New Research Highlights the Gradual Nature Of The Eukaryote Endosymbiosis Event

The position of the lokiarchaeota within the tree of life

The evolutionary transition from prokaryote endosymbiote to true eukaryote is a puzzle which we are only just beginning to understand in detail.

Recent advances in molecular technologies have allowed us to pinpoint the origins of eukaryotic genes and cell components within the antecedant bacterial and archaeal cells. Similarly, recent advances in our understanding of the tree of life have given us new insight into the transition.

Alphaproteobacteria and cyanobacteria have been known to be the ancestors of mitochondria and chloroplasts respectively for a while, but the nature of the archaeal host cell was only vaguely understood until the discovery of the lokiarchaeota. Discovered just last year around a deep sea hydrothermal vent, the lokiarchaeota have been identified as the closest relatives of the eukaryotes and the group from which the archaeal host cell prevailed.

Despite having the start and end points of the transition, the lack of true intermediates has nevertheless hindered our understanding of the difficulties involved. Previously it was assumed that the endosymbiotic event was rapid and harmonious. It has become ever more apparent, however, that it was a much more laboured process. Nick Lane's The Vital Question is particularly good at highlighting the issues involved, such as genetic parasitism on the host genome and intracellular competition between endosymbionts.

Now, a paper published just a few weeks ago, focusing on the genomes of bacteria and the lokiarchaeota, has further demonstrated that the origin of the eukaryotes was likely to be 'the result of a long, slow dance between kingdoms, and not a quick tryst.'

One major difference between eukaryotic and prokaryotic cell is their size and corresponding degree of organisation. Prokaryotes are small enough for diffusion to act as a sufficient means of transporting substances around their cells. As such they require very little in the way of internal organisation. By contrast the massive cells of eukaryotes are divided into multiple compartments linked by elaborate molecular transport systems.

Analysis of the genomes of the lokiarchaeota showed that they contain a greater number of eukaryotic signature proteins (ESPs) than any other prokaryote group, hence the placement of the lokiarchaeota next to the eukaryotes in the tree of life.

A diagram demonstrating the gradual acquisition of key elements
of the eukaryotic cell from antecedant prokaryotes

Crucially, however, a number of these proteins (small Raf and Arf-type GTPases), are critical components of eukaryotic intracellular transport systems. They are unlikely to perform a similar function in the lokiarchaeota, however, as they lack the enzymes required for the association of GTPases with membranes or components of membrane transport systems. Yet the genes and membrane lipids required to make these associations possible can be found in bacteria.

'The (archaeal) genome can be seen as 'primed' for eukaryogenesis,' said Buzz Baum from University College London. 'With the acquisition of a number of key genes and lipids from a bacterial symbiont, it would be possible for loki-type cells to evolve a primitive membrane trafficking machinery and compartmentalization.'

Subsequently, the gradual transfer of genes from bacterial symbiont to archaeal host would have led to the development of a eukaryotic transport system.

'We believe it will be very difficult to crack the mysteries of eukaryogenesis without first understanding the archaeal cell biology,' said Gautam Dey, also from University College London.

The researchers say that their next step will be to study the cell cycle and cellular morphology of the related archaeon Sulfolobus acidocaldarius (loki type cells have yet to be cultured in the laboratory) to better determine just how close the structure of such archaea is to true eukaryotic cells.