|
In molecular biology a pan-genome (or supra-genome) describes the full complement of genes in a clade (typically applied to species in bacteria and archaea), which can have large variation in gene content among closely related strains). It is the union of the gene sets of all the strains of a clade (e.g. species). The significance of the pan-genome arises in an evolutionary context, especially with relevance to metagenomics, but is also used in a broader genomics context. The pan-genome includes the "core genome" containing genes present in all strains, a "dispensable genome" containing genes present in two or more strains, and finally "unique genes" specific to single strains.〔 However, these distinctions are not completely objective, since they depend on which genomes are included in the analysis. == Examples == The original pan-genome concept was developed by Tettelin et al. when they sequenced six strains of ''Streptococcus agalactiae'' which could be described as a core genome shared by all isolates, accounting for approximately 80% of any single genome, plus a dispensable genome consisting of partially shared and strain-specific genes. Extrapolation suggested that the gene reservoir in the ''S. agalactiae'' pan-genome is vast and that new unique genes will continue to be identified even after sequencing hundreds of genomes.〔 A similar pattern was found in ''Streptococcus pneumoniae'' when 44 strains were sequenced (see figure). With each new genome sequenced fewer new genes were discovered. In fact, the predicted number of new genes dropped to zero when the number of genomes exceeds 50 (note, however, that this is not a pattern found in all species). The main source of new genes in ''S. pneumoniae'' was ''Streptococcus mitis'' from which genes were transferred horizontally. The pan-genome size of ''S. pneumoniae'' increased logarithmically with the number of strains and linearly with the number of polymorphic sites of the sampled genomes, suggesting that acquired genes accumulate proportionately to the age of clones.〔 Another example for the latter can be seen in a comparison of the sizes of the core and the pan-genome of ''Prochlorococcus''. The core genome set is logically much smaller than the pan-genome, which is used by different ecotypes of ''Prochlorococcus''. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Pan-genome」の詳細全文を読む スポンサード リンク
|