MCA: The role of symbiont toxins in a defensive mutualism

动物与微生物之间的共生关系在自然界中无处不在，并且可以强烈地影响动物利用资源和应对不同环境压力和疾病的能力。共生中一个新兴的令人兴奋的主题是微生物，特别是细菌，保护宿主免受天敌侵害的能力。这种防御性联系与农业和人类健康高度相关，因为它们可以以消极和积极的方式影响针对各种害虫和虫媒病原体的措施的有效性。尽管这种防御联系的记录迅速积累，但这些细菌如何保护宿主的知识却很落后。研究这种相互作用是困难的，因为通常不可能在昆虫独立的情况下培养和操纵细菌伴侣。该项目利用果蝇（Drosophila melanogaster）模型系统的强大研究工具来了解由遗传细菌（螺原体）产生的毒素的作用，该毒素可以保护苍蝇免受寄生蜂的侵害。该项目还将利用细菌-苍蝇-黄蜂“三方”相互作用中令人兴奋的自然变异，揭示某些黄蜂如何能够避免细菌造成的损害。这些研究将有助于确定自然界中共生体介导的防御的一般原则。该项目将通过为本科生和研究生提供研究经验，以及课堂和外联活动，扩大代表性不足的群体对科学的参与，提高科学素养。在防御关联中记录的宿主、共生体和天敌的分类多样性广泛且快速增长，但对防御机制本身的理解大多局限于涉及单个细菌群（变形菌门）成员的少数关联。由于缺乏用于非模式宿主、共生体和天敌的遗传和实验室资源，研究进展受到阻碍，因此无法确定共生体介导的防御的一般原则。该项目研究了一个涉及有影响力的细菌谱系（螺原体属：Mollicutes类）的系统，该系统不仅高度分化，而且与研究得最好的防御性共生体不同。一些证据表明，螺原体阻止寄生果蝇的黄蜂成功发育的机制与螺原体编码的毒素有关。然而，这种毒素是否足以抑制黄蜂，并没有得到证实；由于无法成功转化螺原体，这项任务受到了阻碍。该项目将使用无细菌转基因果蝇，仔细检查螺旋体编码的毒素是否能使寄生果蝇幼虫的黄蜂失能。补充实验将研究某些黄蜂物种能够避免螺旋体损害的机制。因此，该项目将对作为模型系统出现的三方共同进化军备竞赛的机制基础产生新的见解。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Symbioses between animals and microbes are ubiquitous in nature and can strongly influence the ability of animals to use resources and cope with different environmental stressors and disease. An emerging and exciting theme in symbiosis is the ability of microbes, particularly bacteria, to protect their hosts against natural enemies. Such defensive associations are highly pertinent to agriculture and human health, as they can influence, in negative and positive ways, the effectiveness of measures targeted at diverse insect pests and insect-vectored pathogens. Although records of such defensive associations have rapidly accrued, knowledge of how these bacteria defend their hosts is lagging. Studying such interactions is difficult because it is usually not possible to grow and manipulate the bacterial partner independently of the insect. This project capitalizes on the powerful research tools available with the fruit fly (Drosophila melanogaster) model system to understand the role of toxins produced by an inherited bacterium (Spiroplasma) that protects flies against parasitic wasps. This project will also leverage exciting natural variation in the ‘three-way’ bacterium-fly-wasp interaction to uncover how certain wasps are able to avoid damage caused by the bacterium. These studies will help identify the general principles governing symbiont-mediated defense in nature. This project will broaden the participation of underrepresented groups in science and enhance scientific literacy, through research experiences for undergraduate and graduate students, as well as in-class and outreach activities.  The taxonomic diversity of hosts, symbionts, and natural enemies recorded in defensive associations is broad and rapidly growing, but understanding of the defensive mechanisms themselves is mostly restricted to a few associations involving members of a single bacterial group (Proteobacteria). Research progress has been hampered by the paucity of genetic and laboratory resources available for non-model hosts, symbionts, and natural enemies, precluding identification of general principles governing symbiont-mediated defense. This project investigates a system involving an influential bacterial lineage (genus Spiroplasma: Class Mollicutes) that is not only highly divergent but is also distinct from the best studied defensive symbionts. Several lines of evidence suggest that the mechanism by which Spiroplasma prevents the successful development of wasps that parasitize Drosophila, involves a Spiroplasma-encoded toxin. However, whether the toxin is necessary and sufficient to inhibit wasps has not been demonstrated; a task that is hampered by the inability to successfully transform Spiroplasma. This project will use bacteria-free transgenic Drosophila, to carefully examine whether the Spiroplasma-encoded toxin can disable wasps that parasitize Drosophila larvae. Complementary experiments will investigate the mechanism(s) by which certain wasp species are able to avoid damage by Spiroplasma. Therefore, this project will yield novel insights into the mechanistic underpinnings of a tripartite coevolutionary arms race that is emerging as a model system.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.