Category: Symbiosis & Endosymbiosis

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A symbiosis, derived from the Greek words "syn" (meaning with) and "biosis" (meaning living), is an intimate and often long-term association between members of different species. It is not necessarily mutualistic, i.e. beneficial to both partners, but can also be commensal (one species benefits, while the other is unaffected) or parasitic (one species benefits at a cost to the other). In a narrower sense, however, the term symbiosis refers to mutualistic relationships only. One partner can either live on (ectosymbiosis) or inside the other (endosymbiosis), and the association between them can either be essential for survival (obligate symbiosis) or not (facultative symbiosis).

Numerous animals rely on bacterial endosymbionts. Examples are shipworms (Teredo) and Xylophagus, which show wood-eating convergences to termites and some rodents (porcupine, beaver), the lucinid bivalves, which have evidently acquired symbiotic bacteria several times, and the vestimentiferans, deep-sea tubeworms that live in hydrothermal vents and cold seep areas. These worms have no functional gut and are entirely dependent on nutrients supplied by chemoautotrophic sulphur bacteria housed in so-called trophosomes in the worms' body cavity. Better known for their bacterial symbioses are insects such as sap-sucking aphids and psyllids, which have independently recruited different γ-proteobacteria (Buchnera and Carsonella) to provide them with essential amino acids. Aphids are involved in another fascinating mutualism that has evolved multiple times independently. As a result of their sugar-rich diet, aphids excrete vast quantities of honeydew, which is an important food source for many species of ants as well as a Madagascan gecko.

Insects are also famous for their symbioses with fungi, particularly attine ants (e.g. leaf-cutter ants), which tend fungal gardens and feed on the sugar-rich tips of the hyphae. Similar fungal cultivation has evolved at least twice in non-attine ants as well as in the termites and some beetles (notably ambrosia beetles). Interestingly, both leaf-cutter ants and ambrosia beetles carry bacterial symbionts that secrete antibiotics to prevent contamination of their fungal gardens with virulent fungal parasites.

In terms of associations, fungi are probably best known in lichens, where they form a symbiosis with a photosynthetic partner (either a cyanobacterium or a green alga and once a marine brown alga). Lichens provide prime examples of evolutionary convergence, as confirmed not only by the variety of partners and the precise nature of the symbiosis (some verge on parasitism), but also because different fungi have independently become lichen symbionts.

While many aspects of the origin of the eukaryotic cell are still very controversial, it is generally agreed that chloroplasts and mitochondria are the result of primary endosymbiosis, being derived from cyanobacteria and α-proteobacteria, respectively. In the case of chloroplasts there is some evidence that this endosymbiosis may have occurred more than once, and this view is supported by the ongoing symbiotic association between the amoeba Paulinella and a cyanobacterium that is not closely related to those that evolved into the chloroplasts. Not only are there intriguing parallels in the story of gene loss in chloroplasts and mitochondria, but we also find the re-invention of bacterial pathways, such as quinol oxidation.

Various marine animals such as foraminiferans, radiolarians, sea slugs and, most famously, corals have acquired endosymbiotic flagellate protozoans, the zooxanthellae. Most are autotroph and provide the host with energy in the form of photosynthetic products, in return for protection, nutrients and CO2.

Amongst the numerous examples of plant-insect symbioses is the famous association between the yucca plant and its obligate pollinator, the yucca moth, which has evolved at least twice. Ants have repeatedly evolved multiple mutualistic relationships with so-called ant plants, which they protect from pests and competitors in exchange for shelter.

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Topic Title Teaser text Availablity
Zooxanthellae in corals and other animals n/a Not Available
Agriculture in wood wasps

The most famous hymenopteran farmers are, without doubt, the attine ants. Rightly so, but they are not the only ones...

Agriculture in gall midges (Diptera)

Flies, fungi, farming - sounds interesting? Read on if you want to learn about some rather different gall midges...

Agriculture in beetles

Think of weevils and most likely you'll think of spoiled food. But some weevils have turned to farming...

Agriculture in aquatic snails

Termites and ants are famous for tending fungal gardens, but did you know that also a marine snail farms a fungus? And this is not the only example of agriculture in this group…

Agriculture in damselfish

Don’t be tempted to think human agriculture is unique. On many coral rocks, there are very similar things going on…

Mitochondrial genome convergences

Most likely, mitochondria have a single evolutionary origin, but that doesn't mean they are immune to convergence...

Mitochondrial TOM proteins n/a Not Available
Dicyemids and chromidinids: enigmatic endoparasites

Dicyemids and chromidinids are tiny, worm-like or 'vermiform' creatures that typically live inside the kidneys ('renal organs') of cephalopod molluscs such as octopus, squid and cuttlefish.

Lichens: fungal association with cyanobacteria and green algae n/a Not Available
Fungus-Insect associations

It is clear than an ascomycete yeast in a planthopper has been acquired quite independently of the beetles which employ a true yeast.

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Zygomycetes: convergent forms and symbioses

Zygomycete fungi show convergent evolution, and one of the most striking examples is the reproductive morphologies associated with asexual reproduction in the mucoraleans.

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Light sensitivity and optics in sponges

Some of the silica spicules of glass sponges are very long, and extraordinarily have a striking similarity to the optical fibres employed in the telecommunications industry.

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Endosymbiosis in wasps n/a Not Available
Gut fermentation in herbivorous animals

Ever tried eating a newspaper? Don't. Plant cell walls contain cellulose, which is notoriously difficult to digest. Considering that all vertebrates lack the enzymes to attack this polysaccharide, how do so many of them manage to survive on a plant diet?

Sap feeding and honey-dew production in insects

Interestingly, it has now been shown that the saliva of the aphids has an analogue to the anti-coagulant properties of blood suckers, subverting the wound repair mechanism of the plant.

Chloroplast and mitochondrial plastid origins

Not only are there intriguing parallels in the story of gene loss in chloroplasts and mitochondria, but there is also the re-invention of bacterial pathways, such as oxidation of quinols.

Butterflies and moths: insights into convergence

Some moths feed on the secretions from the tear-ducts of mammals, and some moths in Madagascar have evolved this independently, but instead of mammals they frequent birds.

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Recruitment of endosymbiotic bacteria in insects

Independently the sap-feeding aphids and psyllids have recruited γ-proteobacteria, respectively best known from Buchnera and Carsonella, in an intimate and obligate symbiotic relationship.

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Agriculture: from ants to dugongs

Human farmers tending their fields are a familiar sight. But don't forget about those fungus-farming termites or the fish with a garden of algae…

Beetles: insights into convergence

The beetles are probably the most diverse animal group on earth, so it is not at all surprising that they provide many fascinating insights into convergence.

Agriculture in ants: leaf-cutters (attines) and non-attines

In some species, special squads leave the nest early each day, ascend the tree-trunks and then spend hours cutting out pieces of leaf that are dropped to other units on the ground.

Bivalve molluscs: convergent shells and symbioses

Despite their range of shell types, there is evidence of extensive convergence, notably amongst the fresh water swan-mussels (unionids), mytilids and anomalodesmatans.

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