Phenoloxidase activity and pathogen resistance in yellow dung flies Scathophaga stercoraria.

Phenoloxidase (PO) is an important component of the insect immune system and is frequently used to measure an individual's immune defence ability. However, evidence documenting positive correlations between the immune assay and resistance against pathogens is scarce and contradictory. We used replicate lines of yellow dung flies Scathophaga stercoraria (L.) with different PO levels to investigate whether PO levels affect resistance against parasitic mites and entomopathogenic fungi. Prevalence of flies exposed to pathogens was the same in all selection regimes, although pathogens clearly negatively affected fitness. PO measurements alone therefore do not necessarily predict overall resistance against pathogens. Furthermore, under starvation lines selected for high PO levels did not survive longer than those selected for low PO levels, irrespective of exposure to pathogens. This suggests that even if elevated immune levels increase an individual's ability to combat pathogens, the benefits may not outweigh the costs of increased investment in immunity.

Sex and immunity in the yellow dung fly Scathophaga stercoraria.

The immune system is of increasing interest to evolutionary biologists. Immunity may trade-off against other fitness components, with recent work suggesting reproduction in particular impinges on immune defence. There may also be sex differences in the immune system. Additionally, while life history traits typically have low heritability, little is known about additive genetic variance of immunity. An insect's major defence against multicellular pathogens is to encapsulate the invader. Phenoloxidase (PO) is a key enzyme in the cascade resulting in the melanized capsule, and is often used to estimate resistance to an immune insult. We examined the effects of copulation, egg laying, sex and age on PO in Scathophaga stercoraria. We also measured the heritability of PO activity. The sexes differed in haemolymph PO activity and PO was significantly affected by age, but not by copulation or egg laying. There was significant heritable variation for haemolymph PO.

An experimental field approach to parasitism and immune defence in voles.

The fox tapeworm Echinococcus multilocularis is typically perpetuated in a cycle with red foxes as definitive hosts and various rodent species as intermediate hosts. In this study, foxes were baited with a highly efficient drug against cestodes (praziquantel) in 5 blocks of 1 km2. Voles, Arvicola terrestris, the most abundant intermediate host species, were trapped in the 5 baited blocks and in 5 non-baited control blocks. Baiting the foxes reduced the prevalence of E. multilocularis in fox faecal samples in the baited blocks, but voles trapped in the two blocks did not differ in their infection rates. However, voles from the baited blocks had significantly smaller spleen masses and were more likely to be infested with mites than those from the control blocks, possibly reflecting different immunological activities. Our study suggests that the environmental contamination with E. multilocularis eggs, and perhaps those of other tapeworms, influences the immune system of the intermediate host species A. terrestris in the wild.