Data from: Slowing them down will make them lose: a role for attine ant crop fungus in defending pupae against infections?

Fungus-growing ants (Attini) have evolved an obligate dependency upon a basidiomycete fungus that they cultivate as their food. Less well known is that the crop fungus is also used by many attine species to cover their eggs, larvae and pupae. The adaptive functional significance of this brood covering is poorly understood. One hypothesis to account for this behaviour is that it is part of the pathogen protection portfolio when many thousands of sister workers live in close proximity and larvae and pupae are not protected by cells, as in bees and wasps, and are immobile. We performed behavioural observations on brood covering in the leaf-cutting ant Acromyrmex echinatior and we experimentally manipulated mycelial cover on pupae and exposed them to the entomopathogenic fungus Metarhizium brunneum to test for a role in pathogen resistance. Our results show that active mycelial brood covering by workers is a behaviourally plastic trait that varies temporally, and across life stages and castes. The presence of a fungal cover on the pupae reduced the rate at which conidia appeared and the percentage of pupal surface that produced pathogen spores, compared to pupae that had fungal cover experimentally removed or naturally had no mycelial cover. Infected pupae with mycelium had higher survival rates than infected pupae without the cover, although this depended upon the time at which adult sister workers were allowed to interact with pupae. Finally, workers employed higher rates of metapleural gland grooming to infected pupae without mycelium than to infected pupae with mycelium. Our results imply that mycelial brood covering may play a significant role in suppressing the growth and subsequent spread of disease, thus adding a novel layer of protection to their defence portfolio.菌耕蚁（Attini族）已演化出与一种担子菌（basidiomycete fungus）的专性依赖关系，它们会将该真菌作为食物进行栽培。鲜为人知的是，诸多菌耕蚁物种还会利用这种栽培真菌覆盖其卵、幼虫与蛹。 该护幼覆盖行为的适应性功能意义迄今仍未得到充分阐释。有假说提出，当数千只姊妹工蚁紧密聚居时，幼虫与蛹既不像蜜蜂、胡蜂那样受巢室保护，自身又无法移动，此时这种覆盖行为便属于病原体防护策略组合的组成部分。 我们针对切叶蚁Acromyrmex echinatior的护幼覆盖行为开展了行为学观察，并通过实验调控蛹体表的菌丝覆盖层，随后将蛹暴露于虫生真菌布氏绿僵菌（Metarhizium brunneum）环境中，以验证该菌丝覆盖层在病原体抗性中的作用。 研究结果表明，工蚁主动为幼体覆盖菌丝的行为属于行为可塑性性状，其表达水平随时间、生命阶段以及蚁类品级发生变化。与实验移除菌丝覆盖层或天然无菌丝覆盖的蛹相比，蛹体表的真菌覆盖层可降低分生孢子（conidia）的生成速率，以及产生病原菌孢子的蛹体表占比。携带菌丝的受感染蛹存活率高于未覆盖菌丝的受感染蛹，但这一效应取决于成年姊妹工蚁与蛹发生互动的时间节点。最后，相较于覆盖有菌丝的受感染蛹，工蚁会对无菌丝覆盖的受感染蛹更为频繁地开展后胸侧板腺（metapleural gland）梳理行为。 本研究结果暗示，菌丝护幼覆盖行为可能在抑制病原菌生长及后续病害传播中发挥关键作用，从而为菌耕蚁的防御策略组合增添了一层全新的保护机制。