Unraveling the genomic mosaic of a ubiquitous genus of marine cyanobacteria.
BACKGROUND
The picocyanobacterial genus Synechococcus occurs over wide oceanic expanses, having colonized most available niches in the photic zone. Large scale distribution patterns of the different Synechococcus clades (based on 16S rRNA gene markers) suggest the occurrence of two major lifestyles ('opportunists'/'specialists'), corresponding to two distinct broad habitats ('coastal'/'open ocean'). Yet, the genetic basis of niche partitioning is still poorly understood in this ecologically important group.


RESULTS
Here, we compare the genomes of 11 marine Synechococcus isolates, representing 10 distinct lineages. Phylogenies inferred from the core genome allowed us to refine the taxonomic relationships between clades by revealing a clear dichotomy within the main subcluster, reminiscent of the two aforementioned lifestyles. Genome size is strongly correlated with the cumulative lengths of hypervariable regions (or 'islands'). One of these, encompassing most genes encoding the light-harvesting phycobilisome rod complexes, is involved in adaptation to changes in light quality and has clearly been transferred between members of different Synechococcus lineages. Furthermore, we observed that two strains (RS9917 and WH5701) that have similar pigmentation and physiology have an unusually high number of genes in common, given their phylogenetic distance.


CONCLUSIONS
We propose that while members of a given marine Synechococcus lineage may have the same broad geographical distribution, local niche occupancy is facilitated by lateral gene transfers, a process in which genomic islands play a key role as a repository for transferred genes. Our work also highlights the need for developing picocyanobacterial systematics based on genome-derived parameters combined with ecological and physiological data.
Dufresne
Alexis
A
Université Paris 6 and CNRS, UMR 7144, Station Biologique, 29682 Roscoff, France. dufresne@sb-roscoff.fr
Ostrowski
Martin
M
Scanlan
David J
DJ
Garczarek
Laurence
L
Mazard
Sophie
S
Palenik
Brian P
BP
Paulsen
Ian T
IT
de Marsac
Nicole Tandeau
NT
Wincker
Patrick
P
Dossat
Carole
C
Ferriera
Steve
S
Johnson
Justin
J
Post
Anton F
AF
Hess
Wolfgang R
WR
Partensky
Frédéric
F
D022761:Gene Transfer, Horizontal; D016680:Genome, Bacterial; D012623:Seawater / Q000382:microbiology*; D046940:Synechococcus / Q000145:classification* / Q000235:genetics*