Insects often harbour heritable symbionts that provide defence against specialized natural enemies, yet little is known about symbiont protection when hosts face simultaneous threats. In pea aphids (
Heritable symbionts are often observed at intermediate prevalence within host populations, despite expectations that positive fitness feedbacks should drive beneficial symbionts to fixation. Intermediate prevalence may reflect neutral dynamics of symbionts with weak fitness effects, transient dynamics of symbionts trending towards fixation (or elimination), or a stable intermediate outcome determined by the balance of fitness effects and failed symbiont transmission. Theory suggests that these outcomes should depend on symbiont‐conferred demographic effects and vertical transmission efficiency, which may both depend on environmental context. We established experimental populations of winter bent grass We found evidence for all three proposed mechanisms for intermediate symbiont prevalence, but the outcome differed qualitatively across years and precipitation treatments. In the first year, symbionts trended towards fixation under drought conditions but drifted neutrally under elevated precipitation. Fixation likely arose from symbiont‐conferred recruitment benefits outweighing reproductive costs under the drought conditions, while elevated precipitation tempered these effects. In the second transition year, we inferred stable intermediate prevalence across both precipitation treatments, which indicated a balance between symbiont conferred recruitment benefits that allowed low‐prevalence populations to increase and imperfect transmission that caused high‐prevalence populations to decrease.
- NSF-PAR ID:
- 10381409
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of Ecology
- Volume:
- 109
- Issue:
- 2
- ISSN:
- 0022-0477
- Page Range / eLocation ID:
- p. 847-859
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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