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Symbiosis is an association lasting for at least one part of the biological
cycle, between two or several specifically distinct organisms. This association
leads to the formation of a new biological entity, the symbiocosm, itself
subject to natural selection. On the physiological plan, symbiosis generally
allows for a better adaptation to the environment and on the genetic side
it is a sophisticated way of acquiring new genes by lateral transfer.
The integration of the symbiont in the metabolism of the host is often
so advanced that the symbiont is like a new cytoplasmic organelle.
The symbiont can be likened to a group of genes acquired suddenly and
ready to act; it is generally transmitted to descendants maternally. As
these genes are usually advantageous, the symbiont tends to extend rapidly
through the whole population and even through the whole species. It is
a much more efficient method of modifying the global genome than mutagenesis
or by chromosome rearranging. Secondly, the genes can be transferred from
the symbiont towards the nuclear genome of the host, as proposed for the
mitochondria and the plasts. The integrated symbiont is an organelle,
in some way completely domesticated by its host. The host can also be
considered as a parasite with regard to the symbiont, symbiosis being
considered as the ultimate mechanism of genetic predation invented by
the eukaryotes. Seen from this point of view, it seems to be one of the
most efficient mechanisms of biological innovation existing, and its role
not only in the formation of the host cell but also throughout evolution
must be admitted.
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