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Dating analyses were conducted on the 100 phylograms topologically identical to the MAP tree.The trees were calibrated by fixing the origin of the cyanobacterial lineage at 3500 MYA, based on the age of oldest fossils represented by stromatolites (Schopf and Packer, 1987), and at 2700 MYA, the time at which oxygen-evolving cyanobacteria had likely quite originated due to reports of steranes in carbonaceous shales of northwestern Australia (Brocks et al., 1999).

Minimal age constraints of 16 MYA were applied to the heterocyst-forming lineage and to the origin of plastids, respectively, derived from a preliminary analysis in which the ages of these lineages were estimated without imposing minimal age constraints.

Point estimates of age from each of the 100 phylograms were used to obtain mean and standard deviations of ages of nodes across the tree.

Bayesian inferences of phylogenetic relations between cyanobacteria and chloroplasts with 16S r DNA and rbc L genes, plus the concatenated set, produced the following results: (1) chloroplasts constitute a monophyletic lineage and are most closely related to N-fixing unicellular cyanobacteria and (2) heterocyst-forming cyanobacteria are their sister group (Figure 1).

Although the initial suggestion that cyanobacteria are the ancestors of chloroplasts was greeted with skepticism, the idea is now widely accepted.

Here we attempt to resolve and date the cyanobacterial ancestry of the chloroplast using phylogenetic analysis and molecular clocks.

Phylogenetic relationships of cyanobacteria and chloroplasts rooted with (a) Gloeobacter violaceus (rbc L and concatenated set), (b) Chlorobium tepidum and (c) Chloroflexus aurantiacus (16S r DNA).

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