Chapter 2 : Minimal Genomes and Reducible Complexity

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A review of Aristotelian reality, represented by the diversity of bacterial cells with small and reduced genomes which evolved naturally, can help delineate the Platonic idea of a hypothetical minimal cell. The study of minimal cells can benefit enormously from the study of present-day organisms with small genomes by showing how relatively simple biological systems have evolved and currently operate. Thus, cells and reduced genomes of endosymbionts, parasites, and free-living organisms are examples of naturally evolved minimal gene sets. The diversity of lineages, nutritional strategies, and ecological niches occupied by these free-living organisms with small genomes is noteworthy. The smallest photosynthetic cells, represented by , have slightly larger genomes (1,660 kb and 1,765 genes). Genomes with fewer genes than the smallest free-living prokaryote belong to parasitic or endosymbiotic organisms and are found in 15 different orders among currently sequenced genomes. The reduction of the flagellar apparatus in is a wonderful lesson in evolutionary tinkering that shows its convoluted history in two main respects. First, certain components of the retained flagellar genes serve not for bacterial mobility but, rather, to export proteins that could eventually become involved in infecting new surrounding host cells, ovaries, or embryos, thereby enabling to be vertically transmitted to its host’s offspring. Second, and probably more importantly, has recovered functions that were present in the ancestors of current free-living relatives, which possess a complete flagellum. If this is the case, the flagellum would be an example of reducible complexity.

Citation: Moya A. 2012. Minimal Genomes and Reducible Complexity, p 17-23. In Kolter R, Maloy S (ed), Microbes and Evolution. ASM Press, Washington, DC. doi: 10.1128/9781555818470.ch2
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Figure 1

The flagellum, an example of tinkering in bacterial evolution. (A) flagellum. Protein and structural components are indicated: filament, hook-filament junction, hook, and basal body. The last is formed by a set of rings anchoring the inner and outer membranes and the cell wall, the rod, the stator, the rotor, and the export apparatus (including one ATPase). (B) The reduced flagellar apparatus in BCc, from the cedar aphid. Elements involved in the formation of the flagellum filament and hook and the stator/motor have been lost. None of them are necessary for a nonmotile bacterium. Many elements involved in the export apparatus anchorage to the cell envelope have also been lost, consistent with the absence of a well-structured outer membrane and cell wall. All the retained proteins have a homolog of the type III secretion system, supporting the hypothesis that this structure is used by the bacterium as a protein export system to facilitate the invasion of new host cells. doi:10.1128/9781555818470ch2f1

Citation: Moya A. 2012. Minimal Genomes and Reducible Complexity, p 17-23. In Kolter R, Maloy S (ed), Microbes and Evolution. ASM Press, Washington, DC. doi: 10.1128/9781555818470.ch2
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