Insects share an intimate relationship with their gut microflora and this symbiotic association has developed into an essential evolutionary outcome intended for their survival through extreme environmental conditions. While it has been clearly established that insects, with very few exceptions, associate with several microbes during their life cycle, information regarding several aspects of these associations is yet to be fully unraveled. Acquisition of bacteria by insects marks the onset of microbial symbiosis, which is followed by the adaptation of these bacterial species to the gut environment for prolonged sustenance and successful transmission across generations. Although several insect–microbiome associations have been reported and each with their distinctive features, diversifications and specializations, it is still unclear as to what led to these diversifications. Recent studies have indicated the involvement of various evolutionary processes operating within an insect body that govern the transition of a free-living microbe to an obligate or facultative symbiont and eventually leading to the establishment and diversification of these symbiotic relationships. Data from various studies, summarized in this review, indicate that the symbiotic partners, i.e., the bacteria and the insect undergo several genetic, biochemical and physiological changes that have profound influence on their life cycle and biology. An interesting outcome of the insect-microbe interaction is the compliance of the microbial partner to its eventual genome reduction. Endosymbionts possess a smaller genome as compared to their free-living forms, and thus raising the question what is leading to reductive evolution in the microbial partner. This review attempts to highlight the fate of microbes within an insect body and its implications for both the bacteria and its insect host. While discussion on each specific association would be too voluminous and outside the scope of this review, we present an overview of some recent studies that contribute to a better understanding of the evolutionary trajectory and dynamics of the insect-microbe association and speculate that, in the future, a better understanding of the nature of this interaction could pave the path to a sustainable and environmentally safe way for controlling economically important pests of crop plants.

昆虫与其肠道菌群有着密切的关系，这种共生关系已发展成为昆虫在极端环境条件下生存的重要进化结果。虽然已经明确表明，除了极少数例外，昆虫在其生命周期中与多种微生物有关联，但有关这些关联的几个方面的信息尚未完全阐明。昆虫获取细菌标志着微生物共生的开始，随后这些细菌物种适应肠道环境，以实现长期维持并成功代代相传。尽管已经报道了几种昆虫-微生物组关联，并且每种关联都有其独特的特征、多样化和专业化，但目前还不清楚是什么导致了这些多样化。最近的研究表明，昆虫体内涉及各种进化过程，控制着自由生活的微生物向专性或兼性共生体的转变，并最终导致这些共生关系的建立和多样化。本综述总结的各种研究数据表明，共生伙伴，即细菌和昆虫，会经历一些遗传、生化和生理变化，这些变化对其生命周期和生物学产生深远影响。昆虫与微生物相互作用的一个有趣的结果是微生物伙伴对其最终基因组减少的顺应性。与自由生活的形式相比，内共生体拥有更小的基因组，因此提出了一个问题，是什么导致了微生物伙伴的还原进化。 这篇综述试图强调昆虫体内微生物的命运及其对细菌及其昆虫宿主的影响。虽然对每个具体关联的讨论过于冗长，超出了本综述的范围，但我们概述了一些最近的研究，这些研究有助于更好地理解昆虫-微生物关联的进化轨迹和动态，并推测，在未来，更好地了解这种相互作用的性质可以为控制经济上重要的作物害虫的可持续和环境安全的方法铺平道路。