On the identity of the amoeboflagellates Didascalus thorntoni and Adelphamoeba galeacystis

Didascalus thorntoni, Singh 1952 has been classified alternately as a separate genus or as a species of Naegleria. In the 18th edition of the American Type Culture Collection catalogue it is classified as Naegleria thorntoni. To resolve the question of its identity we have used riboprinting and sequencing of the small subunit ribosomal DNA. The results indicate that D. thorntoni does not belong to the genus Naegleria. The sequence of the small subunit ribosomal DNA differs only in 20 nucleotides (1%) from that of Paratetramitus jugosus. The difference is much smaller than between some species of Nageleria. Therefore, it is not clear whether D. thorntoni should be considered as a species of Paratetramitus or as a separate genus. The strain used in different laboratories as the type strain of Adelphamoeba galeacystis has been identified as a Naegleria strain. We believe that the type strain of A. galeacystis was mislabeled prior to submission to the American Type Culture Collection and to the Culture Collection of Algae and Protozoa. A recent isolate, which on the basis of morphology was identified as a strain of A. galeacystis, has the identical small subunit ribosomal DNA sequence as D. thorntoni. Our results prove Page was right when he stated that Adelphamoeba might be a synonym of Didascalus.