Free-Living Flagellates From Anoxic Habitats and the Assembly of the Eukaryotic Cell

Abstract

DAVID J. PATTERSON, ALASTAIR G. B. SIMPSON, AND NIMALIKA WEERAKOON School of Biological Sciences, University of Sydney, NSW 2006, Australia Comparative studies of protists conducted from the 1970s to 1990s led to models of their evolutionary relationships and, in turn, to models of the sequence in which eukaryotic cells were assembled (1, 2). In part, this was a result of ultrastructural studies of cell organization which had re- vealed that many familiar clusters of protists were polyphy- letic. Such studies provided evolutionary biologists with a fairly robust catalog of about 70 types of eukaryotes (2, 3). Spurred on by ideas that early eukaryotes were created by symbiotic associations of previously independent organ- isms, investigators explored the relationships among many of these groups by a combination of ultrastructural and molecular comparisons; the latter were mostly based on the small subunit ribosomal RNA (4). An important feature of the emerging tree was that a number of lineages had arisen from early, pre-mitochondrial stages in eukaryote evolution and have survived to the present. These purportedly primi- tively amitochondriate protists included the microsporidia, pelobionts, diplomonads, retortamonads, oxymonads, ent- amoebae, trichomonads, and other parabasalids. Some or all of these taxa, in various combinations, have been referred to as the “Archezoa.” This group has proven to be composi- tionally unstable, and it is paraphyletic. Nonetheless, the identification of this group assisted in the emergence of models of how eukaryotic cells might have developed. The models involved the autogenous emergence of the endoplas- mic reticulum, nuclei, and dictyosomes and the autogenous or xenogenous evolution of flagella and microtubular and non-microtubular cytoskeletal structures, before the symbi- otic acquisition of mitochondria and plastids. One strength