Diversity and evolution of pigment types and the phycobilisome rod gene region of marine Synechococcus cyanobacteria

Synechococcus picocyanobacteria are ubiquitous and abundant photosynthetic organisms in the marine environment and contribute for an estimated 16% of the ocean net primary productivity. Their light-harvesting complexes, called phycobilisomes (PBS), are composed of a conserved allophycocyanin core from which radiates six to eight rods with variable phycobiliprotein and chromophore content. This variability allows Synechococcus to optimally exploit the wide variety of spectral niches existing in marine ecosystems. Seven distinct pigment types or subtypes have been identified so far in this taxon, based on the phycobiliprotein composition and/or the proportion of the different chromophores in PBS rods. Most genes involved in their biosynthesis and regulation are located in a dedicated genomic region called the PBS rod region. Here, we examined the variability of gene sequences and organization of this genomic region in a large set of sequenced isolates and natural populations of Synechococcus representative of all known pigment types. All regions start with a tRNA-PheGAA and some possess mobile elements including tyrosine recombinases, suggesting that their genomic plasticity relies on a tycheposon-like mechanism. Comparison of the phylogenies obtained for PBS and core genes revealed that the evolutionary history of PBS rod genes differs from the rest of the genome and is characterized by the co-existence of different alleles and frequent allelic exchange. We propose a scenario for the evolution of the different pigment types and highlight the importance of population-scale mechanisms in maintaining a wide diversity of pigment types in different Synechococcus lineages despite multiple speciation events.