Parasite virulence affects both the temporal dynamics of host-parasite relationships and the degree to which parasites regulate host populations. If hosts can compensate for parasitism, then parasites may exhibit condition-dependent virulence, with high virulence being seen only when the host is under conditions of stress. Despite their usually low level of virulence, theory suggests that such parasites may still affect host population dynamics. We tested whether a trypanosome intestinal parasite of bumblebees, Crithidia bombi, expresses condition-dependent virulence. Hosts were infected with the parasite and then kept under either favourable or starvation (stressed) conditions. Under favourable conditions the infection caused no mortality, while when hosts were starved the infection increased the host mortality rate by 50%. In addition, we found a parasiterelated change in host resource allocation patterns. Infected bees invested relatively more resources into their fat body and less into their reproductive system than did non-infected bees. Whether this reallocation is parasite-driven, to enhance transmission, or a hostresponse to parasitism, remains unknown.Brown et al: Condition-dependent virulence 3

-Experimental infections of adult and larval workers and adult males of Bombus terrestris with the microsporidium Nosema bombi showed all stages and both sexes to be susceptible. On average 19-29 % of infections were successful and no significant differences among these host categories were found. Different sources of Nosema spores differed in their success in infecting different host colonies, suggesting genotype-genotype interactions at the level of colonies and parasite sources. In a second experiment, N. bombi obtained from B. terrestris were found to be infective for workers of B. lapidarius and B. hypnorum, although less so in these foreign hosts. On the other hand, case mortality was significantly higher in foreign hosts than in B. terrestris. Infection and high spore loads correlated with early death of the host. In addition, a factorial analysis showed that variation among-colonies-within-species explained more of the variation in infection success than the factor species per se. © Inra/DIB/AGIB/Elsevier, Paris parasite / brood / host specificity / Bombus / Nosema bombi

Hosts vary in both their strength of response to a general immunological insult and in their specific susceptibility to different parasite species or different strains of the same parasite. The variation in the general immune response is considered a result of the costs imposed by selection on defended individuals. The variation in the specific response may originate from variation in host and parasite genotypes and is a requirement for frequency-dependent selection. The relationship between these two fundamental aspects of defense has only rarely been studied. Using the bumblebee Bombus terrestris and its gut trypanosomal parasite Crithidia bombi we found that the host's specific response profile toward different strains correlates negatively with its level of response to a general insult. This is the opposite result one would expect if the level of general response were simply a measure of immunological quality (immunocompetence). Rather, it suggests that there is some form of a trade-off between these two fundamental aspects of the immune system. These results, therefore, shed an important light on the possible constraints that affect the evolution of the immune system and particularly the trade-off between different arms of the immune system.

Hosts vary in both their strength of response to a general immunological insult and in their specific susceptibility to different parasite species or different strains of the same parasite. The variation in the general immune response is considered a result of the costs imposed by selection on defended individuals. The variation in the specific response may originate from variation in host and parasite genotypes and is a requirement for frequency-dependent selection. The relationship between these two fundamental aspects of defense has only rarely been studied. Using the bumblebee Bombus terrestris and its gut trypanosomal parasite Crithidia bombi we found that the host's specific response profile toward different strains correlates negatively with its level of response to a general insult. This is the opposite result one would expect if the level of general response were simply a measure of immunological quality (immunocompetence). Rather, it suggests that there is some form of a trade-off between these two fundamental aspects of the immune system. These results, therefore, shed an important light on the possible constraints that affect the evolution of the immune system and particularly the trade-off between different arms of the immune system.