In current theories, symbiosis is seen as a critical step in the evolution of higher organisms such as plants and animals. The cellular energy "factories" of plants, chloroplasts, where the capture of light energy takes place, and mitochondria, where production of energy by the metabolism of fixed carbon compounds occurs, are both thought to have originated as symbiotic bacteria within the cells of other primitive organisms. These ancient symbioses imparted enormous advantages to the partners; allowing them to develop novel characteristics, become more complex and specialized and consequently, to colonize habitats that as individuals were unavailable to them. Animal cells also contain mitochondria. Both plants and animals have evolved to occupy many diverse habitats, owing in part to their ancestral symbiotic association with other organisms. From this perspective, many scientist view symbiotic relationships not as a curiosity, but rather as a key to understanding the nature of life on Earth.
What is Symbiosis?
provided the first description of the symbiotic organs of Cycads, coralloid roots. It is now known that these specialized roots are inhabited by nitrogen fixing cyanobacteria of the genus Nostoc. Cyanobacteria form symbiotic relationships with all known Cycads. Cycads are the only gymnosperms known to participate in mutualistic symbiosis with cyanobacteria, however, other gymnosperms are well known for their association with symbiotic fungi, the mycorrhizae.
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All life strategies, including symbiosis, have costs and benefits associated with them. Under some conditions, the cost is outweighed by the benefit, and visa versa. Environmental factors determine whether or not an individual that is capable of facultative symbiosis adopts such a strategy. Just like balancing a checkbook, plants must make sure that what comes in is greater than what goes out, if their "account" is to grow. The cost of a strategy is usually weighed by the amount of fixed carbon resources that must be extended to support the strategy. A strategy is termed beneficial if it increases an organism's fitness in the particular environment in which it lives. Fitness is a measure of the genetic contribution of an organism to future generations, relative to others living within the same environment.
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