Symbiosis

Symbiosis describes the relationship, close association, or interaction between two organisms of different species. Although the term is often used to describe a relationship that benefits both species, there are different types of symbiosis. Symbiotic relationships that have occurred over very long periods of time can sometimes result in evolutionary changes in the organisms involved in the relationship.

Symbiosis literally means "living together," and there are many examples in nature of organisms of entirely different species that are involved

LYNN MARGULIS

American biologist Lynn Margulis (1938– ) has suggested some of the more revolutionary ideas in the history of modern biology. Her symbiotic theory of evolution has offered a new approach to both evolution and the origin of cells within the nucleus. She also subscribes to the "Gaia" hypothesis, which states that the Earth acts a superorganism, or single living system, that can regulate itself.

Lynn Margulis was born in Chicago, Illinois. Her parents, Morris and Leone Alexander, had three other daughters. An exceptional student, Margulis was fifteen when she completed her second year at Hyde Park High School and was accepted into an early entrant program at the University of Chicago. There she was immediately inspired by her science courses and took to reading the original works of the world's great scientists. She also became interested in the deeper aspects of heredity and genetics. While at Chicago, she met Carl Sagan (1934–1996), who would become an astronomer (a person who studies the universe beyond Earth) and one of the best-known scientists in the world. Sagan was a graduate student and Margulis was nineteen in the year she both received her bachelor's degree and married Sagan. She then entered the University of Wisconsin to pursue a joint master's degree in zoology and genetics, and in 1960 she and Sagan moved to the University of California at Berkeley where she conducted genetic research for her doctoral dissertation. The marriage to Sagan produced two sons but ended before she received her Ph.D. in 1965. After teaching at Brandeis University, she joined Boston University and married crystallographer (a person who studies crystal structure) Thomas N. Margulis, with whom she had two children before they divorced in 1980. Since 1988, Margulis has been a distinguished university professor at the University of Massachusetts at Amherst.

Margulis has regularly questioned the commonly accepted theories of genetics yet she has been called the most gifted theoretical biologist of her generation by numerous colleagues. As a graduate student she became interested in what is called non-Mendelian inheritance, which is when the genetic makeup of a cell's descendants cannot be traced solely to the genes in the cell's nucleus (the cell's control center). This puzzling phenomenon led her to search for genes in the cytoplasm of cells, that is, inside the cell but not inside its nucleus. In the early 1960s, Margulis actually found deoxyribonucleic acid (DNA, which is the carrier of genetic information) in the cytoplasm (jelly-like substance) of plant cells, suggesting that heredity in higher organisms might not be totally determined by genetic information carried only in the cell nucleus. This led her to eventually formulate her most startling idea, called the serial endosymbiotic theory (SET). Margulis stated that prokaryotes (cells that do not have a nucleus, such as very simple life forms like bacteria), which simply carry their genetic information inside the cell's cytoplasm, were the evolutionary forerunners of the more complex eukaryotes (which are cells that have a separate nucleus). All plants and animals have eukaryotic cells. She argued that eukaryotes evolved from prokaryotes when different types of prokaryotes formed symbiotic systems to increase their chances of survival. Symbiosis means that they had some sort of relationship, usually a type of partnership in which both members benefitted. An example of this, she says, is a cell's mitochondria (specialized structures inside a cell that break down food and release energy) that process oxygen.

Most scientists now agree that these cell structures evolved from oxygen-using bacteria, which joined with fermenting bacteria. Margulis is unique in her argument that traditional evolutionary theory cannot explain what she calls the "creative novelty" of life. Margulis also extends her concept of symbiosis to the entire biosphere (that part of Earth that contains life) and therefore accepts the Gaia hypothesis put forth by English chemist James E. Lovelock. This theory states that all life, and Earth itself, including its oceans and the atmosphere, are parts of a single, all-encompassing symbiosis that in turn form a single "organism," or a single living system. For Margulis, the concept of symbiosis is a powerful explanatory tool.

in some form of close, beneficial relationship or association. In fact, there are some symbiotic relationships that are necessary for the survival of the participating organisms. There are three types of symbiosis—mutualism, commensalism, and parasitism—depending on the nature of the relationship. As with any classification system, there are always exceptions, and sometimes it is difficult to categorize a certain situation. It is also a mistake to make a judgment of one type of symbiosis being better than another, since each is simply an organism's adaptation to survive.

Mutualism is a type of symbiotic relationship that results in a mutual benefit. Both species realize some type of gain by living together and cooperating within the same habitat. An example of mutualism would be the close relationship between a certain bacteria (Rhizobia) that lives under the soil and is attached to the roots of certain plants like peas, beans, clover, and alfalfa. These bacteria are nitrogen-fixing, meaning they are able to take in nitrogen gas that exists in the atmosphere and change it into nitrates that plants can use. This plant/bacteria relationship is mutualistic because both organisms benefit: the plant gains the necessary nitrogen in a usable form, and the bacteria gains access to a source of energy (using the plant's ready-made glucose). This is also an example of what is called "obligatory mutualism," since both partners are completely dependent on each other. Another example of this type of mutualism is the lichen, which is really made up of a fungus (plural, fungi), and an alga (plural, algae) living together. An alga can make its own food but can only live in wet places. A fungus cannot make its own food but can store a great deal of water. Together, they can live anywhere since the alga makes food (and lives inside the fungus), while the fungus provides it with its necessary water.

The other form of mutualism is called "facultative" and describes a relationship in which both partners benefit, but which each could still survive if the relationship did not exist. The relationship between the oxpecker (also called the tickbird) of Africa and the black rhinoceros is a good example, since these birds spend most of their time clinging to the bodies of large animals like the rhinoceros and eating ticks and maggots that infest the rhinoceros' hides. The birds also make a hissing sound that alerts the rhinos to possible danger. The rhinoceros benefits by having blood-sucking insects removed from its body, as well as having an early warning system. However, although both animals benefit from their relationship, the bird could obtain insects elsewhere if the rhino were to vanish, and the rhino could survive being infested with ticks.

Commensalism is the second type of symbiosis and describes a relationship in which one species benefits while the other experiences basically no effect (it neither benefits nor suffers). A bromeliad (an air plant) growing on the high branch of a rainforest tree is an example of such a relationship since it benefits by being closer to the sunlight while the tree is not harmed in any way (it also does not receive anything beneficial from the bromeliad). Another example is the tiny mollusks or crustaceans called barnacles that attach themselves to the body of a humpback whale enjoy the benefit of being moved through the water so they can filter microscopic food. The whale is neither bothered nor benefitted by the mollusks. Commensalism is usually practiced by one species on another to obtain something it cannot provide for itself such as transportation, protection, or nutrition.

Finally, a symbiotic relationship is described as parasitism if it results in the host organism being somehow harmed. In this type of relationship, the organism that benefits is called the parasite, while the organism that the parasite lives in or on is called the host. Disease-producing organisms are probably the best examples of parasitism. Such is the case with a tapeworm that lives inside the digestive organs of mammals. Because the tapeworm takes nutrition from the host, the host is left weakened and may also suffer tissue damage. An example of a strange and interesting form of parasitism is that conducted by a brood parasite. In this phenomenon, one species of animal uses the adult or parent of another species to raise its young. The common cuckoo bird does this regularly by laying its eggs in the nest of a species with similar-looking eggs. As soon as the cuckoo hatches, it pushes all the other eggs from the nest and eats all the food provided by its foster parents (which have been tricked into raising the cuckoo as their own).

[See alsoParasite ]