EVOLUTION OF THE PROTISTS AND PROTISTAN PARASITES FROM THE PERSPECTIVE OF MOLECULAR SYSTEMATICS

Unlike prokaryotes, the Protista are rich in morphological and ultrast ructure information. Their amazing phenotypic diversity permits assign ment of many protists to cohesive phyletic assemblages but sometimes b lurs relationships between major lineages. With the advent of molecula r techniques, it became possible to test evolutionary hypotheses that were originally formulated according to shared phenotypic traits. More than any other gene family, studies of rRNAs changed our understandin g of protist evolution. Stramenopiles (oomycetes, chrysophytes, phaeop hytes, synurophytes, diatoms, xanthophytes, bicosoecids, slime nets) a nd alveolates (dinoflagellates, apicomplexans, ciliates) are two novel , complex evolutionary assemblages which diverged nearly simultaneousl y with animals, fungi, plants, rhodophytes, haptophytes and a myriad o f independent amoeboid lineages. Their separation may have occurred on e billion years ago and collectively these lineages make up the ''crow n'' of the eukaryotic tree. Deeper branches in the eukaryotic tree sho w 16S-like rRNA sequence variation that is much greater than that obse rved within the Archaea and the Bacteria. A progression of independent protist branches, some as ancient as the divergence between the two p rokaryotic domains, preceded the sudden radiation of ''crown'' groups. Trichomonads, diplomonads and Microsporidia are basal to all other eu karyotes included in rRNA studies. Together with pelobionts, oxymonads , retortamonads and hypermastigids, these amitochondriate taxa compris e the Archaezoa. This skeletal phylogeny suggested that early branchin g eukaryotes lacked mitochondria, peroxisomes and typical stacked Golg i dictyosomes. However, recent studies of heat shock proteins indicate that the first eukaryotes may have had mitochondria. When evaluated i n terms of evolution of ultrastructure, lifestyles and other phenotypi c traits, the rRNA phylogenies provide the most consistent of molecula r trees. They permit identification of the phylogenetic affinity of ma ny parasitic groups as well as a means to integrate molecular and cell biological information from diverse eukaryotes. We must place greater emphasis upon improved phylogenetic inference techniques and investig ations of genomic diversity in protists. (C) 1998 Australian Society f or Parasitology. Published by Elsevier Science Ltd.