AUTHOR=Sánchez-Baracaldo Patricia , Bianchini Giorgio , Di Cesare Andrea , Callieri Cristiana , Chrismas Nathan A. M. 

TITLE=Insights Into the Evolution of Picocyanobacteria and Phycoerythrin Genes (mpeBA and cpeBA)

JOURNAL=Frontiers in Microbiology

VOLUME=10

YEAR=2019

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2019.00045

DOI=10.3389/fmicb.2019.00045

ISSN=1664-302X

ABSTRACT=<p>Marine picocyanobacteria, <italic>Prochlorococcus</italic> and <italic>Synechococcus</italic>, substantially contribute to marine primary production and have been the subject of extensive ecological and genomic studies. Little is known about their close relatives from freshwater and non-marine environments. Phylogenomic analyses (using 136 proteins) provide strong support for the monophyly of a clade of non-marine picocyanobacteria consisting of <italic>Cyanobium, Synechococcus</italic> and marine Sub-cluster 5.2; this clade itself is sister to marine <italic>Synechococcus</italic> and <italic>Prochlorococcus</italic>. The most basal lineage within the Syn/Pro clade, Sub-Cluster 5.3, includes marine and freshwater strains. Relaxed molecular clock (SSU, LSU) analyses show that while ancestors of the Syn/Pro clade date as far back as the end of the Pre-Cambrian, modern crown groups evolved during the Carboniferous and Triassic. Comparative genomic analyses reveal novel gene cluster arrangements involved in phycobilisome (PBS) metabolism in freshwater strains. Whilst PBS genes in marine <italic>Synechococcus</italic> are mostly found in one type of phycoerythrin (PE) rich gene cluster (Type III), strains from non-marine habitats, so far, appear to be more diverse both in terms of pigment content and gene arrangement, likely reflecting a wider range of habitats. Our phylogenetic analyses show that the PE genes (<italic>mpeBA</italic>) evolved via a duplication of the <italic>cpeBA</italic> genes in an ancestor of the marine and non-marine picocyanobacteria and of the symbiotic strains <italic>Synechococcus spongiarum</italic>. A ‘primitive’ Type III-like ancestor containing <italic>cpeBA</italic> and <italic>mpeBA</italic> had thus evolved prior to the divergence of the Syn/Pro clade and <italic>S. spongiarum</italic>. During the diversification of <italic>Synechococcus</italic> lineages, losses of <italic>mpeBA</italic> genes may explain the emergence of pigment cluster Types I, II, IIB, and III in both marine and non-marine habitats, with few lateral gene transfer events in specific taxa.</p>