Some herbivorous insects possess the ability to synthesize phytohormones and are considered to use them for manipulating their host plants, but how these insects acquired the ability remains unclear. We investigated endogenous levels of auxin (IAA) and cytokinins (iP and tZ), including their ribosides (iPR and tZR), in various terrestrial arthropod taxa. Surprisingly, IAA was detected in all arthropods analysed. In contrast, tZ and/or tZR was detected only in some taxa. Endogenous levels of IAA were not significantly different among groups with different feeding habits, but gall inducers possessed significantly higher levels of iPR, tZ and tZR. Ancestral state reconstruction of the ability to synthesize tZ and tZR revealed that the trait has only been acquired in taxa containing gall inducers. Our results strongly suggest critical role of the cytokinin synthetic ability in the evolution of gall-inducing habit and IAA has some function in arthropods.

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Recent studies have revealed that relationships between plant pathogens and their vectors differ depending on species, strains and associated host plants. Turnip mosaic virus (TuMV) is one of the most important plant viruses worldwide and is transmitted by at least 89 aphid species in a non-persistent manner. TuMV is fundamentally divided into six phylogenetic groups; among which Asian-BR, basal-BR and world-B groups are known to occur in Japan. In Kyushu Japan, basal-BR has invaded approximately 2000 and immediately replaced the predominant world-B virus group. To clarify the relationships between TuMV and vector aphids, we examined the effects of the TuMV phylogenetic group on the population growth of aphid vectors in turnip plants. The population growth of a generalist aphid, Myzus persicae, was not significantly different between non-infected and TuMV-infected treatments. The population growth of a specialist aphid, Lipaphis erysimi, was higher in TuMV-infected plants than non-infected ones. Similar results were obtained in experiments using world-B and basal-BR groups of TuMV. Therefore, we conclude that L. erysimi is more mutualistic with TuMV than M. persicae, and differences in TuMV phylogenetic groups do not affect the growth of aphid vectors on turnip plants.

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