Archamoebae: the ancestral eukaryotes?

Abstract

The archezoan phylum Archamoebae Cavalier-Smith, 1983 is here modified by adding a new order Phreatamoebida (presently containing only Phreatamoeba) and removing the family Entamoebidae. Entamoebidae are instead tentatively placed as a class Entamoebea together with the classes Heterolobosea, Percolomonadea and Pseudociliatea in the new protozoan phylum Percolozoa Cavalier-Smith, 1991. Thus emended the phylum Archamoebae is more homogeneous; it is more distinguished from the other two phyla of the primitively amitochondrial kingdom and superkingdom Archezoa (i.e. Metamonada and Microsporidia) by having kinetids with only a single flagellum and basal body and a flagellar root consisting of a cone of evenly spaced microtubules. This unikont character of the archamoebae suggests that they may be ancestral to the tetrakont Metamonada, from which the non-flagellate Microsporidia possibly evolved. Higher eukaryotes (superkingdom Metakaryota) probably evolved from a tetrakont metamonad by the symbiotic origin of mitochondria and peroxisomes. If so, the Archamoebae are the most primitive extant phylum of eukaryotes; if molecular phylogenetic studies confirm this idea, Archamoebae will deserve intensive study, which could reveal much about the origin of the eukaryote condition and also establish what is truly universal among eukaryotes. Archamoebae, like other Archezoa, lack mitochondria and peroxisomes and have no obvious Golgi dictyosomes. Their evolutionary significance is discussed and a detailed classification is presented in which the two earlier classes are merged into a single one: Pelobiontea Page, 1976 stat. nov., containing two orders Mastigamoebida Frenzel, 1892 (Syn. Rhizo-Flagellata Kent, 1880 non Rhizomastigida auct.) (including Mastigamoeba, Mastigina, Mastigella, Pelomyxa and probably a few other genera, which have one or more flagella or cilia (motile or immotile, 9 + 2 or otherwise) in the amoeboid trophic phase), and Phreatamoebida ord. nov. (including only Phreatamoeba in the new family Phreatamoebidae, which has alternating phases of non-flagellate amoebae and uniflagellate cells). Mastigamoebida are divided into three families: Mastigamoebidae Goldschmidt, 1907; Mastigellidae fam. nov.; Pelomyxidae Schulze, 1877. Archamoebae may be uni- or multi-nucleate and either gut parasites or (more usually) free-living in soil, freshwater, or marine habitats. Some can form cysts that would probably fossilize; the earliest (1450 My old) smooth-walled fossil cells large enough to be probable eukaryotes might therefore be archamoebal cysts.