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Phylogenetic analysis of genes encoding photosynthesis proteins in cyanophage isolates and natural virus communities Chénard, Caroline

Abstract

Cyanophages infecting cyanobacteria of the genera Synechococcus and Prochlorococcus are abundant and ubiquitous in aquatic environments. Sequencing of some cyanophage isolates has revealed homologous genes to psbA and psbD that encode key proteins for photosynthesis. Using molecular techniques, this thesis explored the phylogenetic diversity of these genes in cyanophage isolates and natural virus communities. First, I amplified cyanophages psbA and psbD genes fragments from myoviruses infecting marine Synechococcus strain DC2 (=WH7803) and compared them phylogenetically. I demonstrated that psbA was present in all cyanophage genomes, while psbD was presented in only half of them. Moreover, gene-based phylogenies revealed similar tree topologies for both genes. This suggests that psbA and psbD were acquired coordinately but psbD was lost during multiple events. Next, I compared the genetic diversity of viral psbA from a range of environments with the goal of determining if sequences cluster based upon their environments. Phylogenetic reconstruction showed that viral psbA sequences from fresh waters have an evolutionary history distinct from their marine equivalents. Moreover, photosynthesis sequences from cyanophages infecting neither Prochlorococcus nor ' Synechococcus were distinct, as were sequences from different phage families (e.g., podoviruses vs myoviruses). This thesis confirmed that viral psb genes have their own evolutionary history that is distinct from that of their host.

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