Polymorphisms in the sequences of Marteilia internal transcribed spacer region of the ribosomal RNA genes (ITS-1) in Spain: genetic types are not related with bivalve hosts B Novoa 1 , D Posada 2 and A Figueras 1 1 Instituto de Investigaciones Marinas, CSIC, Vigo, Spain 2 Facultad de Ciencias, Universidad de Vigo, Campus Universitario Lagoas-Marcosende, Vigo, Spain Abstract Marteilia refringens is a protozoan parasite causing a disease notifiable to the Office International des Epizooties (OIE) and its distribution has implica- tions for the transfer of live animals. The internal transcribed spacer-1 (ITS-1) from Marteilia clones contains polymorphism. Digestion with HhaI reveals two different restriction profiles, previously referred as ÔOÕ (Marteilia from oyster or Marteilia refringens) and ÔMÕ (Marteilia from mussels or Marteilia maurini). The aim of the present work was to determine whether the two previously des- cribed Marteilia molecular types (O and M) exist in the Iberian Peninsula and the strictness of the association with their bivalve host species. The sequence variability in the ITS-1 of Marteilia species was studied in mussels, Mytilus galloprovincialis, and flat oysters, Ostrea edulis, from different geograph- ical locations in Spain, to establish the existence and the distribution of different species or molecular types. Although there were two distinct evolution- ary lineages that corresponded more or less strictly with the ÔMÕ and ÔOÕ types, it was evident from the estimated phylogeny that some ÔOÕ types have switched to ÔMÕ type, and vice versa. Moreover, ÔOÕ types were found in mussels and ÔMÕ types were found in oysters, which suggests that there have been several cross-species transmissions of Marteilia between mussels and oysters. Keywords: bivalve, ITS, Marteilia maurini, Marte- ilia refringens, Mytilus galloprovincialis, Ostrea edu- lis. Introduction Marteilia refringens is a member of the Phylum Paramyxea (Berthe, Le Roux, Peyretaillade, Peyret, Rodriguez, Gouy & Vivares 2000), associated with serious and recurrent mortalities in the European flat oyster, Ostrea edulis. The disease caused by this parasite is currently listed by the Office Interna- tional des Epizooties (OIE) (2000) as a notifiable disease. Although the production of flat oysters in the European aquaculture is relatively low (5000 ton year )1 ), infection by M. refringens still has serious consequences. The presence of M. refringens in other bivalve species such as mussels, Mytilus galloprovincialis (production 650 000 ton year )1 ), means that these species are effectively considered as carriers for the pathogen, and as a consequence their movement between countries can be restricted. Marteilia refringens has been found in France, Greece, Italy, Morocco, Portugal and Spain. In its early developmental stages, the parasite is 5–8 lm in size reaching 40 lm during sporulation. In the bivalve host, the life cycle takes place mainly in the digestive tubules and in the epithelia of stomach and intestine. Interestingly, whilst the prevalence of Marteilia has remained stable in France, it has almost disappeared in Spain. Two different species of Marteilia have been reported in Europe, M. refringens, parasitic in the Journal of Fish Diseases 2005, 28, 331–338 Correspondence A Figueras, Instituto de Investigaciones Marinas, CSIC, Eduardo Cabello 6, 36208 Vigo, Spain (e-mail: pato1@iim.csic.es) 331 Ó 2005 Blackwell Publishing Ltd