Abstract. 1. Many insects host secondary bacterial symbionts that are known to have wide-ranging effects on their hosts, from host-plant use to resistance against natural enemies. This has been most widely studied in aphids, which have become a model system to study insect-bacteria interactions.2. While there is an increasing understanding of the role of symbionts in aphids from controlled laboratory studies, we are only beginning to explore the impact of hosting these symbionts on eco-evolutionary dynamics in natural systems. To date, many research groups have identified bacterial symbionts from various aphid species, providing us with a bank of literature on aphid-symbiont associations in natural populations.3. The role of secondary symbionts in aphids is discussed, and the taxonomic and geographical distribution of symbionts among aphids are summarised, and the potential reasons for the patterns observed. The need to test for multiple symbiont species (and co-infections) across many individuals and the whole distribution range of an aphid is highlighted, including sampling on all known host-plant species.4. It is further important also to consider variation within the symbiont, the aphid-host and the surrounding community, e.g. host-plants or the natural enemies, to understand how these have the potential to mediate aphid-symbiont interactions.5. Finally, the knowledge gained from experimental work should now be used to understand the role of aphid secondary symbionts in field systems, to fully understand the potentially far-reaching consequences of aphid endosymbionts on community and ecosystem processes.

Tansy plants (Tanacetum vulgare L.) exhibit high chemical variation, particularly in mono- and sesquiterpenes that are stored in specialised glands on the plant surface. In the present work we investigated the effects of terpene chemotypes on Metopeurum fuscoviride, an aphid species specialised on tansy, and their tending ants, at the field scale. Previous studies have chemotyped tansy by assessing dominant compounds; here we propose a method of chemotyping using all volatile compounds that are likely emitted from the storage glands. The analysis is based on two extraction methods: GC-MS analysis of leaf hexane extracts and SBSE analysis of headspace emissions. In an initial screening we identified the subset of compounds present in both chemical patterns, labelled as ‘compounds likely emitted from storage’. In a large field survey we could show that the putative chemotypic emission pattern from storage pools significantly affected the early aphid colonisation of tansy. Moreover, the statistical analyses revealed that minor compounds exerted a stronger influence on aphid and tending-ant presence than dominant compounds. Overall we demonstrated that within the enormous chemotypic variation of terpenes in tansy plants, chemical signatures of volatile terpenes can be related to the occurrence of insects on individual plants in the field.

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Urbanization transforms environments in ways that alter biological evolution. We examined whether urban environmental change drives parallel evolution by sampling 110,019 white clover plants from 6169 populations in 160 cities globally. Plants were assayed for a Mendelian antiherbivore defense that also affects tolerance to abiotic stressors. Urban-rural gradients were associated with the evolution of clines in defense in 47% of cities throughout the world. Variation in the strength of clines was explained by environmental changes in drought stress and vegetation cover that varied among cities. Sequencing 2074 genomes from 26 cities revealed that the evolution of urban-rural clines was best explained by adaptive evolution, but the degree of parallel adaptation varied among cities. Our results demonstrate that urbanization leads to adaptation at a global scale.

Animal associations with microbes are widespread across the natural world and can play key roles in the biology of their hosts.Thanks to important innovations in molecular techniques, the last two decades have provided deep insights into these diverse and often intricate host-microbe interactions (McFall-Ngai et al., 2013). Insects are the most abundant group of species in terrestrial systems and have evolved symbiotic associations with various microbes (Brownlie & Johnson, 2009;Feldhaar, 2011;Frago et al., 2020). Herbivorous insects in particular host complex communities of bacteria and fungi in their guts that enable them to feed on low-quality and toxin-laden leaf material (Dillon & Dillon, 2004;Hammer & Bowers, 2015). Many insects acquire beneficial symbionts from their surroundings each generation (Kikuchi et al., 2011), while others host more permanent endosymbionts within specialised insect cells, which are transferred vertically from mother to offspring (Douglas, 1998). Plant-sucking insects feed on impoverished diets, and have co-evolved with specialised bacteria that synthesise essential nutrients they cannot acquire directly from the plant (Bennett & Moran, 2015). Over time, this coevolution can lead to genomic erosion resulting in an obligate

It is well known that plant volatiles influence herbivores in their selection of a host plant; however, less is known about how the nonvolatile metabolome affects herbivore host selection. Metabolic diversity between intraspecific plants can be characterized using non-targeted mass spectrometry that gives us a snapshot overview of all metabolic processes occurring within a plant at a particular time. Here, we show that non-targeted metabolomics can be used to reveal links between intraspecific chemical diversity and ecological processes in tansy (Tanacetum vulgare). First, we show that tansy plants can be categorized into five subgroups based up on their metabolic profiles, and that these "metabotypes" influenced natural aphid colonization in the field. Second, this grouping was not due to induced metabolomic changes within the plant due to aphid feeding but rather resulted from constitutive differences in chemical diversity between plants. These findings highlight the importance of intraspecific chemical diversity within one plant population and provide the first report of a non-targeted metabolomic field study in chemical ecology.

Genetic differences among tree species, their hybrids and within tree species are known to influence associated ecological communities and ecosystem processes in areas of limited species diversity. The extent to which this same phenomenon occurs based on genetic variation within a single tree species, in a diverse complex ecosystem such as a tropical forest, is unknown. The level of biodiversity and complexity of the ecosystem may reduce the impact of a single tree species on associated communities. We assessed the influence of within-species genetic variation in the tree Brosimum alicastrum (Moraceae) on associated epiphytic and invertebrate communities in a neotropical rainforest. We found a significant positive association between genetic distance of trees and community difference of the epiphytic plants growing on the tree, the invertebrates living among the leaf litter around the base of the tree, and the invertebrates found on the tree trunk. This means that the more genetically similar trees are host to more similar epiphyte and invertebrate communities. Our work has implications for whole ecosystem conservation management, since maintaining sufficient genetic diversity at the primary producer level will enhance species diversity of other plants and animals.

1. Spatially distributed resources can lead to the formation of metapopulations, where individual subpopulations are often small and can experience frequent local extinction events followed by recolonisation. An example of terrestrial metapopulations are specialised phytophagous insects on their patchily distributed host plants.2. The present study investigated the population dynamics of a specialised aphid (Metopeurum fuscoviride) on its patchily distributed host plant (Tanacetum vulgare) and associated community of mutualistic ants and predators in a small-scale field site. Furthermore, aphid habitat differences (plant size, C/N ratio, location and surrounding vegetation) were quantified, and seasonal timing and precipitation were considered.3. Seasonal timing and precipitation both had effects on aphid colonisation, extinction events and aphid colony persistence. Towards the end of the season, and after higher precipitation, aphid colonisation events decreased and extinction events increased. Plant size and location as well as aphid within-field dispersal determined the spatio-temporal distribution of aphid colonies.4. Mutualistic ants (Lasius niger and Myrmica rubra) increased the chance of establishment of aphid colonies. However, when M. rubra was tending, aphid colony persistence was reduced. Aphid persistence and extinction were dependent on aphid abundance, as a higher colony size reduced the probability of extinction by predation.5. The results emphasise the importance of dispersal limitation, population growth and the presence of mutualists when studying the spatio-temporal dynamics of tansy aphids, particularly in a small-scale field site.