title: Condition-dependent alteration of cellular immunity by secondary symbionts in the pea aphid, Acyrthosiphon pisum.
abstract: Endosymbionts can fundamentally alter host physiology. Whether such changes are beneficial or detrimental to one or both partners may depend on the dynamics of the symbiotic relationship. Here we investigate the relationship between facultative symbionts and host immune responses. The pea aphid, Acyrthosiphon pisum, maintains an obligate primary symbiont, but may also harbour one or more facultative, secondary symbionts. Given their more transient nature and relatively recent adoption of a symbiotic lifestyle compared to primary symbionts, secondary symbionts may present a challenge for the host immune system. We assessed the response of several key components of the cellular immune system (phenoloxidase activity, encapsulation, immune cell counts) in the presence of alternative secondary symbionts, investigating the role of host and secondary symbiont genotype in specific responses. There was no effect of secondary symbiont presence on the phenoloxidase response, but we found variation in the encapsulation response and in immune cell counts based largely on the secondary symbiont. Host genotype was less influential in determining immunity outcomes. Our results highlight the importance of secondary symbionts in shaping host immunity. Understanding the complex physiological responses that can be propagated by host-symbiont associations has important consequences for host ecology, including symbiont and pathogen transmission dynamics.
authors.affiliation: Biology Department, Emory University, O. Wayne Rollins Research Center, 1510 Clifton Road NE, Atlanta, GA 30322, USA; School of Biological and Chemical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS, UK. Electronic address: a.laughton@qmul.ac.uk.
authors.forename: Alice M
authors.initials: AM
authors.lastname: Laughton
authors.affiliation: Biology Department, Emory University, O. Wayne Rollins Research Center, 1510 Clifton Road NE, Atlanta, GA 30322, USA; Department of Biology, Washington University in St. Louis, One Brookings Drive, Campus Box 1137, St. Louis, MO 63130, USA.
authors.forename: Justine R
authors.initials: JR
authors.lastname: Garcia
authors.affiliation: Biology Department, Emory University, O. Wayne Rollins Research Center, 1510 Clifton Road NE, Atlanta, GA 30322, USA.
authors.forename: Nicole M
authors.initials: NM
authors.lastname: Gerardo
mesh_terms: D000818:Animals; D001042:Aphids / Q000201:enzymology / Q000276:immunology* / Q000382:microbiology*; D005260:Female; D007111:Immunity, Cellular; D014442:Monophenol Monooxygenase / Q000378:metabolism; D012705:Serratia / Q000502:physiology*; D013559:Symbiosis
chemical_list: D014442:Monophenol Monooxygenase
keywords: Ecological immunity; Encapsulation; Endosymbiont; Host-microbe; Immune cell count; Symbiosis