Inbreeding and measures of immune function in the cricket Teleogryllus commodus

Studies of sexual selection and immunity in invertebrates often assay components of the immune system (e.g., encapsulation response, hemocyte counts) to estimate disease resistance. Because increased disease resistance is thought to enhance fitness in most cases, we might expect a positive relationship between fitness and measured immune function. Indeed, several studies have shown that measures of immunity are correlated with fitness enhancing traits. We used inbreeding to investigate the relationship between fitness and 2 commonly used assays of insect immunity in the cricket Teleogryllus commodus. Previous studies in T. commodus have shown inbreeding depression for several life history and sexually selected traits. We compared the lysozyme-like activity of the hemolymph and hemocyte counts of inbred (full-sibling mating) and outbred crickets. If these measures of immune function are positively correlated with fitness, we expect both measures to decline with inbreeding. However, there was no change in lysozyme-like activity and a significant increase in hemocyte counts with inbreeding. Our results demonstrate that it is not always the fittest individuals that have highest measured immune function.