CERCOMONADS AND THEIR RELATIONSHIP TO THE MYXOMYCETES

The typical cercomonad structure (based on original investigations of Heteromita sp., Cercomonas agilis and literature data) is as follows: the amoeboflagellate cell is covered by only a distinct glycocalyx; th ere are two flagella, extrusomes, microbodies, mitochondria with vesic ular cristae, and the nucleus has a sharp end directed towards the bas al bodies. Absolute orientation of the flagellar apparatus reveals an anterior basal body directed in a forward or ventral direction, and th e posterior basal body is located on the ventral side and is directed to the right. There are 2-4 lateral microtubular rootlets, a layered s tructure and a nuclear fibrillar rootlet. A dictyosome occupies the do rsal position. The life-cycle of cercomonads includes a plasmodial sta ge. Comparison of cercomonad morphology and life-cycles with those of myxomycetes (predominantly with zoospores of protostelids) and Hyperam oeba flagellata (a fresh-water protist of uncertain taxonomic position ) reveals the homology of structures in the flagellar rootlet system; the same absolute orientation of the flagellar apparatus and dictyosom e; and mitochondria with vesicular cristae. Homology of the main stage s in the life-cycles of all three groups has been also proposed. The r elationship of cercomonads and protostelids has been confirmed by new ultrastructural data, and H. flagellata is considered as a bridge betw een these two groups. This view contradicts molecular data which usual ly demonstrates a long distance between cercomonads and myxomycetes.