Genome Reduction in Obligate Endosymbiotic Species
Obligate endosymbiotic species are characterized by a complete inability to survive external to their host environment. These species have become a considerable threat to human health, as they are often highly capable of evading human immune systems and manipulating the host environment to acquire nutrients. A common explanation for these keen manipulative abilities is the compact and efficient genomic structure consistently found in obligate endosymbionts. This compact genome structure is the result of massive losses of extraneous DNA - an occurrence that is exclusively associated with the loss of a free-living stage. In fact, as much as 90% of the genetic material can be lost when a species makes the evolutionary transition from a free-living to obligate intracellular lifestyle. Common examples of species with reduced genomes include: Buchnera aphidicola, Rickettsia prowazekii and Mycobacterium leprae. One obligate endosymbiont of psyllid, Candidatus Carsonella ruddii, has the smallest genome currently known among cellular organisms at 160kb. It is important to note, however, that some obligate intracellular species have positive fitness effects on their hosts. (See also mutualists and parasites.)
The reductive evolution model has been proposed as an effort to define the genomic commonalities seen in all obligate endosymbionts. This model illustrates four general features of reduced genomes and obligate intracellular species:
- ‘genome streamlining’ resulting from relaxed selection on genes that are superfluous in the intracellular environment;
- a bias towards deletions (rather than insertions), which heavily affects genes that have been disrupted by accumulation of mutations (pseudogenes);
- very little or no capability for acquiring new DNA; and
- considerable reduction of effective population size in endosymbiotic populations, particularly in species that rely on vertical transmission.
Based on this model, it is clear that endosymbionts face different adaptive challenges than free-living species.
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