ON THE SYMBIOTIC ORIGIN OF PROTISTS, THEIR DIVERSITY, AND THEIR PIVOTAL ROLE IN TEACHING SYSTEMATIC BIOLOGY

All multicellular organisms - animals, fungi and plants - are derived from unicellular protists which themselves arose from symbiotic associ ations of various prokaryotes. The position of protists in relation to other living beings and their tremendous genetic diversity is discuss ed in relation with studies by Sogin and others on comparative sequenc ing of small subunit ribosomal RNA. Current views on the symbiotic ori gin of protists and their organelles (nucleus, 9 + 2 flagellum, mitoch ondria and plastids) are reviewed with particular reference to extant model organisms from phototrophic dinoflagellates and cryptomonads whi ch are examples of ''eukaryotes within eukaryotes''. It is emphasised that in addition to mutation and selection, symbiosis is an equally im portant motor of evolution in general. Protists currently seem to fall into three major groups. The early group seems to lack mitochondria p rimarily and is represented by the Microspora. Giardia, certain amoebo flagellates, trichomonads and polymastigotes. But recent discovery of mitochondrial chaperons throws some doubts on this view. The second, m iddle group contains the Euglenozoa (kinetoplastids, euglenoids) and t he majority of the former rhizopods. The last one, called crown group, due to a rapid radiation, gave rise to the alveolates (which contain the apicomplexean sporozoans and the dinoflagellates, forming together the sister-group of the ciliates), the heterokont algae including bro wn algae, plus oomycetes and net-slime molds, the cryptomonads, the ha ptomonads, and the three main branches with multicellular organisms, e .g. plants and animals and true fungi, the latter being the sister gro up of the animals. Finally, some ideas are developed for a unifying co ncept of reaching ''protistology'' in modern systematic biology. Proti sts do nor form a monophyletic taxon, but exemplify a particular, unic ellular organisational level; their classification should avoid any hi erarchical categories, but search for true sister-group relationships.