Abstract
Symbiotic interactions between insects and bacteria have long fascinated ecologists. Aphids have emerged as the model system on which to study the effect of endosymbiotic bacteria on their hosts. Aphid‐symbiont interactions are ecologically interesting as aphids host multiple secondary symbionts that can provide broad benefits, such as protection against heat stress or specialist natural enemies (parasitic wasps and entomopathogenic fungi). There are nine common aphid secondary symbionts and individual aphids host on average 1–2 symbionts. A cost‐benefit trade‐off for hosting symbionts is thought to explain why not all aphids host every possible symbiont in a population. Both positive and negative associations between various symbionts occur, and this could happen due to increased costs when cohosting certain combinations or as a consequence of competitive interactions between the symbionts within a host. In this issue of Molecular Ecology, Mathé‐Hubert, Kaech, Hertaeg, Jaenike, and Vorburger (2019) use data on the symbiont status of field‐collected aphids to inform a model on the evolution of symbiont co‐occurrence. They vary the effective female population size as well as the rate of horizontal and maternal transmission to infer the relative impact of symbiont‐symbiont interactions versus random drift. Additional data analysis revisits an association between two symbionts in a fruit fly species using a long‐term data set to highlight that such interactions are not limited to aphids.
Highlights
One step forward is provided by Mathé-Hubert et al (2019) in this issue of Molecular Ecology. These authors explore the role of drift and symbiont-symbiont interactions to explain associations between various symbiont species in field populations
Mathé-Hubert et al (2019) analysed a European data set on symbiont occurrence from 498 pea aphids (Acyrthosiphon pisum), identifying seven symbiont species: Hamiltonella defensa, Regiella insecticola, Serratia symbiotica, Rickettsia, Rickettsiella, Spiroplasma and X-type ( Fukatsuia symbiotica)
They found similar average numbers of symbionts within aphids collected from two different host plant species (0.97 symbionts per aphid on Trifolium spp. and 0.77 on Medicago sativa, with similar sample sizes)
Summary
The effect sizes associated with these interactions varied widely depending on the species and genotype of both symbiont and aphid. The building evidence that host-symbiont dynamics are strongly influenced by maternal and horizontal transmission rates, host and symbiont genetic variation, and the wider interacting community, highlights the need to focus efforts on combining laboratory experimental work with field studies. These authors explore the role of drift and symbiont-symbiont interactions to explain associations between various symbiont species in field populations.
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