symbiosis

in the narrow sense, the living together of individuals of two species whereby both partners interact directly with the environment (Sh. D. Moshkovskii, 1946; V. A. Dogel’, 1947). The relationship with the environment is regulated by the joint activity of both organisms. In the broad sense, symbiosis refers to all forms of close association between organisms of different species, including parasitism, which is called antagonistic symbiosis.

Symbiosis is usually mutualistic, that is, living together is mutually beneficial for both organisms (symbionts). It arises in the course of evolution as a means of adaptation to the conditions of existence. Symbiosis may occur both in multicellular organisms and in individual cells (intracellular symbiosis). A symbiotic relationship may exist between plants and plants, plants and animals, animals and animals, plants and microorganisms, animals and microorganisms, and microorganisms and microorganisms.

The term “symbiosis” was introduced in 1879 by the German botanist H. A. De Bary, who applied it to lichens. An excellent example of symbiosis among plants is mycorrhiza, or the living together of mycelium of a fungus with the roots of a higher plant. The hyphae wind around the roots and enable them to absorb water and minerals from the soil; some orchids cannot live without mycorrhiza. One type of symbiosis between animals and plants is the living together of unicellular algae and certain animals, including protozoans, coelenterates (hydras, anthozoans), and turbellarians. It has been proved that unicellular algae that settle in the cells of anthozoans are vital for the normal growth and development of the latter.

Hermit crabs and actinians participate in a symbiotic relationship whereby the actinians settle on the shell in which the crab conceals its abdomen. The nematocysts of the actinian’s tentacles protect both symbionts, and the actinian feeds on the remains of the food caught by the crab. Symbiosis is very common between animals (including man) and microorganisms, for example, those constituting normal intestinal flora. In some insects cellulose is digested by enzymes secreted by yeast cells living in special depressions of the insect’s intestine. Symbiosis with nodule bacteria is important for legumes. There are many different forms of symbiosis between protozoans and certain prokaryotic organisms, including bacteria and rickettsias, which are usually localized in cytoplasm. Infusorians and bacteria inhabiting their nuclei may also be symbionts; some bacterial species are found only in the macronucleus, whereas others are found only in the micronucleus.

In the early 20th century the Russian scientists K. S. Mere-zhkovskii and A. S. Famintsyn advanced the theory that sy-biosis is a major factor in the progressive evolution of the organic world (symbiogenesis theory). For example, they regarded angiospermous chloroplasts as modified symbiotic algae. This concept, which for a time was forgotten, has currently been revived. Many contemporary scientists, including microbiologists and cytologists, link the origin of eucaryotic cells in evolution to symbiosis. Specifically, they consider mitochondria to be modified prokaryotic symbionts, emphasizing the resemblance between mitochondrial DNA and the DNA of a prokaryotic ring chromosome.

Symbiosis is slightly distinguished from other forms of close association, such as parasitism, commensalism, and predation, between which several transitional forms exist.