Tomitani_etal2006_EvolCyano - The evolutionary...

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The evolutionary diversification of cyanobacteria: Molecular–phylogenetic and paleontological perspectives Akiko Tomitani †‡ , Andrew H. Knoll §¶ , Colleen M. Cavanaugh § , and Terufumi Ohno The Kyoto University Museum, Kyoto University, Yoshida-honmachi, Sakyo, Kyoto 606-8501, Japan; and § Department of Organismic and Evolutionary Biology, Harvard University, 16 Divinity Avenue, Cambridge, MA 02138 Contributed by Andrew H. Knoll, February 6, 2006 Cyanobacteria have played a significant role in Earth history as primary producers and the ultimate source of atmospheric oxygen. To date, however, how and when the group diversified has remained unclear. Here, we combine molecular phylogenetic and paleontological studies to elucidate the pattern and timing of early cyanobacterial diversification. 16S rRNA, rbcL , and hetR genes were sequenced from 20 cyanobacterial strains distributed among 16 genera, with particular care taken to represent the known diversity of filamentous taxa. Unlike most other bacteria, some filamentous cyanobacteria evolved a degree of cell differentiation, producing both specialized cells for nitrogen fixation (heterocysts) and rest- ing cells able to endure environmental stress (akinetes). Phyloge- netic analyses support the hypothesis that cyanobacteria capable of cell differentiation are monophyletic, and the geological record provides both upper and lower bounds on the origin of this clade. Fossil akinetes have been identified in 1,650- to 1,400-mega-annum (Ma) cherts from Siberia, China, and Australia, and what may be the earliest known akinetes are preserved in ± 2,100-Ma chert from West Africa. Geochemical evidence suggests that oxygen first reached levels that would compromise nitrogen fixation (and hence select for heterocyst differentiation) 2,450–2,320 Ma. Inte- grating phylogenetic analyses and geological data, we suggest that the clade of cyanobacteria marked by cell differentiation diverged once between 2,450 and 2,100 Ma, providing an internal bacterial calibration point for studies of molecular evolution in early organisms. molecular evolution ± phylogeny ± Proterozoic C yanobacteria are oxygenic photosynthetic bacteria that are widely distributed in aquatic and terrestrial environments, including such extreme habitats as hot springs, deserts, and polar regions (1). They are globally important primary producers today and have been through much of our planet’s history (2, 3). Some diazotrophic cyanobacteria are reported to be important agents in the global nitrogen budget (3, 4); therefore, the group plays a significant role in the nitrogen cycle as well as in the cycles of oxygen and carbon. Moreover, it is generally accepted that the chloroplasts of plants and algae are derived from a cyanobac- terial ancestor (5), implicating the blue–green bacteria in eu- karyotic evolution. Without doubt, then, cyanobacteria are key to any understanding of Earth’s early biological and environ- mental history. Cyanobacteria are monophyletic but morphologically diverse.
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This note was uploaded on 08/04/2009 for the course BIOL 4125 taught by Professor Christner during the Spring '08 term at LSU.

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Tomitani_etal2006_EvolCyano - The evolutionary...

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