Introduction to Symbiogenesis

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Symbiogenesis is the theory that describes speciation that arises by symbiosis. Symbiosis is the beneficial association of two or more different species of organisms. For convenience, this discussion will just consider the cases of the symbiosis of two species with the understanding that situations exist in nature that involve more than two species. This association differs in intimacy from simply living in close proximity where each species derives some benefit from the other to becoming so closely intertwined that neither species can survive for long without the other except through artificial means. A cleaner shrimp that pick tiny food scraps from the mouth of a moray eel is one example of a very loose symbiotic relationship. The shrimp gains a safe an easy source of food and the eel gains good oral hygiene. Some species of lichen are the archetypal representatives of a very intimate association between an alga (or cyanobacterium) and fungus where if the two are separated they would only survive for a short time in the wild.

This theory originated in tsarist Russia in the latter half of the 19^th century and early part of the 20^th century. Andrey Famintsyn did much of the seminal research and was one of the first to enunciate the theory. Konstantin Merezhkovsky first publish the theory in 1905 in his paper, "The Nature and Origins of Chromatophores in the Plant Kingdom." Famintsyn published his version two years later also on the topic of chromatophores. Although the two men communicated frequently prior to the publication of either paper and Merezhkovsky referred to experimental work done by Famintsyn, the two men appear to have arrived independently at the conclusion that chromatophores in plants were the result of symbiotic incorporation of bacteria into another species.

Russia's plentiful and varied lichen biota provided a natural laboratory for the investigation of symbiogenesis. Gelatinous lichens in particular were found to range from loose associations to symbiosis so intimate that it was difficult to separate the alga and fungus as separate organisms. In the process of cataloging and studying the rich variety of lichens, scientists were able to see the progression from two species into a new third species. In addition Famintsyn's and Merezhkovsky's studies of plastids, such as chloroplasts, in plant cells when compared to certain very simple species of cyanobacteria provided direct evidence that these plastids had once lived as separate species and had been symbiotically incorporated into another species to give rise to a third species. Russian botanists in particular continued to investigate and refine this theory into the 1960s and 1970s. However, their papers were only published in Russian and so this theory was unknown in Western Europe and the USA.

Lynn Margulis rediscovered this important mechanisms of how evolution can proceed. It augments but does not replace the accumulation of mutations as a mechanism of speciation. Margulis is a microbiologist and developed her own hypothesis of symbiogenesis - speciation through symbiosis - based on her research into the structure of protists and protoctists where she was able to establish that chloroplasts, mitochondria, flagella, and other cell organelles had once been independently living organisms that had been incorporated by another to produce new species. Her work in many ways paralleled the work of the two Russians, Famintsyn and Merezhkovsky. It was only after she had published her work that she became aware of the earlier work in Russia.

In the case of simple organisms, protoctists in particular, symbiogenesis is well established as a scientific fact. The chlorophyllic plastids in alga, for example, were once independent cyanobacteria. The nuclei of eukaryotes have been demonstrated to have arisen from flagellant bacteria as have cilia (see Acquiring Genomes, A Theory of the Origins of Species by Lynn Margulis and Dorian Sagan for more information). This mechanism of speciation is in addition to the gradual accumulation of mutations that is one of the first proposed mechanisms. Natural selection works on both of these sources of variation, symbiosis and mutation, to favor the individuals and populations that adapt best to the environment in which they live.

The fossil record actually supports this and similar phenomena as important mechanisms of evolution. The punctuated equilibrium theory developed by Stephen Jay Gould and Niles Eldredge is more consistent with symbiogenesis than phyletic gradualism, in my opinion. This theory describes long periods of stasis (little change in the biota as evidenced by fossils) interrupted by periods of relatively rapid speciation (think of the demise of nonavian dinosaurs and the adaptive radiation of mammals in the few million years around the Cretaceous-Tertiary boundary demarcated by the iridium-rich layer signifying the asteroid impact at about 65 mya (million years ago).

In addition to the research concerning microbes, there are well documented cases of underwater slugs (Elysia viridis, for example). The ancestors of this newly evolved slug are gray animals that eat algae. In some of them, instead of being digested, the alga became part of the animal - a plant animal hybrid in fact. Elysia viridis are always green and they do not eat as adults. They derive all their nourishment from the symbiotic algae and so spend their time sunbathing instead of grazing on algae like their gray ancestors. Based on studies of the genomes of the green and gray slugs, it is clear that the green slugs evolved from the gray slugs by symbiotically incorporating the same algae that serve as the fodder of the gray slugs. This is a very clear case of evolution by symbiogenesis.

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