Important ultrastructural aspects

Genus-level Identification
Genera		*Spironucleus*		*Hexamita*		*Octomitus*
Flagellar pockets (cytostomal canals)^a		+		+		-
Central axis formed by recurrent axonemes, microtubular bands, endoplasmic reticulum		-		-		+
Two terminal spikes		-		-		+
Shape of nuclei		S-shaped		Spherical		Reniform
Location of kinetosomes relative to nuclei		Sub-apical		External structure		Between
Position of recurrent flagella relative to nuclei		Medial		Lateral		Medial
Supra-nuclear microtubular band		+		+		Reduced
Infra-nuclear microtubular band		+		+		Reduced
Species-level Identification
Species	*S. barkhanus /S. salmonicida*		*S. salmonis*		*S. torosa*		*S. vortens*
Flagellar pocket ornamentation	Crescent-shaped (barkhan)		No ornamentation		Ring-shaped swellings (torus)		Counter-crossing ridges
Microtubular pattern around flagellar pocket	3 radiate bands		-		3 concentric bands		3 staggard bands
Distinctive organelles	-		Electron-dense bodies / membrane-bound sac of free ribosomes		-		Hydrogenosomes^b

^aWhen present, the recurrent flagella are ensheathed
^bMay also be present in other Spironucleus spp. but currently only documented in S. vortens

Table 2: Important ultrastructural aspects for genus and species level identification of diplomonad genera and Spironucleus spp. respectively. Adapted from Poynton and Sterud [1] with permission from John Wiley & Sons Ltd.

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