Chapter 26 : Microbial Symbiosis and Evolution

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This chapter talks about the symbiosis of microbes and mitochondria. Mutations, including deletions of DNA, happen constantly, so the unused genes have long since disappeared from the genomes of mitochondria that inhabit our cells. In fact, mutations are still raining down on the genomes of our mitochondria, and these continue throughout our lives, contributing to various diseases and disorders. These malfunctions are the main reason we even think about our mitochondria. Another reason the mitochondria can live with so few genes is that many of the ancient genes of the mitochondrial ancestor have changed addresses, moving from the chromosome of the mitochondrion to our own chromosomes but sending their working products back to the mitochondrial homeland to carry out needed work. Some symbiotic bacteria have fewer than 200 genes, and the symbionts we call organelles can have even fewer, as in our own mitochondria with their miserly 15. uses its ancestral genes for making the amino acids tryptophan and leucine, which are nutrients that a host needs. But these genes have been copied many times on tiny extra chromosomes (plasmids), an arrangement that allows hyperproduction of the amino acid products. Both symbionts and domesticated breeds can evolve extreme features over short periods on an evolutionary time scale. These extremes are produced mostly by exaggerating some functions, rather than by acquiring anything truly novel. And just like our domesticated animals and plants, symbionts themselves can be large, typically much larger than their free-living wild relatives.

Citation: Moran N. 2012. Microbial Symbiosis and Evolution, p 191-196. In Kolter R, Maloy S (ed), Microbes and Evolution. ASM Press, Washington, DC. doi: 10.1128/9781555818470.ch26
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