Abstract
The last ten years have witnessed increasing interest in host–pathogen interactions involving invertebrate hosts. The invertebrate innate immune system is now relatively well characterised, but in a limited range of genetic model organisms and under a limited number of conditions. Immune systems have been little studied under real-world scenarios of environmental variation and parasitism. Thus, we have investigated expression of candidate innate immune system genes in the water flea Daphnia, a model organism for ecological genetics, and whose capacity for clonal reproduction facilitates an exceptionally rigorous control of exposure dose or the study of responses at many time points. A unique characteristic of the particular Daphnia clones and pathogen strain combinations used presently is that they have been shown to be involved in specific host–pathogen coevolutionary interactions in the wild. We choose five genes, which are strong candidates to be involved in Daphnia–pathogen interactions, given that they have been shown to code for immune effectors in related organisms. Differential expression of these genes was quantified by qRT-PCR following exposure to the bacterial pathogen Pasteuria ramosa. Constitutive expression levels differed between host genotypes, and some genes appeared to show correlated expression. However, none of the genes appeared to show a major modification of expression level in response to Pasteuria exposure. By applying knowledge from related genetic model organisms (e.g. Drosophila) to models for the study of evolutionary ecology and coevolution (i.e. Daphnia), the candidate gene approach is temptingly efficient. However, our results show that detection of only weak patterns is likely if one chooses target genes for study based on previously identified genome sequences by comparison to homologues from other related organisms. Future work on the Daphnia–Pasteuria system will need to balance a candidate gene approach with more comprehensive approaches to de novo identify immune system genes specific to the Daphnia–Pasteuria interaction.
Highlights
Parasitic infection is often associated with genetic differences between hosts
Using sequence information from D. pulex, the first crustacean to have its genome sequenced; we used a candidate gene approach to assess the relevance of five putative immune effector genes in D. magna to one of its major pathogens, the bacterium P. ramosa
The host and pathogen samples used were well studied with respect to phenotypic responses to infection (e.g. Decaestecker et al, 2007; Carius et al, 2001), and the infectivity results obtained presently were in accordance with expectations
Summary
Parasitic infection is often associated with genetic differences between hosts. Resistant genotypes should increase in frequency, but evolution towards greater immunity in hosts can be matched by counter-adaptation in pathogens, possibly leading to cycles of adaptation and counter adaptation, i.e. coevolution. Frequency-dependent selection is extensively documented in the snail Potamopyrgus antipodarum during trematode infections This has been achieved via experimental manipulations under semi-natural conditions, as well as through field collections where recently common clones often are the most heavily infected, and tend to decline in frequency (Koskella and Lively, 2009; Jokela et al, 2009). A unique alternative is the resurrection of host and pathogens in diapause from a layered seed bank This was achieved with the crustacean Daphnia where carefully controlled hatching from different layers allowed host and pathogens from different time points to be compared in a ‘time-shift’ experiment, reconstructing host–parasite coevolution from the past (Decaestecker et al, 2007). There are not enough longitudinal or time-shift studies completed to conclude that frequency dependent is to commonest mode of coevolution
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