the California group may reflect limited anthropogenic movement of a host species, Ostrea lurida. The extensive geographic distribution of B. exitiosa parasites belonging to the cosmopolitan and Atlantic Coast groups may relate to both natural and anthropogenic dispersal of a single host, O. stentina, which is distributed from the eastern Americas tothe Mediterranean and African coast to New Zealand — that is, in most regions where B. exitiosa has been found to occur