Sirand-Pugnet Pascal, Lartigue Carole, Marenda Marc, Jacob Daniel, Barré Aurélien, Barbe Valérie, Schenowitz Chantal, Mangenot Sophie, Couloux Arnaud, Segurens Beatrice, de Daruvar Antoine, Blanchard Alain, Citti Christine
Université Bordeaux 2, UMR1090, Villenave d'Ornon, France.
PLoS Genet. 2007 May 18;3(5):e75. doi: 10.1371/journal.pgen.0030075.
Mycoplasmas are commonly described as the simplest self-replicating organisms, whose evolution was mainly characterized by genome downsizing with a proposed evolutionary scenario similar to that of obligate intracellular bacteria such as insect endosymbionts. Thus far, analysis of mycoplasma genomes indicates a low level of horizontal gene transfer (HGT) implying that DNA acquisition is strongly limited in these minimal bacteria. In this study, the genome of the ruminant pathogen Mycoplasma agalactiae was sequenced. Comparative genomic data and phylogenetic tree reconstruction revealed that approximately 18% of its small genome (877,438 bp) has undergone HGT with the phylogenetically distinct mycoides cluster, which is composed of significant ruminant pathogens. HGT involves genes often found as clusters, several of which encode lipoproteins that usually play an important role in mycoplasma-host interaction. A decayed form of a conjugative element also described in a member of the mycoides cluster was found in the M. agalactiae genome, suggesting that HGT may have occurred by mobilizing a related genetic element. The possibility of HGT events among other mycoplasmas was evaluated with the available sequenced genomes. Our data indicate marginal levels of HGT among Mycoplasma species except for those described above and, to a lesser extent, for those observed in between the two bird pathogens, M. gallisepticum and M. synoviae. This first description of large-scale HGT among mycoplasmas sharing the same ecological niche challenges the generally accepted evolutionary scenario in which gene loss is the main driving force of mycoplasma evolution. The latter clearly differs from that of other bacteria with small genomes, particularly obligate intracellular bacteria that are isolated within host cells. Consequently, mycoplasmas are not only able to subvert complex hosts but presumably have retained sexual competence, a trait that may prevent them from genome stasis and contribute to adaptation to new hosts.
支原体通常被描述为最简单的自我复制生物，其进化主要特征是基因组缩小，推测的进化过程类似于昆虫内共生菌等专性细胞内细菌。迄今为止，支原体基因组分析表明水平基因转移（HGT）水平较低，这意味着在这些最小的细菌中DNA获取受到强烈限制。在本研究中，对反刍动物病原体无乳支原体的基因组进行了测序。比较基因组数据和系统发育树重建显示，其小基因组（877,438 bp）中约18%已与系统发育上不同的类菌质体簇发生HGT，该簇由重要的反刍动物病原体组成。HGT涉及通常成簇出现的基因，其中一些编码脂蛋白，这些脂蛋白通常在支原体与宿主的相互作用中起重要作用。在无乳支原体基因组中还发现了类菌质体簇成员中也描述过的一种衰退形式的接合元件，这表明HGT可能是通过动员相关遗传元件发生的。利用现有的测序基因组评估了其他支原体之间发生HGT事件的可能性。我们的数据表明，除上述情况外，支原体物种间的HGT水平很低，在较小程度上，两种禽类病原体鸡毒支原体和滑液支原体之间的HGT水平也很低。对处于相同生态位的支原体之间大规模HGT的首次描述，挑战了普遍接受的进化观点，即基因丢失是支原体进化的主要驱动力。后者明显不同于其他小基因组细菌，特别是在宿主细胞内分离的专性细胞内细菌。因此，支原体不仅能够颠覆复杂的宿主，而且可能保留了有性能力，这一特性可能使它们避免基因组停滞，并有助于适应新宿主。
