Zooflagellate Phylogeny and the Systematics of Protozoa.

Abstract

In the six kingdom system of life, the kingdom Protozoa occupies a pivotal position between the ancestral kingdom Bacteria and the four derived eukaryotic kingdoms, Animalia, Plantae, Fungi, and Chromista (1). The diversification of the protozoans is fundamentally important for understanding both the early evolution of eukaryotes as a whole and the origins of these higher kingdoms. Recent advances in molecular phylogeny have led to many changes from the earlier protozoan system (2). Archezoa are now treated, not as a separate kingdom (2, 3), but as a subkingdom of the Protozoa, comprising only the two zooflagellate protozoan phyla, Metamonada and Parabasalia (I), which have no mitochondria and are microaerophilic or anaerobic. Parabasalia are clearly secondarily amitochondrial, since their hydrogenosomes probably evolved from mitochondria by the loss of cytochromes and DNA; protein phylogeny ((4); Hasegawa, pers. comm.) suggests that metamonads are also secondarily amitochondrial. The kingdom Protozoa is currently divided into 13 phyla, of which eight contain zooflagellates (Table I; fuller details