title: The smallest known genomes of multicellular and toxic cyanobacteria: comparison, minimal gene sets for linked traits and the evolutionary implications.
abstract: Cyanobacterial morphology is diverse, ranging from unicellular spheres or rods to multicellular structures such as colonies and filaments. Multicellular species represent an evolutionary strategy to differentiate and compartmentalize certain metabolic functions for reproduction and nitrogen (N(2)) fixation into specialized cell types (e.g. akinetes, heterocysts and diazocytes). Only a few filamentous, differentiated cyanobacterial species, with genome sizes over 5 Mb, have been sequenced. We sequenced the genomes of two strains of closely related filamentous cyanobacterial species to yield further insights into the molecular basis of the traits of N(2) fixation, filament formation and cell differentiation. Cylindrospermopsis raciborskii CS-505 is a cylindrospermopsin-producing strain from Australia, whereas Raphidiopsis brookii D9 from Brazil synthesizes neurotoxins associated with paralytic shellfish poisoning (PSP). Despite their different morphology, toxin composition and disjunct geographical distribution, these strains form a monophyletic group. With genome sizes of approximately 3.9 (CS-505) and 3.2 (D9) Mb, these are the smallest genomes described for free-living filamentous cyanobacteria. We observed remarkable gene order conservation (synteny) between these genomes despite the difference in repetitive element content, which accounts for most of the genome size difference between them. We show here that the strains share a specific set of 2539 genes with >90% average nucleotide identity. The fact that the CS-505 and D9 genomes are small and streamlined compared to those of other filamentous cyanobacterial species and the lack of the ability for heterocyst formation in strain D9 allowed us to define a core set of genes responsible for each trait in filamentous species. We presume that in strain D9 the ability to form proper heterocysts was secondarily lost together with N(2) fixation capacity. Further comparisons to all available cyanobacterial genomes covering almost the entire evolutionary branch revealed a common minimal gene set for each of these cyanobacterial traits.
authors.affiliation: Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany.
authors.forename: Karina
authors.initials: K
authors.lastname: Stucken
authors.forename: Uwe
authors.initials: U
authors.lastname: John
authors.forename: Allan
authors.initials: A
authors.lastname: Cembella
authors.forename: Alejandro A
authors.initials: AA
authors.lastname: Murillo
authors.forename: Katia
authors.initials: K
authors.lastname: Soto-Liebe
authors.forename: Juan J
authors.initials: JJ
authors.lastname: Fuentes-Valdés
authors.forename: Maik
authors.initials: M
authors.lastname: Friedel
authors.forename: Alvaro M
authors.initials: AM
authors.lastname: Plominsky
authors.forename: Mónica
authors.initials: M
authors.lastname: Vásquez
authors.forename: Gernot
authors.initials: G
authors.lastname: Glöckner
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics*; D001427:Bacterial Toxins / Q000378:metabolism; D000458:Cyanobacteria / Q000145:classification / Q000235:genetics* / Q000378:metabolism; D046891:Cylindrospermopsis / Q000166:cytology / Q000235:genetics* / Q000648:ultrastructure; D019143:Evolution, Molecular; D016680:Genome, Bacterial / Q000235:genetics*; D046529:Microscopy, Electron, Transmission; D005810:Multigene Family / Q000235:genetics; D009586:Nitrogen Fixation / Q000235:genetics; D010802:Phylogeny; D012091:Repetitive Sequences, Nucleic Acid / Q000235:genetics; D017422:Sequence Analysis, DNA; D013045:Species Specificity; D026801:Synteny
chemical_list: D001426:Bacterial Proteins; D001427:Bacterial Toxins