Presentation Detail

Disease Ecology and Evolution

Parker, Benjamin James [1], Gerardo, Nicole [1].

Context-dependency of the immune response: Implications for host fitness.

Researchers have started to recognize the importance of ecological context when studying an organism's immune response. A first principle of this field, which has been termed ecological immunology, is that immune responses are costly for hosts, leading to a tradeoff between investment in parasite defense and other life-history traits. However, the tradeoff between immune costs and life history traits is context dependent, and an immune response may only impact individual fitness under certain conditions, e.g. when resources are limited or other energetic demands are high.
Pea aphids (Acyrthosiphon pisum) produce genetically identical winged and unwinged morphs under different environmental conditions. Winged aphids have different energetic requirements than their clonal unwinged sisters since they must also invest in wings and their associated musculature. We first show that when winged aphids are exposed to a heat-killed fungal pathogen, they show a strong reduction in the number of offspring they are able to produce. However, unwinged aphids show no drop in reproductive capacity upon fungal exposure. Furthermore, winged aphids, but not unwinged aphids, show a strong depletion of circulating immune cells upon exposure to heat-killed fungus.
Two alternative hypotheses explain these results. The first possibility is that winged aphids invest more strongly in an immune response, explaining the decrease in reproductive capacity and the depletion of immune cells. A second possibility is that because the production of wings is energetically expensive for aphids, winged aphids cannot maintain high reproductive capacity or high numbers of circulating immune cells upon pathogen exposure. In contrast, unwinged aphids are able to mount and maintainan immune response without a trade-off with fitness.
We test these alternative hypotheses by conducting live pathogen infections in order to measure survival and pathogen load, and by using transcriptome sequencing to measure gene expression. Our data show that unwinged aphids are much more resistant to fungal infection, and more strongly express putative immune genes than winged aphids. These findings support our second hypothesis, and show how polyphenism is an important context for immunity. By linking fitness with specific immune traits, these data enhance our mechanistic understanding of the costs of immunity, and more generally, contribute to our understanding of the factors that influence investment in immunity.

1 - Emory University, Biology, O. Wayne Rollins Research Center, 1510 Clifton Road NE, Room 2006, Atlanta, GA, 30322, United States

Keywords:
Host-pathogen interactions
ecological immunology
polyphenism
tradeoffs
Pea aphids.

Presentation Type: Regular Oral Presentation
Session: 48
Location: Superior B/Cliff Lodge
Date: Saturday, June 22nd, 2013
Time: 4:45 PM
Number: 48006
Abstract ID:975
Candidate for Awards:W.D. Hamilton Award for Outstanding Student Presentation