CALIBRATING THE MOLECULAR RECORD OF CYANOBACTERIAL EVOLUTION (Invited Presentation)

Diagnostic morphological fossils can be used to time calibrate the phylogenies reconstructed from cyanobacterial genomes. Better representation of deeply diverging cyanobacterial taxa, taxa that are not simple coccoids and filaments, and benthic taxa in the current genome databases can help understand which cyanobacterial groups diversified before or after the great oxygenation event (GOE) and when plastids diverged from Cyanobacteria. We address this by targeted sequencing of genomes of 27 cyanobacteria taxa that include deep-branching cyanobacteria, as well as taxa with distinctive and diagnostic morphologies that can be directly compared to some of the oldest cyanobacterial fossils. Phylogenetic trees constructed using 30 concatenated sequences of ribosomal proteins from the newly sequenced species and 34 previously sequenced cyanobacterial genomes, 30 plastid genomes and 47 bacterial outgroups are consistent with nine previously established cyanobacterial groups. The tree also reveals one new group of benthic filamentous cyanobacteria, shows that the ancestor of plastid lineages diverged early, and that plastid lineages evolved at faster rates than crown group cyanobacteria. Molecular clock analyses were used to calibrate the tree. A root prior was established by assuming that the bacterial ancestor existed 3.9 +/- 0.4 Ga. These models calibrate two nodes within Cyanobacteria using two different ~ 0.8 Ga cyanobacterial fossils, and either omit plastid lineages due to their fast rates of evolution, or constrain the divergence of plastids using the ~ 1.1 Ga red algal fossil Bangiomorpha. These molecular clock analyses are independent of geochemical calibrations such as the GOE, mutually consistent, and show that Cyanobacteria diverged between 3 and 2.4 Ga, plastids diverged between 2.4 and 1.9 Ga and heterocystous cyanobacteria diverged between 1.6 and 1.1 Ga. Further genomic sequencing and molecular clock analyses will enable us to produce a robust model of cyanobacterial diversification. This model will be independent of the geochemical record, and can be compared to the latter record to better understand the interplay between biological and geochemical evolution before and after the GOE.