But what we are talking about in the evolution of cells is something that crossed a point at which both partners decided that once they have gone towards co-operation it’s hard to go back. The first few steps commit you to one path or the other. So the very early interactions must have been crucial in determining whether it would be a fight to the death, or whether is was going to be two billion years of happy co-existence and co-evolution.
Mitochondria and chloroplasts are the powerhouses of eukaryotic cells. Mitochondria appear in varying numbers within cells, bounded by a double membrane. They contain DNA and many different enzyme systems, which produce vital energy in the form of the chemical ATP (adenosine triphosphate), usually by utilising oxygen. Chloroplasts are the organelles in plant and algal cells in which photosynthesis takes place. Solar energy is used, in two photochemical reactions and with the participation of a number of intermediates including ATP, to convert water and carbon dioxide into sugars and oxygen. Chloroplasts also contain their own DNA. Both chloroplasts and mitochondria have similarities with simple prokaryotic organisms we still see today. Given that they contain complex biochemical systems for producing energy, it’s obvious why they’d be useful to another cell struggling to survive in a hostile environment. Rather than constantly producing your own raw materials or energy, why not just incorporate an independent, living generator, particularly if you can offer your partner something attractive in return!
The idea that mitochondria and chloroplasts started life as free-living bacteria is not new. In 1905, a Russian scientist called Mereschkowsky proposed that “chromatophores”, or colour carriers, of plant cells had an evolutionary origin in bacteria and other simple organisms such as blue green algae (this was before the term cyanobacteria. existed). For nearly 100 years the idea has been knocking about that cells with a nucleus and distinct organelles had somehow incorporated other organisms. This idea was based on observations of cell structure, the biochemistry taking in the organelles and the fact that mitochondria and chloroplasts had unique genes that did not appear in the cell nucleus. It was an interesting theory that took over seven decades to become respectable.
In the 1960’s, the biologist Lynn Margulis put forward what was to become known as the Endosymbiont Hypothesis. In 1970 her book “Origin if Eukaryotic Cells” was published, followed by “Symbiosis in Cell Evolution” in 1981. Although now accepted as mainstream, the idea was treated with scepticism and even ridicule for several years, according to Professor John Allen: - “To a student in the 1960’s and 1970’s it was still a hypothesis. It was still a bit wacky, still on the fringe. People might want to talk about it over coffee, but you wouldn’t set examination questions on it, because it wasn’t real! It’s mainstream stuff now. These things were once free-living bacteria. It’s quite inconceivable that it could be otherwise. And it raises questions: what sort of bacteria and what evolutionary forces were at work? And why did they lose so many genes? And why didn’t they lose all their genes?”
It was the advent of molecular biology and the ability to study DNA in detail that enabled the idea of endosymbiosis to gain credence. Professor John Raven explains why DNA sequencing proved a turning point. The problem with the earlier analyses of whether the endosymbiotic hypotheses were a runner for the origin of mitochondria (and chloroplasts were I think rather a clearer cut case) was parallel evolution. Two genes that were very, very different and could have evolved independently could have carried out the same function. It is only with the arrival of sequencing that we have been able to demonstrate that in fact it is the same gene, doing the same thing, in the prokaryotic ancestor and the organelles. But of course this is still a very controversial field and more time will have to be taken to see into this in far more detail.
Professor John Allen, agrees. “It is the explosion of whole genome information which has enabled scientists to say ‘let’s take the kind of bacterium that the chloroplast once was, where the chances are there is a whole complete genome we can describe in complete detail, all the genes for all the proteins it’s got.’ And people have come along saying, for example, ‘I’ve found the mitochondrion. It is alive and well.
Analysis of nuclear, mitochondrial and chloroplast DNA and comparison with bacteria which scientists believe are similar to the original endosymbionts reveals that many genes found in the original free-living organisms have been moved or removed over time. Some genes have been lost altogether from the mitochondria and the chloroplasts. Others have been transferred to the nucleus of the eukaryotic cell. Understanding why these changes have taken place, and why certain genes should remain in mitochondria and chloroplasts.
Examples in which the genome of a freely living organism have perhaps begun to make the transition to that of an organelle. These included cyanobacteria that have symbiotic relationships with eukaryotes; plastids; and intracellular bacterial symbionts of insects. Genomes of all these have now been analysed in detail in an attempt to unravel the evolutionary history and to better understand the processes and constraints at work in symbiosis and gene function.
What have bacterial genes contributed to the origin and evolution of mitochondria? Researchers have found that often the mitochondrial proteins encoded in the nucleus of eukaryotic cells are different from those of found in bacteria. This suggests that evolution of some mitochondrial genes took place after their ancestors had been assimilated into eukaryiotic cells. Analysis of the functional genes of yeast supports this theory. Yeasts have two systems for ATP transport, probably with two different origins – one bacterial and one that arose within the eukaryotic cell. The advantage of this would have been allowing the cells the ability to switch between respiration and glycolysis.
During the history of plant cells, complex rearrangements of the genes responsible for photosynthetic machinery have taken place. These have included the loss and gain as well as the transfer of genetic material between the original endosymbionts
Co-evolution took place of genes in the organelles and in the nucleus. As a result, genes and intracellular signalling are very specific, tailored to each individual partnership. Consequently, organelle changes between even closely related species can result in disturbance of the intra cellular genetic balance because of incompatibility.
Throughout the evolution of plastids, genes were transferred to the nucleus of the host cell with major consequences for the regulation of gene expression. Although the genes themselves have been lost to the nucleus, the plastid retains a role in the manufacturing of proteins necessary for photosynthesis, the plastids and the nucleus is involved in regulating the expression a set of nuclear genes that encode proteins required for photosynthesis and related processes.
Two and a half billion years of co-existence and co-evolution are summed up by a theory known as endosymbiosis. It explains the origin of organelles seen inside eukaryotic cells, notably mitochondria and chloroplasts. In a nutshell, the endosymbiotic theory is that mitochondria evolved from bacteria living within their host cell. Chloroplasts evolved from endosymbiotic cyanobacteria; that is from prokaryotic organisms able to synthesise organic constituents from inorganic sources.
