Department of Entomology, Cornell University, Ithaca, NY 14853, USA.
Department of Entomology, Cornell University, Ithaca, NY 14853, USA; Department of Molecular Biology and Genetics, Cornell University, Ithaca, NY 14853, USA.
J Insect Physiol. 2019 May-Jun;115:33-39. doi: 10.1016/j.jinsphys.2019.04.002. Epub 2019 Apr 3.
Hemolymph has long been recognized as a key mediator of nutritional and immunological homeostasis in insects, with the tacit understanding that hemolymph is a hostile environment for microorganisms, and microbiologically sterile in healthy insects. Recent research is overturning the conventional wisdom, and there is now overwhelming evidence that various non-pathogenic microorganisms can stably or transiently inhabit hemolymph in a diversity of insects. Most is known about Spiroplasma, especially in Drosophila species, and secondary symbionts of the Enterobacteriaceae, notably Hamiltonella defensa, in aphids. These bacteria require many nutrients, representing a likely drain on host nutritional resources, and they persist in the hemolymph by a combination of evasion and tolerance of insect immune effectors. These traits can be costly to the insect host. For some hemolymph microorganisms, these costs are balanced by other traits beneficial to the insect, notably protection against natural enemies mediated by specific toxins or competition for key nutrients. Three key priorities for future research are: to investigate the prevalence and taxonomic diversity of hemolymph microorganisms in insects; to establish the role of host nutritional and immune factors as determinants of the abundance and proliferation rates of hemolymph microorganisms; and to integrate the developing understanding of these microorganisms and their impacts (both costs and benefits) on insect nutrition and immune function into the wider study of insect physiology.
血淋巴一直被认为是昆虫营养和免疫稳态的关键介质，人们默认血淋巴是微生物的敌对环境，健康昆虫的血淋巴是无菌的。最近的研究颠覆了传统观念，现在有压倒性的证据表明，各种非致病性微生物可以在多种昆虫中稳定或短暂地栖息在血淋巴中。关于螺旋体，特别是在果蝇属物种中，以及肠杆菌科的次要共生菌，特别是在蚜虫中的防御性汉森氏菌，人们了解得最多。这些细菌需要许多营养物质，这可能会消耗宿主的营养资源，它们通过逃避和耐受昆虫免疫效应物的组合在血淋巴中持续存在。这些特征对昆虫宿主来说可能是代价高昂的。对于一些血淋巴微生物来说，这些代价可以通过对昆虫有益的其他特征来平衡，特别是通过特定毒素来保护昆虫免受天敌的侵害，或通过竞争关键营养物质来实现。未来研究的三个关键重点是：调查昆虫血淋巴微生物的流行程度和分类多样性；确定宿主营养和免疫因素作为血淋巴微生物丰度和增殖率决定因素的作用；并将对这些微生物及其对昆虫营养和免疫功能的影响（成本和收益）的不断发展的理解纳入对昆虫生理学的更广泛研究中。
