Direct and indirect pathways of fitness-impact in a protozoan-infected kissing bug

Abstract

Parasites can reduce host fitness through short-term mortality, complete or partial castration, or slight reductions in host fecundity. Hosts may reduce reproductive effort as an adaptive strategy to tolerate parasitism. However, host fitness reduction may be unrelated to host adaptation but represent a pathological sideeffect of infection. The present study evaluates experimentally the direct and indirect impact of the protozoan parasite Trypanosoma cruzi on the investment of female kissing bugs in reproductive tissue. The presence of the parasite decreases gonad weight but this effect disappears when body size is included as covariate. To examine in more detail the set of causal relationships involved, a structural equation modelling analysis is performed using body size, moulting time and nutrition as predictor variables on gonad weight in the presence and absence of the protozoan. The results obtained indicate that, irrespective of the pathway and status of infection, female kissing bugs showing a slow development tend to have lighter gonads. On the other hand, the importance of blood ingestion for gonad weight is dependent on body size and contingent on the status of infection. Uninfected individuals tend to invest more in reproductive tissue when ingesting more blood during their ontogeny, and the opposite situation is observed for infected insects. These results indicate that gonad weight reduction in T. cruzi -infected Mepraia spinolai (Porter, 1934) is a consequence of nutrition curtailment and body size reduction rather than an adaptive strategy to cope with infection.