Chapter 2 : Minimal Genomes and Reducible Complexity

Abstract:

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 Prochlorococcus marinus, 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 Buchnera 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 Buchnera to be vertically transmitted to its host’s offspring. Second, and probably more importantly, Buchnera 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.