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RESEARCH PRODUCT

Genomic Changes in Bacteria: From Free-Living to Endosymbiotic Life

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Symbiosis is the association between two or more distinct organisms during at least one part of their lifecycle. Although this term is sometimes used in a narrower sense, it includes for most authors a set of different situations such as mutualism, parasitism and commensalism. Mutualism is defined as an association in which both partners derive benefit from living together. Parasitism is an association in which one of the partners benefits, while the other is harmed. Finally, commensalism is an association in which one of the two members benefits while the other is neither harmed nor obtains an advantage. In most cases, the association is established between a pluricellular eukaryote and a microorganism such as a bacterium or unicellular fungus. These microbial symbionts establish either facultative or obligate associations with their hosts. In the latter, the symbionts are always required to be together. The terms endosymbiosis and endosymbionts are applied to those symbionts that live inside their hosts (the other partner in the association). This association may also be narrower when the endosymbiont lives inside the host’s cells. Endocytobiosis is the term applied to intracellular symbiosis [1]. Many bacterial species have been reported to be able to infect and produce a pathogenic effect on insects. The pathogenic relation between the sporeforming bacterium Bacillus thuringiensis and many species of lepidoptera, coleoptera or diptera is well-known, but there are many other parasitic or pathogenic relations involving species of the genera Clostridium, Serratia, Pseudomonas, Photorhabdus, Wolbachia, etc. There are also many mutualistic associations between insects and microorganisms, and in most of them the endosymbiont adapted to live intracellularly in a specialized host cell called mycetocyte when it contains a fungal endosymbiont, and bacteriocyte when it contains a bacterial symbiont. These cells are derived from different parts and tissues of the insect such as fat body or Malpighian tubules and form organs called mycetome or bacteriome depending on the endosymbiont type. Bacteriocyte-associated endosymbionts have been described in many