We all stand together
In symbiotic relationships organisms do not evolve independently but evolve as a pair – or even as a group. Eventually, they may even merge to become a single entity
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 (see ‘Coral reefs’).
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 instance.
Often, organisms evolve together, as in the case of the ant and fungi in this illustration.
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 in its contribution to the relationship, at the expense of the things it gains. Evidence of this can be seen in their genes.
Illustration © Glen McBeth
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 typical of symbiosis. Presumably, there’s no selective pressure to keep genes active if the partner is meeting an organism’s needs. Loss of metabolic genes in Amanita mushrooms, for example, has left them unable to digest dead organic matter, and they are dependent on host plants for their carbon needs.
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 now cannot survive on its 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 that are 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 several such endosymbionts. The neat tree of life diagram is thus misleading, as some early boughs branched off but later merged with others. And, as Carsonella illustrates, branches continue to merge today.
One major recent development has been the sequencing of multiple bacterial species at the same time – known as metagenomics. An important advantage of metagenomics is that organisms don’t have to be cultured (more than 90 per cent of bacteria have never been grown in the lab). This kind of work has provided much deeper insight into microbial ecosystems in natural environments.
This technique has also been used to investigate the bacterial communities that live in different parts of our body. Notably, the nature of the communities can have a substantial impact on our biology and our health. The microbial communities of our gut have been implicated in obesity, cardiovascular disease, inflammatory disease and even autism.
Hence, some people talk about a human ‘super-organism’, which encompasses the billions of microbes that live within us and shape our biology.
- Microbe Wiki: Leafcutter ants, fungi and bacteria
- Metabolic complementarity and genomics of the dual bacterial symbiosis of sharpshooters (2006)
- Learning how to live together: genomic insights into prokaryote–animal symbioses (2008)
- The irreversible loss of a decomposition pathway marks the single origin of an ectomycorrhizal symbiosis (2012)
- Symbiosis insights through metagenomic analysis of a microbial consortium (2006)
- The frontier between cell and organelle: genome analysis of Candidatus Carsonella ruddii (2007)
- The history of endosymbiosis
- Nature: The five steps of horizontal gene flow (2005)
- Gut microbiome–host interactions in health and disease (2011)
- An ecological and evolutionary perspective on human-microbe mutualism and disease (2007)
Questions for discussion
- Do you think the bacteria within you are part of ‘you’?