title: Accelerated evolution associated with genome reduction in a free-living prokaryote.
abstract: BACKGROUND
  Three complete genomes of Prochlorococcus species, the smallest and most abundant photosynthetic organism in the ocean, have recently been published. Comparative genome analyses reveal that genome shrinkage has occurred within this genus, associated with a sharp reduction in G+C content. As all examples of genome reduction characterized so far have been restricted to endosymbionts or pathogens, with a host-dependent lifestyle, the observed genome reduction in Prochlorococcus is the first documented example of such a process in a free-living organism.
  
  
  RESULTS
  Our results clearly indicate that genome reduction has been accompanied by an increased rate of protein evolution in P. marinus SS120 that is even more pronounced in P. marinus MED4. This acceleration has affected every functional category of protein-coding genes. In contrast, the 16S rRNA gene seems to have evolved clock-like in this genus. We observed that MED4 and SS120 have lost several DNA-repair genes, the absence of which could be related to the mutational bias and the acceleration of amino-acid substitution.
  
  
  CONCLUSIONS
  We have examined the evolutionary mechanisms involved in this process, which are different from those known from host-dependent organisms. Indeed, most substitutions that have occurred in Prochlorococcus have to be selectively neutral, as the large size of populations imposes low genetic drift and strong purifying selection. We assume that the major driving force behind genome reduction within the Prochlorococcus radiation has been a selective process favoring the adaptation of this organism to its environment. A scenario is proposed for genome evolution in this genus.
authors.affiliation: Station Biologique, UMR 7127 CNRS et Université Paris 6, BP74, 29682 Roscoff Cedex, France. dufresne@sb-roscoff.fr
authors.forename: Alexis
authors.initials: A
authors.lastname: Dufresne
authors.forename: Laurence
authors.initials: L
authors.lastname: Garczarek
authors.forename: Frédéric
authors.initials: F
authors.lastname: Partensky
mesh_terms: D000222:Adaptation, Physiological; D019943:Amino Acid Substitution; D001426:Bacterial Proteins / Q000235:genetics; D001483:Base Sequence; D004260:DNA Repair; D019143:Evolution, Molecular; D016680:Genome, Bacterial; D042822:Genomic Instability; D023281:Genomics; D008969:Molecular Sequence Data; D048929:Prochlorococcus / Q000235:genetics*; D026801:Synteny
chemical_list: D001426:Bacterial Proteins