Weird and wonderful symbioses

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Rhizobia bacteria inside these specialised nodules fix nitrogen

At the John Innes Centre, many of us work on the interactions between plants and bacteria. Most people automatically think of these interactions as harmful and disease-causing (The Sainsbury Laboratory, amongst others, study these). However, plenty of us are more interested in interactions that benefit the plant, not hinder it. These interactions include symbioses. Symbiosis, from the Greek meaning ‘working together’ describes an interaction between two different species. An example of this that is investigated here in Norwich is the rhizobia-legume symbiosis. In this symbiosis the bacteria ‘fix’ nitrogen gas into a biologically useful form that the plant can use. In return the plant provides sugars for the bacteria to use for energy. Since two of us on the blog team work in this area (Izzy and Jo), we thought it would be fun to write something about some of the more unusual symbioses that are out there.

Fig and Wasp

Figs and fig wasps are an example of obligate mutualism – both symbiotic partners need the other partner in order to reproduce. They have co-evolved a unique way in which they rely on each other: figs need the wasps to become pollinated and the fig wasps both need the figs to reproduce and to feed on. Figs (also known as a syconium) are in fact made up of lots of tiny female flowers that cover the inner surface and these are pollinated by female fig wasps. In order to do this, the female wasp crawls inside the fig (through a tube known as an ostiole) and at the same time she will also lay her eggs. Male and female wasps then hatch out into larvae inside the fig and then they reproduce.  Male wasps in fact do not survive well outside of the fig, so spend their lives trapped inside the fig! Female wasps will mature and become covered in male pollen from the inside of the fig, and when they tunnel out and escape to find a new fig will pollinate that fig again and the cycle continues. This in itself is enough to put me off eating figs for life, however you will be pleased to know that figs do produce an enzyme which breaks down the wasps that die inside the fig so you aren’t actually eating lots of wasp bodies.

Jo was too disgusted by a picture of a bee inside a fig, so we chose this picture instead.

Jo was too disgusted by a picture of a wasp inside a fig, so we chose this picture instead.

Siboglinid worms

Siboglinid worms deep in the oceans have no digestive system, relying on bacteria living inside them

Inside hydrothermal vents and cold seeps deep on the ocean floor live sibloginid worms. Siboglinid, or beard worms are a type of ringed worm which occur (and dominate) a wide range of habitats across the globe. Unlike most animals, however, they completely lack a digestive system. Siboglinid worms are nutritionally dependent on endosymbiotic (meaning one symbiotic partner lives within the other) bacteria, which they acquire from their environment and house in a unique storage organ called a trophosome.

The siboglinid worm has a ‘plume’, an organ which is able to exchange compounds with its environment. Through this plume the worm is able to acquire gases including carbon dioxide, oxygen, hydrogen sulphide and methane. The bacteria within the trophosome are able to convert these gases into organic molecules, which can then be used by the worm as food. The ‘chemosynthetic’ bacteria are also able to convert nitrate into ammonium ions, which can then be used to make amino acids, also provided to the worm.

Mitchondria

Nearly every cell in the human body (red blood cells being an exception) contains endosymbionts – or at least, what were once endosymbionts. Mitochondria, the organelle where respiration occurs, were once free-living bacteria, engulfed by the single-celled organisms that are our ancestors. This event is predicted to have happened around 1.5 billion years ago, and led to the first eukaryotic cells. The same theory can also be applied to chloroplasts – which were seen to resemble free-living cyanobacteria. The engulfed bacteria survived, and their ability to respire/photosynthesize was utilized by the host cell. The theory that a eukaryotic cell is a symbiotic union of prokaryotic cells was proposed by Lynn Margulis in 1966 although she initially met criticism for this. Since the theory was put forward, evidence has mounted that strengthens the case for  the endosymbiotic theory

The endosymbiotic theory

The endosymbiotic theory

We won’t list all of the evidence for it here, since we would probably need an entire blog post to do it, but we thought we’d name a few. One of the most interesting features of these organelles is that they have their own genome, different from the DNA found in the cell nucleus. Mitochondrial genomes show similarity to bacterial genomes, as do the mitochondrial ribosomes. Mitochondria also have double phospholipid bilayer as their organelle membrane, similar to the membrane found on the outside of a cell. The mitochondria also reproduce via binary fission, which is similar to bacteria. If mitochondria are destroyed, the cell is unable to produce new ones using the genes from the nucleus.

Interestingly, many of the genes found in the nucleus are thought to have once been found in the mitochondria, but at some point in the past 1.5 million years have moved location. This ‘horizontal gene transfer’ can be seen in other symbioses too – for example, homocitrate, which is essential to the rhizobia-legume symbiosis, is produced by the plant, despite being required for the bacterial nitrogenase enzyme.

Here we have described just three of the interesting symbioses that are out in the world. However, these three just scratch the surface. There are loads of other examples of different species interacting with each other in a way that benefits both partners. We plan to blog again in the future about some more of the weird and wonderful symbioses out there.

By Jo Harrison and Izzy Webb

 References:

Thornhill et al. 2008, Commun Integr Biol 1 (2) 163-166
Sagan 1967 J Theor Biol 14(3): 255–274.

Image credits:

Author’s own photo
Wikimedia Commons
explorebiochemistryworld.wordpress.com
Never Eat Anything Bigger Than Your Head & Other Drawings, B. Kliban

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