Evidence for multiple species within the endoparasitic dinoflagellate Amoebophrya ceratii as based on 18S rRNA gene-sequence analysis

Parasitism within the group of dinoflagellates is a widespread phenomenon. Whether the parasitic dinoflagellates exhibit specificity in their infectio n is not well known, but this possibility has become an important issue in the development of biological control of harmful algal blooms. The 18S rDNA sequences from the parasite Amoebophrya sp. and its dinoflagellate host Di nophysis norvegica were determined and compared with the published sequence of Amoebophrya sp. infecting Gymnodinium sanguineum and other dinoflagella tes. The results showed that the sequence from the parasite within D. norve gica was clustered with that of the one from G. sanguineum with 100% bootst rap support in a maximum-likelihood analysis. The observed identity between these two sequences was 93%, which indicates that they are not identical s pecies. The two sequences from Amoebophrya sp. were deeply branched within the group of dinoflagellate sequences and represent the earliest diverging dinoflagellates. The sequence from the parasite Parvilucifera infectans, al so infecting D. norvegica, was not closely related to the Amoebophrya sp. s equences. The sequence from D. norvegica appeared as a sister group to a cl uster containing Prorocentrum lima and Alexandrium spp. without significant bootstrap support. The data presented herein support the hypothesis that A . ceratii comprises more than one species, and this opens the possibility t hat infections of harmful algal species might involve more than one Amoebop hrya species.