Morphological investigation of three Tetramitus spp. which are phylogenetically very closely related: Tetramitus horticolus, Tetramitus russelli n. comb. and Tetramitus pararusselli n. sp.

Abstract Two vahlkampfiid amoebae have been isolated which have the same 5.8S rDNA sequence as Tetramitus horticolus, but which differ in internal transcribed spacer (ITS) sequences from this species. The two new strains, 123R and Rhodos, are identical with each other in ITS sequences, except for a 1 bp substitution in the ITS1 and an insertion–deletion sequence (indel) of 4 bp in the ITS2. Because of the close phylogenetic relationship between the two new isolates and T. horticolus we compared both isolates with the latter species by light and electron microscopy. While T. horticolus has a cyst pore structure not seen in any other vahlkampfiid, the two new strains have no pores in their cysts. Strains 123R and Rhodos are very similar in morphology, but significantly different in cyst diameter. Neither strain could be induced to form flagellates under laboratory conditions. Strain 123R seems to correspond to the description of Vahlkampfia russelli. Because of this, and its molecular identification as a strain of Tetramitus, we propose that this strain should become the neotype strain of V. russelli (Singh, 1952), now classified as T. russelli n. comb. Because of the small (but significant) differences of strain Rhodos from strain 123R we propose the former as the type strain of a new species, T. pararusselli n. sp. The addition of T. horticolus, T. russelli and T. pararusselli confirms that the phenotypic definition of the genus Tetramitus is obsolete. Instead the genus can be differentiated by the significant differences from other vahlkampfiid genera in rDNA sequence.

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