We all stand together
Coral is among the Earth's most beautiful organisms. Partly this is because it is a symbiosis between an animal and algae, often strikingly coloured. When subject to environmental stress, corals may 'bleach' as the algae die.
Symbiosis describes a relationship in which both partners benefit from a close attachment. Sometimes this might be a simple exchange – protection in exchange for a supply of food, for example.
But when relationships become established, organisms begin to evolve together and the relationship becomes one of dependence. Indeed, each member of the partnership begins to specialise on its contribution to the relationship, at the expense of the things it gains. Evidence of this has can be seen in their genes.
A remarkable example is the three-way love-in between an insect (the glassy-winged sharpshooter) and two species of bacteria. Unusually, the sharpshooter lives off water-carrying xylem rather than nutrient-rich phloem. Genome sequence analysis of the bacteria reveals that one species has the enzymatic machinery to make vitamins but not essential amino acids, while the other has a very small genome but can make the amino acids needed by itself and its partners.
Loss of genes is turning out to be common in symbiosis. Presumably, there is no selective pressure to keep genes active if the partner is meeting an organism's needs.
Bulk genetics
A major recent development has been the sequencing of multiple bacterial species at the same time – known as metagenomics. A major advantage is that organisms don't have to be cultured (more than 90 per cent of bacteria have never been grown in the lab).
The marine worm Olavius algarvensis has, bizarrely, no mouth, gut or kidney-like structures. Sequencing of four symbiotic bacteria revealed a remarkable division of labour between the organisms. The bacteria supply the worm with nutrients and digest its waste products. The adaptations are linked to the highly specialised chemical environments of the sediment in which the worm lives.
Symbioses raise interesting questions about the nature of an organism. Olavius cannot now survive on it own – it lives as a mutually dependent organismal 'superassembly' with the bacteria.
Inside the cell
A similar thing often happens when bacteria take up residence inside other cells. Typically, over time, the bacteria start to lose genes. The bacterial endosymbiont Carsonella, for example, has a tiny genome of just 160 000 nucleotides (E. coli has 5 million). It has lost many genes considered essential to life, so it is perhaps a transitional form between free-living organism and organelle.
Indeed, key structures in the cell – the mitochondrion and, in plants, the chloroplast – are thought to be the remnants of formerly free-living organisms. They may have started out as partners in a symbiotic relationship, but gradually they have lost more and more of their own identity and now have only a fraction of their original genetic material (mitochondrial DNA and chloroplast DNA).
Throughout nature there are a number of such endosymbionts. The neat tree of life diagram is thus misleading, as some early boughs branched off but then merged with others. And, as Carsonella illustrates, branches continue to merge today.