Parasite-mediated sexual selection in a damselfly

Sexual selection can improve population fitness and purge deleterious mutation from the gene pool by promoting condition-dependent mate selection. One ecological factor that reduces individual condition is parasitism. Parasitism tends to increase hosts' mutation load and likely indicates inferior host genetic quality. Parasite-mediated selection, therefore, should favour the mating success of parasite-resistant individuals over parasitised individuals. We tested this hypothesis in male Agriocnemis pygmaea damselflies, which are parasitised by Arrenurus water mites. We calculated frequency (i.e. the proportion of parasitism) and intensity (i.e. the number of parasites per parasitised individual) of parasitism in free-flying males and males in copula in seven natural populations. We predicted that males observed mating will be less likely to be parasitised than expected based on the frequency of parasitism in the population. We further predicted that parasite intensity would be lower in males caught in copula than single males. We found that parasitised males were significantly less likely to be found in copula than non-parasitised males, independent of their occurrence frequency. However, there was no difference in the average parasite load between males captured in copula or free-flying males. Our study shows that in addition to natural selection, sexual selection is a strong agent against parasitism and implies that it could promote local adaptation to counteract parasite driven extinction risks.