Parallel Evolution in the Integration of a Co-obligate Aphid Symbiosis

David Monnin, Raphaella Jackson, E. Toby Kiers, Marie Bunker, Jacintha Ellers, Lee M. Henry*

*Corresponding author for this work

Research output: Contribution to JournalArticleAcademicpeer-review

Abstract

Insects evolve dependence-often extreme-on microbes for nutrition. This includes cases in which insects harbor multiple endosymbionts that function collectively as a metabolic unit [1-5]. How do these dependences originate [6], and is there a predictable sequence of events leading to the integration of new symbionts? While co-obligate symbioses, in which hosts rely on multiple nutrient-provisioning symbionts, have evolved numerous times across sap-feeding insects, there is only one known case in aphids, involving Buchnera aphidicola and Serratia symbiotica in the Lachninae subfamily [7-9]. Here, we identify three additional independent transitions to the same co-obligate symbiosis in different aphids. Comparing recent and ancient associations allow us to investigate intermediate stages of metabolic and anatomical integration of Serratia. We find that these uniquely replicated evolutionary events support the idea that co-obligate associations initiate in a predictable manner-through parallel evolutionary processes. Specifically, we show how the repeated losses of the riboflavin and peptidoglycan pathways in Buchnera lead to dependence on Serratia. We then provide evidence of a stepwise process of symbiont integration, whereby dependence evolves first. Then, essential amino acid pathways are lost (at ∼30-60 mya), which coincides with the increased anatomical integration of the companion symbiont. Finally, we demonstrate that dependence can evolve ahead of specialized structures (e.g., bacteriocytes), and in one case with no direct nutritional basis. More generally, our results suggest the energetic costs of synthesizing nutrients may provide a unified explanation for the sequence of gene losses that occur during the evolution of co-obligate symbiosis.

Original languageEnglish
Pages (from-to)1949-1957.e6
Number of pages9
JournalCurrent Biology
Volume30
Issue number10
Early online date2 Apr 2020
DOIs
Publication statusPublished - 18 May 2020

Bibliographical note

Copyright © 2020 The Author(s). Published by Elsevier Inc. All rights reserved.

Keywords

  • aphid
  • Buchnera aphidicola
  • co-obligate
  • evolution of dependence
  • metabolic complementation
  • Serratia symbiotica
  • symbiont

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