Resolving the infection process reveals striking differences in the contribution of environment, genetics and phylogeny to host-parasite interactions

Abstract

BackgroundInfection processes consist of a sequence of steps, each critical for the interaction between host and parasite. Studies of host-parasite interactions rarely take into account the fact that different steps might be influenced by different factors and might, therefore, make different contributions to shaping coevolution. We designed a new method using the Daphnia magna - Pasteuria ramosa system, one of the rare examples where coevolution has been documented, in order to resolve the steps of the infection and analyse the factors that influence each of them.ResultsUsing the transparent Daphnia hosts and fluorescently-labelled spores of the bacterium P. ramosa, we identified a sequence of infection steps: encounter between parasite and host; activation of parasite dormant spores; attachment of spores to the host; and parasite proliferation inside the host. The chances of encounter had been shown to depend on host genotype and environment. We tested the role of genetic and environmental factors in the newly described activation and attachment steps. Hosts of different genotypes, gender and species were all able to activate endospores of all parasite clones tested in different environments; suggesting that the activation cue is phylogenetically conserved. We next established that parasite attachment occurs onto the host oesophagus independently of host species, gender and environmental conditions. In contrast to spore activation, attachment depended strongly on the combination of host and parasite genotypes.ConclusionsOur results show that different steps are influenced by different factors. Host-type-independent spore activation suggests that this step can be ruled out as a major factor in Daphnia-Pasteuria coevolution. On the other hand, we show that the attachment step is crucial for the pronounced genetic specificities of this system. We suggest that this one step can explain host population structure and could be a key force behind coevolutionary cycles. We discuss how different steps can explain different aspects of the coevolutionary dynamics of the system: the properties of the attachment step, explaining the rapid evolution of infectivity and the properties of later parasite proliferation explaining the evolution of virulence. Our study underlines the importance of resolving the infection process in order to better understand host-parasite interactions.

Resistance to a bacterial parasite in the crustacean Daphnia magna shows Mendelian segregation with dominance

“…Thus, hosts show binary resistance patterns either being resistant (no attachment) or susceptible (attachment). Attachment to the host oesophagus is responsible for the strong genotype-genotype interactions in the D. magna-P. ramosa system (Duneau et al, 2011;Luijckx et al, 2011). Thus, the finding of a single host locus controlling susceptibility to P. ramosa C1 suggests that these interactions might have a simple genetic basis.…”

“…The attachment test is a recently developed technique that employs fluorescent microscopy to assess the ability of P. ramosa spores to attach to the oesophagus of Daphnia. Attachment perfectly correlates with susceptibility of the Daphnia host in infection trials (Duneau et al, 2011). Infection trials reflect the outcome of the entire infection process (encounter, attachment, penetration and proliferation within the host), whereas the attachment test only looks at one step in this process, the attachment step.…”

“…P. ramosa is a bacterial endoparasite of D. magna (Ebert et al, 1996). Transmission occurs when hosts ingest waterborne spores, which attach to the oesophagus, penetrate and subsequently cause infection (Duneau et al, 2011). During the infection millions of spores fill the host body cavity; upon death the spores are released from the decaying cadaver and the cycle begins anew (Ebert et al, 1996).…”

“…The attachment test is described in full Duneau et al (2011). In short, for assessment of susceptibility with the attachment test four individuals older than 4 days were taken from stock cultures for each host clone (stocks kept in 100-ml jars and fed 3Â per week with 8Â10 7 cells ml À1 ) and placed singly in 24-well plates in 1 ml of medium.…”

“…This allows us to separate the step where the parasite attaches to the host, which is believed to be the key step in P. ramosa-D. magna co-evolution, from the other steps in the infection process (encounter and proliferation within the host). In addition, it allows for higher sample sizes than classical infection trials (Duneau et al, 2011). Third, we employ a structured Mendelian approach in which D. magna inbred parental clones, a F1 clone, an array of F2 clones, an array of backcrossed clones and selfed parental clones are used to resolve the genetic pattern of inheritance underlying susceptibility.…”