An interview with Professor Martin Embley FRS
Professor Martin Embley FRS is based at the BioSciences Institute at Newcastle University. His research focuses on the investigation of early eukaryotic evolution using genomics, phylogenetics and cell biology, and in this interview, he tells us how discoveries in the field of microbiology have profoundly changed our ideas about eukaryotic origins and evolution.
Tell us more about the 'three domains tree of life' and the origin of mitochondria
In the mid-1990s, two ideas dominated thoughts about early cellular evolution. One was that the 'three domains tree of life,' was an accurate description of the relationships between eukaryotes (archaea) and prokaryotes (bacteria).
In the three domains tree, eukaryotes are a separate group that is most closely related to archaea, and the host for the mitochondrial endosymbiont (a bacterium) was already a eukaryote (i.e. a cell with a nucleus like our own). The other prevailing idea was that some anaerobic and/or parasitic microbial eukaryotes at the base of the three domains tree never had mitochondria because they split from other eukaryotes before the mitochondrial endosymbiosis. Mitochondria are now vitally important for our own cells because they produce most of the energy we need to live.
How has this research evolved?
Over the past 25 years, microbiologists have provided the data used to radically change our ideas about early eukaryotic evolution and the timing of the mitochondrial endosymbiosis. Thus, it now appears that all contemporary eukaryotes are descended from a host cell that contained the bacterial endosymbiont that evolved into mitochondria, and the most conserved function of the organelle across eukaryotic life appears to be a role in the biogenesis of essential Fe/S proteins, not making energy.
Analysis using better phylogenetic methods have also shown that the 'two domains,' or 'eocyte' tree, wherein the eukaryotic nuclear lineage and eventual host for the mitochondrial endosymbiont originated from within the archaea, is a better supported hypothesis than the traditional three domains tree.
In the new tree, the basal split in life is between bacteria and archaea, and eukaryotes have an archaeal parent. The new tree has now received strong confirmation by the discovery of environmental archaea that are more closely related to eukaryotes, and which contain genes previously thought to be eukaryotic-specific.
Why is the study of the new archaea so important to microbiology?
Eukaryotic cells have an internal structural complexity that is not found in prokaryotes, and the origin of the genes that underpin these differences has always been a major evolutionary puzzle. The discovery of archaea that contain the same genes that are used by eukaryotes to build this complexity is extremely exciting and promises to revolutionise our understanding of eukaryogenesis. Notably, microbiologists working in Japan have just succeeded in culturing one of the new archaea, so that genes whose proteins are often considered to underpin the prokaryote-to-eukaryote transition, can now be studied for their roles in cells that are undoubtedly still prokaryotes.
