Molecular evidence for the existence of two species of Marteilia in Europe

Marteilia refringens is one of the most significant pathogens of bivalve mo lluscs. previous sequencing of the small subunit ribosomal RNA gene of M.. refringens isolates derived from the infected mussels (Mytilus edulis and M ytilus galloprovinciallis) and the oyster (Ostrea edulis) in Europe did not reveal genetic polymorphisms despite indications from epizootiological dat a that district types may exist. We investigated the existence of polymorph isms in the internal transcribed spacer region of the ribosomal RNA genes. The sequences of this region proved to be clearly dimorphic among Marteilia from five sampling sites. The distribution of the two genetic types, named "O" and "M", appeared to be linked to the host species, oysters and mussel s, respectively. We therefore support the recognition of two species of Mar teilia in Europe and propose that the "O" type corresponds to M. refringens and the "M" type to M. maurini.