1] Microbial Diversity Institute, Yale University, West Haven, CT, USA [2] Bioproduction Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan.
1] Microbial Diversity Institute, Yale University, West Haven, CT, USA [2] Department of Integrative Biology, University of Texas at Austin, Austin, TX, USA.
ISME J. 2014 Jun;8(6):1237-46. doi: 10.1038/ismej.2013.235. Epub 2014 Jan 9.
Bacterial symbionts that undergo long-term maternal transmission experience elevated fixation of deleterious mutations, resulting in massive loss of genes and changes in gene sequences that appear to limit efficiency of gene products. Potentially, this dwindling of symbiont functionality impacts hosts that depend on these bacteria for nutrition. One evolutionary escape route is the acquisition of a novel symbiont with a robust genome and metabolic capabilities. Such an acquisition has occurred in an ancestor of Philaenus spumarius, the meadow spittlebug (Insecta: Cercopoidea), which has replaced its ancient association with the tiny genome symbiont Zinderia insecticola (Betaproteobacteria) with an association with a symbiont related to Sodalis glossinidius (Gammaproteobacteria). Spittlebugs feed exclusively on xylem sap, a diet that is low both in essential amino acids and in sugar or other substrates for energy production. The new symbiont genome has undergone proliferation of mobile elements resulting in many gene inactivations; nonetheless, it has selectively maintained genes replacing functions of its predecessor for amino-acid biosynthesis. Whereas ancient symbiont partners typically retain perfectly complementary sets of amino-acid biosynthetic pathways, the novel symbiont introduces some redundancy as it retains some pathways also present in the partner symbionts (Sulcia muelleri). Strikingly, the newly acquired Sodalis-like symbiont retains genes underlying efficient routes of energy production, including a complete TCA cycle, potentially relaxing the severe energy limitations of the xylem-feeding hosts. Although evolutionary replacements of ancient symbionts are infrequent, they potentially enable evolutionary and ecological novelty by conferring novel metabolic capabilities to host lineages.
经历长期母体传递的细菌共生体经历了有害突变的固定升高，导致大量基因丢失和基因序列发生变化，这些变化似乎限制了基因产物的效率。潜在地，这种共生体功能的减少会影响依赖这些细菌获取营养的宿主。一种进化上的逃避途径是获得具有强大基因组和代谢能力的新共生体。这种获得发生在斑须蝽（Philaenus spumarius）的一个祖先中，斑须蝽是一种草地沫蝉（昆虫纲：沫蝉科），它已经取代了与古老的微小基因组共生体 Zinderia insecticola（β变形菌）的联系，转而与与 Sodalis glossinidius（γ变形菌）相关的共生体联系在一起。沫蝉仅以木质部汁液为食，这种饮食中必需氨基酸和糖或其他能量产生的基质含量都很低。新的共生体基因组经历了移动元件的增殖，导致许多基因失活；尽管如此，它选择性地维持了基因，这些基因取代了其前身的功能，用于氨基酸生物合成。虽然古老的共生体伙伴通常保留完全互补的氨基酸生物合成途径集，但新型共生体引入了一些冗余，因为它保留了一些也存在于伙伴共生体（Sulcia muelleri）中的途径。引人注目的是，新获得的类似于 Sodalis 的共生体保留了高效的能量产生途径的基因，包括完整的三羧酸循环，这可能会缓解木质部取食宿主的严重能量限制。尽管古老共生体的进化替代并不常见，但它们通过赋予宿主谱系新的代谢能力，为进化和生态新奇性提供了可能。
