The evolutionary importance and affinities of 'amitochondriate' protists

Virginia Edgcomb¹, Andrew Roger¹,
and Alastair Simpson^2
1Woods Hole Oceanographic Institution
²University of Sydney, Australia

Wednesday, November 4, 1998
3:00 p.m.—Pacific Forum

It is now widely accepted that the eukaryotes we call protists are far more diverse in cellular organization than the non-protist eukaryote groups—namely animals, plants and fungi. A variety of heterotrophic protists lack classical mitochondria, inhabiting low- oxygen environments such as the guts and tissues of animals, marine or freshwater sediments, and the lower reaches of stratified water bodies. Over the last two decades these 'amitochondriate' organisms have been of great interest to evolutionary biologists, as some may have diverged before the acquisition of the mitochondrion and consequently represent very early stages in the evolution of the eukaryotic cell. Some amitochondriate groups—particularly the largely parasitic trichomonads, diplomonads, and microsporidia— have indeed tended to form the most basal branches in evolutionary trees of eukaryotes, based on molecular sequence comparisons. However the validity of these deep branches has, of late, been vigorously challenged, as has the contention that these organisms lack any trace of having had mitochondria. To date, almost all of the research into amitchondriate protists has focused on those groups with parasitic members. However surveys of sediments and anoxic water bodies reveal a considerable and drastically understudied diversity of free-living, low-oxygen protists, frequently of unclear affinities. In several instances, electron-microscopical studies indicate the absence of classical mitochondria.

Detailed morphological data in concert with molecular phylogenies, covering both free-living and parasitic taxa, are leading us towards a more authoritative state regarding the affinities of the amitochondriate protists and whether any groups remain candidates for being primitively amitochondriate relicts of early eukaryotic evolution.

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