title: Host age modulates within-host parasite competition.
abstract: In many host populations, one of the most striking differences among hosts is their age. While parasite prevalence differences in relation to host age are well known, little is known on how host age impacts ecological and evolutionary dynamics of diseases. Using two clones of the water flea Daphnia magna and two clones of its bacterial parasite Pasteuria ramosa, we examined how host age at exposure influences within-host parasite competition and virulence. We found that multiply-exposed hosts were more susceptible to infection and suffered higher mortality than singly-exposed hosts. Hosts oldest at exposure were least often infected and vice versa. Furthermore, we found that in young multiply-exposed hosts competition was weak, allowing coexistence and transmission of both parasite clones, whereas in older multiply-exposed hosts competitive exclusion was observed. Thus, age-dependent parasite exposure and host demography (age structure) could together play an important role in mediating parasite evolution. At the individual level, our results demonstrate a previously unnoticed interaction of the host's immune system with host age, suggesting that the specificity of immune function changes as hosts mature. Therefore, evolutionary models of parasite virulence might benefit from incorporating age-dependent epidemiological parameters.
authors.affiliation: Department of Zoology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv 6997801, Israel.
authors.forename: Rony
authors.initials: R
authors.lastname: Izhar
authors.affiliation: Department of Zoology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv 6997801, Israel.
authors.forename: Jarkko
authors.initials: J
authors.lastname: Routtu
authors.affiliation: Department of Zoology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv 6997801, Israel frida@post.tau.ac.il.
authors.forename: Frida
authors.initials: F
authors.lastname: Ben-Ami
mesh_terms: D000818:Animals; D003621:Daphnia / Q000235:genetics / Q000382:microbiology*; D005838:Genotype; D054884:Host-Pathogen Interactions; D058676:Pasteuria / Q000235:genetics / Q000502:physiology*
keywords: Daphnia–Pasteuria system; age-structured interactions; epidemiology; optimal virulence; stage-structured theory; trade-off hypothesis