Mutualism – The Positive interspecific interaction

Mutualism between different species

In mutualism the members of two different species favor the growth and survival of each other and their association is obligatory.

Species A = (+)
Species B = (+)

In this interaction, the members have widely different requirements and so interdependent that they cannot survive separately. It is an extreme type of symbiosis in which symbionts have permanent and obligatory close contacts.

1.Mutualism between animal and animal species

Cleveland (1926) reported the presence of a multi flagellate protozoan – Trichonympha Campanula as a symbiont in the intestine of white ant – termite. 

The ant provides food and shelter to the protozoan which in turn secretes cellulase enzymes to digest the cellulose of wood ingested by the ant. Cellulose is hydrolyzed to sugars which are used by both the partners.

When the gut lining of Termite is ready for molting, Trichonympha undergoes encystment and are passed out with molting.

To ensure infection, the ant eats its molt. Newly hatched termites lick the anus of the older termites to ingest the symbiont.

About 11 families and 40 genera of flagellates have been reported from the intestine of termites.

Ant-aphid mutualism

Some kinds of ants pick up aphids or the aphids eggs from the surface of green plants and shelter them inside their own nest. The ants use the digestive wastes of the aphids as their food while aphids in return get nourishment from the rootlets of the plants ramifying through their nests.

2. Mutualism between plant and animals species

Green alga, Zoochlorella, found as a symbiont in the cytoplasm of  Paramecium bursaria, Parenchyma of flatworm  Convoluta roscoffensis and in the gastro-dermal cells of Hydra viridissima.

In Convoluta, the algal cells not only fill the parenchyma but also invade its tissues, giving the greenish appearance to the flatworm.

Similarly, yellow or brown alga Zooxanthella is found as a symbiont in the cells of exposed mantle edges of a mollusc Tridancnai ( Largest bivalve).

The algal partner, being autotroph, provides food, useful nitrogenous compounds, and oxygen to the animal partner while, in return, gets the suitable matrix for their growth and nutrients from the animal.

Yucca-plants and yucca moth mutualistic relationship

In this the female yucca moth (Pronuba yuccasella) collect a ball of pollens in its specially designed mouth parts, Female yucca moth deposits its eggs in the ovary of the flower of another yucca plant with its ovipositor.

Then it deposits the ball of pollens into the stigma so favoring the pollination. When the larvae hatch out of the eggs, these feed on some of the ovules to form young yucca moth while remaining ovules ripen to form seed.

Pollination of the flower by animals like insects, birds, bats, etc. is also an example of symbiotic association.

The insect-pollination flowers develop certain adaptations to attract the insect’s e.g. scented flowers in Certrum (queen of night), brightly-colored bracts in Bouganinvillea (glory of the garden); edible stamens in Papaver (poppy) and Rosa indica etc.

Insects visit these flowers to feed nectar or pollens. Quite often, the breeding of the insect is timed with a specific flower for pollination e.g.  breeding period of Blastophaga (a wasp) coincides with the ripening of stigmas and anthers of flowers of fig (Ficus carica)

3. Mutualism between animal and bacteria

Symbiotic bacteria like Ruminococcus are found in the rumen part of the compound stomach of cud-chewing mammals like cattle, sheep, goat, camel etc. and secrete cellulase enzyme to digest the cellulose of plant food eaten by the ruminants which provide food and shelter for the bacteria.

Other symbiotic bacteria reported in the stomach of herbivorous mammals are Closteridium cellobioparus, Propionibacterium, Lactobacillus and Streptococcus. Certain ciliate protozoans e.g. Entodinium caudatum are found in the stomach of ruminants.

Similarly, certain symbiotic bacteria reside in the large intestine of man and synthesize vitamins like B12 and K while in return get food and shelter.

4. Mutualism between plants and fungus

Linchens involve the symbiotic interspecific association of a green alga (Phycobiont) and a fungus (Mycobiont).

The core of lichen is formed of the green algal cell (about 5%) form a protective cover around the algal cells. These also provide moisture and minerals to the algal cells. Two partners are so interdependent and morphologically integrated that these form a third kind of organism which resembles neither of the partners.

Mutualism in the lichens is so strong that neither the fungus nor the alga can live independently under natural conditions. The lichens show the evolution of mutualism from the parasite and the host.

In primitive lichens, fungal hyphae actually penetrate into the algal cells and behave as parasites while in advanced lichens, two live in close harmony.

Mycorrhizae represent a mutualistic interaction between the hyphae of a fungus (e.g. Boletus) and roots of higher plants (e.g. Pinus). The roots provide food and shelter to the fungus while the latter helps in the absorption of water, minerals and produce growth promoting chemicals and secrete antibiotics to provide protection from the pathogenic organism.

Mycorrhizae are divided into two categories on the basis of their location:

Ectomycorrhizae (fungal hyphae mainly on the surface of roots) in pines, oaks, beech, and hickories.

Endomycorrhizae ( fungal hyphae mainly located in intercellular spaces of the cortex and from VAM-mycorrhizae) in red maple, and orchids. In spruce roots, the fungal hyphae form clusters around the roots and are called peritrophic mycorrhizae.

5. Mutualism between plants and bacteria

Nitrogen-fixing bacteria like Rhizobium leguminosarum are found in the nodules mainly on the secondary roots of leguminous plants like the pea, gram etc. The bacteria obtained carbohydrate, water, minerals and shelter from the leguminous plants, while in return fix gaseous nitrogen as nitrites and nitrates which are used by the plants in their growth. Nitrogen fixation occurs in the presence of an enzyme-nitrogenase which operates in anaerobic conditions maintained by a red-colored leghaemoglobin which act as an oxygen-scavenger. There are different strains of bacteria which grow on certain specific species of legumes

Nitrogen fixation occurs in the presence of an enzyme-nitrogenase which operates in anaerobic conditions maintained by a red-colored leghaemoglobin which act as an oxygen-scavenger. There are different strains of bacteria which grow on certain specific species of legumes

Similar kind of mutualistic association is found in the water-fern Azolla and the nitrogen-fixing cyanobacterium, Anabaena; between the cyanobacteria – Anabaena and coralloid roots of Cycas; and between the Nostoc and thalloid body of Anthoceros (Hornwort).

6. Mutualism between animals and fungi in fungal gardening

Martin (1970) showed that some ants and termites are known to cultivate and grow ”Fungal gardens” in the specialized chambers of their homes e.g. worker ants of Atta genus cut small piece of green leaves of certain plants, deposit these pieces into their nests, chew these pieces and vomit out the pulpy mass and spread it out as a bed for the growth of a fungus – Rozites gonygylophora. 

 This mycelial growth is used as food by the ants. In return, the excreta of ants act as the preferred food for the fungus. Ant excreta contains certain proteolytic enzymes which the fungus lacks.

So the ants contribute their enzymatic apparatus to the fungus to degrade the proteins. This fungal gardening goes on generation after generation.

A new virgin before leaving out an old nest to move into a new nest carries with it a pellet of fungus in a pocket below the mouth and deposits it in her new royal chamber.