Drivers of sex differences in insect immunity

Sex differences in immunity are common across animal taxa with males typically showing lower immune responses. However, recent studies in diverse taxa seem to challenge this notion of a general female bias in immunity. The mechanisms behind sex differences in immunity remain unclear but differences in how males and females resolve trade-offs between immunity and other fitness related traits may play a role. We performed a meta-analysis of sexual dimorphism in immunity in insects, and examined several mechanistic and evolutionary factors that may shape sex-biased immune competence. The goal was to determine the magnitude and direction of sex differences. With this, we wanted to examine whether sex dimorphisms are related to variables such as immune parameters considered in studies (i.e. type of assay), insect taxa or species-specific characteristics such as sex determination system and invasiveness status to identify potential fitness-related traits that may influence sex-specific investment in immunity across insects.   We collected >1100 published experimental data sets from 156 insect species, investigating sex differences in at least one standard measure of immunity. Additionally, we collected 127 published datasets on sex-specific survival following infection with any pathogen from 10 insect species. Generally, we found that insect females are significantly more immunocompetent than males, however this difference seems to be driven by specific insect orders and disappears when correcting for phylogeny. Likewise, the magnitude of sex-difference depends on how immunity is assayed, for example, antibacterial capacity showing a female biased pattern in the presence of, but not in the absence of, infectious pathogens or virgins showing stronger female bias in phenoloxidase activity compared to mated insects. Conversely, survival and pathogen load did not show any sex bias. Overall, this suggests that some responses may be costlier than others. Interestingly, we found that haplodiploid insects and insect species, where sex is determined via XO/XX chromosomes, tend to show female biased immune responses. This could indicate a role of sex chromosomes in shaping immune response, as observed in mammals. Additionally, species listed as invasive show a female biased immune response, as opposed to non-invasive species, hinting at potential ecological consequences of sex-differences in immunity.