PMID:
     21092259
Authors:
     Bennett JS, Bentley SD, Vernikos GS, Quail MA, Cherevach I, White B,
     Parkhill J, Maiden MC.
Title:
     Independent evolution of the core and accessory gene sets in the genus Neisseria:
     insights gained from the genome of Neisseria lactamica isolate 020-06.
Journal:
     BMC Genomics. 2010 Nov 23;11:652. doi: 10.1186/1471-2164-11-652.
Abstract:
     BACKGROUND: The genus Neisseria contains two important yet very different
     pathogens, N. meningitidis and N. gonorrhoeae, in addition to non-pathogenic
     species, of which N. lactamica is the best characterized. Genomic comparisons of 
     these three bacteria will provide insights into the mechanisms and evolution of
     pathogenesis in this group of organisms, which are applicable to understanding
     these processes more generally. RESULTS: Non-pathogenic N. lactamica exhibits
     very similar population structure and levels of diversity to the meningococcus,
     whilst gonococci are essentially recent descendents of a single clone. All three 
     species share a common core gene set estimated to comprise around 1190 CDSs,
     corresponding to about 60% of the genome. However, some of the nucleotide
     sequence diversity within this core genome is particular to each group,
     indicating that cross-species recombination is rare in this shared core gene set.
     Other than the meningococcal cps region, which encodes the polysaccharide
     capsule, relatively few members of the large accessory gene pool are exclusive to
     one species group, and cross-species recombination within this accessory genome
     is frequent. CONCLUSION: The three Neisseria species groups represent coherent
     biological and genetic groupings which appear to be maintained by low rates of
     inter-species horizontal genetic exchange within the core genome. There is
     extensive evidence for exchange among positively selected genes and the accessory
     genome and some evidence of hitch-hiking of housekeeping genes with other loci.
     It is not possible to define a 'pathogenome' for this group of organisms and the 
     disease causing phenotypes are therefore likely to be complex, polygenic, and
     different among the various disease-associated phenotypes observed.

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