RESUMEN
The phylogenetic relationship of Perkinsus olseni originating from the Ebro Delta, Spain, to other Perkinsus spp. was investigated using the nontranscribed spacer (NTS) region and the internal transcribed spacer (ITS) region (including ITS1, 5.8S and ITS2) of the ribosomal DNA sequences. These 2 molecular markers (NTS and ITS) were sequenced from prezoosporangia of Perkinsus sp. originating from Manila clam Ruditapes philippinarum from the Ebro Delta. The sequence of the 5.8S ITS region of the ribosomal RNA gene was 100% similar to that of P. olseni. Higher genetic variability was found for the NTS sequence, with 80.7 to 81.8% similarity to P. olseni. The NTS sequence of a P. olseni isolate previously detected in R. decussatus from the same area was also obtained and showed 81% identity with our isolate. Evidence obtained from phylogenetic analysis of the 5.8S ITS and NTS aligned sequences appears to indicate that P. olseni strains group together according to their host rather than their geographic origins within a well-resolved P. olseni clade.