Are there any known mutual symbiotic relationships that has more than two agents?

Question

We are all familiar with the examples of symbiotic relationships in nature consisting of two agents, like lichen or the co-evolvement between certain birds and flowering plants.

Do we know of any mutually symbiotic relationships that has three of a even higher number of agents?

Edit: To make a implicit distinction clear, let's specify that the agents in question have distinctive and fully interdependent roles in the symbiosis. In contrast to a scenario where a range of bird species might co-evolve for a single specific flower and have the same role as pollinators in the symbiosis.

This image represents my current models in thinking about this:

The type I'm searching for here is mainly the "3-agent symbiosis", but the bidirectional one is of interest as well.

Answer 1

I think what you are asking for is "Multiple Mutualism". Normally,

Most of the known cases of multiple mutualism involve relationships in which two of the partners are at the same organizational level—usually two bacteria that might be interacting directly—but are both essential for the survival of the host

^{— Entente cordiale: Multiple symbiosis illustrates the intricate interconnectivity of nature -Philip Hunter}

For example, the glassy-winged sharpshooter (Homalodisca coagulata) primarily feeds on xylem-fluid which is low on organic nutrients…so it depends on 2 bacteria, Baumannia cicadellinicola and Sulcia muelleri. The sharpshooter provides these with raw carbon skeletons present in the xylem fluid, and those bacteria in return synthesizes vitamins, essential amino acids, etc. For more detailed study, see Wu et al, 2006.

However, there are also examples of three-way mutualistic symbioses in which all the partners are at different organizational levels. One known case involving a virus, a bacterium and an insect, see Moran NA, Degnan PH, Santos SR, Dunbar HE, Ochman H (2005) The players in a mutualistic symbiosis: insects, bacteria, viruses, and virulence genes. Proc Natl Acad Sci USA 102: 16919–16926

Anyway, the essence as stated by Andreas Brune, Professor of Biogeochemistry at the Max Planck Institute for Terrestrial Microbiology in Marburg, Germany:

Multiple symbiosis is probably more widespread than we think, especially in view of the symbiotic systems involving different organizational levels, for example between bacterium, nematode and insect, or bacterium, protozoan and insect.

Answer 2

The human body contains a few billion microbes (more than human cells), so we live in symbiosis with many different species. Some of them protect us from infections, some of them help in the digestion, and so on... So I think the human body fulfills your requirements.

• wikipedia - human microbiome

Normally we don't live in sterile environment. So we depend on our microflora in order to protect us against pathogens by preventing them to colonize e.g. our intestines. So without a sterile environment we would die in the lack of our microflora e.g. in a C. difficile infection. In exchange we give our microbes shelter (e.g. we protect them from oxygen, UV radiation, temperature changes) and food (e.g. carbohydrates we cannot digest, for example xylitol). So the human-microbiome relationship is a mutualism. Some of these microbes are not detected outside of the (not necessary human) host, so these relationships can be obligate. These microbes do not only support us, but each other as well. E.g. they help each other to digest nutritients (syntrophy), so they can live in mutualism with each other as well. So multipartite symbiosis certainly exists in the human body, and it seems like much more common in the nature than we previously thought.

• 2013 - Integrative analysis of the microbiome and metabolome of the human intestinal mucosal surface reveals exquisite inter-relationships
• 2003 - Honor thy symbionts
• 2014 - Cooperation, communication, and co-evolution: grand challenges in microbial symbiosis research
• 2007 - Interactions and competition within the microbial community of the human colon: links between diet and health
• 2005 - Host-Bacterial Mutualism in the Human Intestine
• 2013 - Metabolic modeling of species interaction in the human microbiome elucidates community-level assembly rules
• 2013 - It Takes a Village: Ecological and Fitness Impacts of Multipartite Mutualism
• 2006 - Entente cordiale: Multiple symbiosis illustrates the intricate interconnectivity of nature

Answer 3

I can think of an example for the 3rd picture in your image "Common Agent With Multiple Symbiotic Relationships."

We have: 1.) a species of parasitoid wasp 2.) a species of plant and 3.) a species of moth.

The moth provides pollination activity for the plant. The plant provides a place for the moth to lay its eggs, and a food source for caterpillars once they hatch. Once the plant begins undergoing herbivory it releases Volatile Organic Compounds (VOCs) which attract the wasp and inform it that there are potential hosts nearby. The parasitoid parasitizes either the eggs or the larvae (depending on the species of wasp) and helps the plant by limiting the damage it sustains due to herbivory (like a body guard).

Common example species could be: 1.) Wasps of the genus Cotesia (parasitize caterpillars) or of the genus Trichogramma (parasitize eggs). 2.) Plants of the family Solanaceae (Datura, tomatoes, or potatoes) 3.) Sphinx moths of the genus Manduca

There is a lot of literature on the subject of VOCs and parasitoids, but if you need a quick reference here is one: http://chemse.oxfordjournals.org/content/30/9/739.short.