Department of Fundamental Microbiology, University of Lausanne, Lausanne, Switzerland.
Department of Ecology and Evolution, University of Lausanne, Lausanne, Switzerland.
Mol Ecol. 2019 May;28(9):2224-2237. doi: 10.1111/mec.15075. Epub 2019 May 29.
Bacteria that engage in long-standing associations with particular hosts are expected to evolve host-specific adaptations that limit their capacity to thrive in other environments. Consistent with this, many gut symbionts seem to have a limited host range, based on community profiling and phylogenomics. However, few studies have experimentally investigated host specialization of gut symbionts and the underlying mechanisms have largely remained elusive. Here, we studied host specialization of a dominant gut symbiont of social bees, Lactobacillus Firm5. We show that Firm5 strains isolated from honey bees and bumble bees separate into deep-branching host-specific phylogenetic lineages. Despite their divergent evolution, colonization experiments show that bumble bee strains are capable of colonizing the honey bee gut. However, they were less successful than honey bee strains, and competition with honey bee strains completely abolished their colonization. In contrast, honey bee strains of divergent phylogenetic lineages were able to coexist within individual bees. This suggests that both host selection and interbacterial competition play important roles in host specialization. Using comparative genomics of 27 Firm5 isolates, we found that the genomes of honey bee strains harbour more carbohydrate-related functions than bumble bee strains, possibly providing a competitive advantage in the honey bee gut. Remarkably, most of the genes encoding carbohydrate-related functions were not conserved among the honey bee strains, which suggests that honey bees can support a metabolically more diverse community of Firm5 strains than bumble bees. These findings advance our understanding of the genomic changes underlying host specialization.
与特定宿主长期关联的细菌预计会进化出宿主特异性的适应能力，从而限制其在其他环境中茁壮成长的能力。根据群落分析和系统基因组学的研究结果，许多肠道共生菌的宿主范围似乎很有限。然而，很少有研究从实验上研究肠道共生菌的宿主特异性，其潜在机制在很大程度上仍未被揭示。在这里，我们研究了社会性蜜蜂的一种主要肠道共生菌——乳杆菌 Firm5 的宿主特异性。我们发现，从蜜蜂和熊蜂中分离出的 Firm5 菌株分为深分支的宿主特异性进化枝。尽管它们的进化方向不同，但定植实验表明，熊蜂菌株能够定植在蜜蜂的肠道中。然而，它们的定植成功率低于蜜蜂菌株，与蜜蜂菌株的竞争完全抑制了它们的定植。相比之下，具有不同进化枝的蜜蜂菌株能够在单个蜜蜂体内共存。这表明宿主选择和细菌间竞争都在宿主特异性中发挥了重要作用。通过对 27 个 Firm5 分离株的比较基因组学分析，我们发现蜜蜂菌株的基因组中与碳水化合物相关的功能比熊蜂菌株更多，这可能为其在蜜蜂肠道中提供了竞争优势。值得注意的是，编码与碳水化合物相关功能的大多数基因在蜜蜂菌株之间并不保守，这表明蜜蜂可以支持比熊蜂更具代谢多样性的 Firm5 菌株群落。这些发现增进了我们对宿主特异性相关基因组变化的理解。
