The interaction between Drosophila and its microsporidian parasite: From the within-host to the evolutionary scale

Abstract

Resistance to parasites is advantageous for most organisms but investing limited resources into defence depends on the selection pressure involved and the nature and magnitude of costs involved. The evolutionary interactions between hosts and their parasites have received much attention but the effect of intracellular parasites in such systems is far less understood. Microsporidia are intracellular parasites of vertebrates and invertebrates that have gained prominence both as a pathogen and a potential biocontrol agent. This thesis investigates evolution of resistance against intracellular parasites and the associated trade-offs in Drosophila melanogaster and Tubulinosema kingi system. Stage-specific susceptibility of the host to T. kingi infection and stage-specific within-host parasite proliferation; host tissue specificity and the impact of T. kingi on host sex ratio were also investigated in this system. Immune responses of D. melanogaster to T. kingi infection were examined. Increased haemocyte density and phenoloxidase activity was observed in infected flies and a negative effect of nitric oxide on parasite density was observed. Five pairs of replicate D. melanogaster lines were used for experimental evolution of resistance to T. kingi. The within-host parasite density decreased significantly in selected lines indicating the evolution of resistance. The early fecundity and longevity of selected lines were significantly higher than of control lines when infected by T. kingi. Trade-offs associated with increased resistance against microsporidia was investigated. The evolved resistance was negatively correlated with fecundity both under normal and stressed conditions of the host; the selected lines were also poor larval competitors for scarce food resources. The haemocyte density and phenoloxidase activity in haemolymph of larvae from selected lines was higher than in control lines. The implications of these results are discussed in relation to other host-parasite systems and the possibility of using the system as a model insect-microsporidia system to explore ecological and evolutionary interactions.