Mechanisms of host plant induced plasticity of cuticular hydrocarbon phenotypes in mustard leaf beetles

角质层碳氢化合物（CHCs）在昆虫物种和配偶识别中起着重要作用。在其他生物和非生物因素中，饮食可能影响昆虫的CHC表型，这些变化可能改变交配行为，导致种群或宿主使用不同的近亲物种之间的生殖隔离。两种寡食芥菜叶甲虫共存于同一生境，但寄主利用方式不同。在自然条件下（不同寄主种），两种物种表现出行为隔离。然而，如果两个物种在同一寄主植物上饲养，行为隔离就会被打破，两个物种都会随机交配。化学分析表明，CHC表型受寄主使用的强烈影响。无论甲虫的种群或物种身份如何，饲养在同一寄主物种上的甲虫的CHC表型比饲养在不同寄主物种上的甲虫的CHC表型更相似。饮食可能对肠道微生物群有显著影响。肠道微生物组的这些变化可以改变CHC表型以及交配行为。本研究项目的目的是：(A)研究宿主特异性选型交配在后代的生产性能和免疫能力方面是否具有适应性，以及(B)阐明芥菜甲虫饮食诱导的CHC表型可塑性背后的机制以及肠道微生物组的潜在作用。在第一步，我们将在不同的寄主植物上饲养这两种甲虫，以说明不同的寄主使用对肠道微生物群的影响。为了确定CHC表型变化的植物相关触发因素，我们将(i)在单一宿主物种上饲养甲虫，我们将对其层球微生物组进行不同的操作，以及（ii）在不同的人工饲料上添加不同的宿主特异性次级代谢物。下一步，我们将跟踪宿主移位后肠道微生物组的变化，将肠道微生物组的时间变化与CHC表型的变化联系起来。此外，我们将使用RNA-seq方法分别分析不同宿主使用和肠道微生物组对甲虫基因表达的影响，并特别关注卵母细胞（CHC生物合成的部位）。随后，选定的候选基因在宿主触发的CHC表型可塑性中的作用将通过RNA干扰（RNAi）进行验证。本项目的研究结果将为了解CHC生物合成的调控机制提供新的基础知识，并强调种间相互作用和表型可塑性在昆虫通讯系统分化中的作用。

Cuticular hydrocarbons (CHCs) play an essential role in species and mate recognition in insects. Among other biotic and abiotic factors, diet could affect the CHC phenotype of an insect and these changes may modify the mating behavior and lead to reproductive isolation between populations or closely related species with divergent host use. The two oligophagous mustard leaf beetle species Phaedon armoraciae and Phaedon cochleariae co-occur in the same habitat, but differ in host use. Under natural conditions (different host species), both species show behavioral isolation. However, if both species were reared on the same host plant species, behavioral isolation breaks down and both species mate randomly. Chemical analyses show that the CHC phenotype is strongly affected by host use. CHC phenotypes of beetles reared on the same host species are more similar than CHC phenotypes of beetles reared on different host species, irrespective of population or species identity of the beetles. Diet could have a significant effect on the gut microbiome. These changes in the gut microbiome could change the CHC phenotype as well as the mating behaviour. The aims of this research project are (A) to investigate whether host-specific assortative mating is adaptive in terms of performance and immunocompetence of the offspring, and (B) to elucidate the mechanisms behind the diet-inducted plasticity of CHC phenotypes in mustard beetles and the potential role of the gut microbiome. In a first step, we will rear both beetle species on different host plant species to illustrate the impact of divergent host use on the gut microbiome. In order to identify the plant-associated trigger for the change in CHC phenotypes, we will rear beetles (i) on a single host species of which we will manipulate the phyllosphere microbiomes differently, and (ii) on different artificial diets that differ in the supplementation with host-specific secondary metabolites. In a next step, we will follow the change of the gut microbiome after a host shift to associate the temporal change of the gut microbiome with the change in the CHC phenotype. Furthermore, we will use an RNA-seq approach to analyse the impact of divergent host use and gut microbiomes, respectively, on gene expression in beetles with a special focus on oenocytes, the site of CHC biosynthesis. Subsequently, the role of selected candidate genes in host-triggered plasticity of CHC phenotypes will be validated by RNA interference (RNAi). The results of our project will provide new fundamental knowledge about the regulatory mechanisms of the CHC biosynthesis and highlight the role of interspecific interactions and phenotypic plasticity in the divergence of communication systems in insects.