The Lineages of Eukaryotes

This description of the diversity of protists is based on two reviews (Patterson, 1994, 1999) . These papers present one of several differing views about the diversity and inter-relationships of the protists and other eukaryotes (e.g. Cavalier-Smith, 1993, 1994; 1998; Corliss, 1984, 1994; Margulis et al., 1990). Patterson argues that the groups which share a common cytological organization (an 'ultrastructural identity' - Patterson and Brugerolle, 1988) generally constitute monophyletic lineages. The concept of ultrastructural identities is used to generate the concept that there are about60 types of eukaryotes.

An ultrastructural identity refers to the appearance of a variety of features. They include:

• the shape of the cristae in the mitochondria (cristae may be tubular - long or bleb shaped or branching; or they may be discoidal with or without a pedicel, or they may be flat plates); the form of cristae varies within a species or group of protists on rare occasions
• the presence or absence of hairs, scales or other excrescences on the flagella
• the component parts of the transition region where the 9+2 organization of the axoneme transforms into the nine triplet structure of the basal bodies
• the length and orientation of the basal bodies and their associations with other organelles
• the nature of the microtubular and other rootlet structures which arise from the basal bodies, and the associations between these and other organelles
• the source and deployment of other microtubular or other cytoskeletal arrays within the cell, or under the cell surface
• the nature, composition, origins and appearance of any extracellular materials
• presence and nature of microtubule organizing centres
• number, nature and heterogeneity of nuclei, intra-envelope, intranuclear and paranuclear inclusions
• the behaviour of the nuclear envelope during mitosis (is it retained, is it completely lost, or is it perforated in regions by the intruding spindle), and where does the spindle nucleate, is it located within the nucleus, or external to the nucleus, or does it start in the cytoplasm and penetrate into the nucleus)
• if there is a chloroplast, how many bounding membranes, how many thylakoids per lamella, contained (e.g. stigma) or associated (e.g. nucleomorph) organelles
• identity and nature of other endomembranous organelles in the cell
• other idiosyncracies

This approach identifies the types of eukaryotes listed below. As new taxa are identified by further exploration of the world around us or by the study of genera with ambiguous identities (see second table below) so new ultrastructural identities are revealed, and this list lengthens. As relationships are agreed through comparative molecular biology or comparative morphology, so the list shortens. Over the last decade the list has shortened from about 85 lineages to about 60 lineages.

Lineages of eukaryotes: groupings which share an ultrastructural identity and have no evident sister group

Genera of protists for which no clear identity has been developed (after Patterson, 1999)

References

Cavalier-Smith, T. 1993. Kingdom Protozoa and its 18 phyla. Microbiol. Rev. 57:953-94.

Cavalier-Smith, T. 1998. A revised six-kingdom system of Life Biological reviews of the Cambridge Philosophical Society, 73: 203-266.

Copeland, H. F. 1956. Classification of the Lower Organisms. Pacific Books, Palo Alto, California.

Corliss, J. O. 1984. The Kingdom Protista and its 45 phyla. BioSystems 17: 87-126.

Corliss, J.O. 1994. An interim ("user-friendly") hierarchical classification and characterization of the protists. Acta Protozoologica, 33:1-51.

Margulis L., Corliss J. O., Melkonian, M. and Chapman D. J. 1990. Handbook of Protoctista. Jones and Bartlett Publishers, Boston.

Patterson, D. J. 1994. Protozoa: Evolution and Systematics. In: Hausmann, K. and Hülsmann, N. (eds.): Progress in Protozoology, (Proceedings of the IX International Congress of Protozoology, Berlin 1993), pp 1-14. Gustav Fischer Verlag, Stuttgart, Jena, New York USA.

Patterson, D. J. 1999. The diversity of eukaryotes. American Naturalist 154 (suppl.):S96-S124.

Patterson, D. J. and Brugerolle, G. 1988. The ultrastructural identity of Stephanopogon apogon and its relatedness to other kinds of protists. European Journal of Protistology, 23, 279-290.