The large percentage of the fly's body devoted to reproduction makes clear its importance; female flies are essentially ovaries with wings. It is therefore unsurprising that an immune response that might compete for energy would have an effect on reproduction. Even a simple immune response to a non-pathogenic elicitor like E. coli will reduce egg-laying (Zerofsky et al, 2005). Experiments with Salmonella typhimurium strains of various levels of virulence suggest that as pathogenicity increases so egg-laying decreases. In infected females, eggs do not appear to progress beyond stage 8 of oogenesis, when yolk deposition begins (Brandt and Schneider, 2007). It should be unsurprising to the reader at this point in the chapter that this is a checkpoint in egg development that is known to be responsive to insulin signalling (Drummond-Barbosa and Spradling, 2001).
Ovaries themselves can become infected with intracellular pathogens like L. monocytogenes and S. typhimurium as these microbes infect mac-rophage-like cells in the oviduct (Brandt and Schneider, 2007). It isn't clear whether this direct infection is responsible for changes in fertility or whether fecundity changes are linked to effects occurring elsewhere in the body. These two models are not mutually exclusive.
A curious phenotype has been observed in male flies linking mating and immunity; McKean and Nunney (2001) demonstrated that mating males have a reduced capacity to clear injected E. coli. Carney (2007) showed that within minutes of mating male flies downregulate AMP expression, which could account for the observed difference in the realized immune response in these flies.
Alterations in circadian rhythm provide another explanation for this phenomenon (Fujii et al., 2007). When male flies are placed in a circadian activity meter they show the standard circadian rhythm of activity during the day and rest at night; however, when they are housed with females they stay active almost continuously, resting only at dusk. Perhaps the sleep disruption resulting from this 24-h activity is enough to alter the fly's immune response.
The act of mating itself induces an immune response in females. This is due to the activity of a sex peptide injected along with the sperm (Peng et al., 2005). This peptide can activate both the Imd and Toll signalling pathways and induces the transcription of AMPs. The purpose of this response is unknown but it is easy to speculate that it could be a prophylactic attempt to block sexual transmission of diseases. The cost of mating on the female in terms of lifespan reduction does not appear to involve changes in diet and is suspected to involve the activity of this sex peptide, perhaps by forcing the activation of the immune response. What is really puzzling is that female flies show an immediate activation of their immune response and this is driven by the male, whereas males flies turn down their own immune system following mating. What evolved for the goose did not evolve for the gander here and the logic behind this difference is unknown.
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