Response to Bacterial Infection in a Butterfly

Parasites and pathogens are ubiquitous and represent a major threat for every individual. A well-functioning immune system therefore is crucial for every species and is under strong selection pressure. Infection risk has shown to differ depending on the season, the climate, density in the population and other environmental conditions that the organism might experience during its development. However, the risk might further differ depending on the way pathogens enter the system. If a parasite breaks through the exoskeleton of insects, which is a protective barrier, it passes the haemocoel and faces the systemic immune response, a complex interaction of cellular and humoral components. Ingestion of pathogens or spores on the other hand do have the potential to infect a host, even though they first have to overcome the gut epithelium.

I wanted to investigate the importance of bacterial infection during the adult stage in the Glanville fritillary butterfly (Melitaea cinxia) via oral infection in comparison to haemocoelic exposure of the same strain. This species is in Finland only present in the Åland Islands where it occurs in a classical metapopulation. Adults disperse from habitat patches to recolonize new patches or fly for foraging and mating. During dispersal events, individuals might encounter a higher infection risk due to changes in quality or quantity of parasites. On the one hand individuals might encounter pathogens based on wounding that might occur due to predators such as ants, spiders and parasitoids. Such wounds allow pathogens to enter directly into the haemocoel. On the other hand, dispersal reflects a resource costly event and individuals might feed on more nectar after dispersal, potentially also increasing the amount of pathogens ingested.

In a series of experiments that I conducted in the Lammi Biological Station, I infected male and female adult butterflies with a bacterial strain to investigate how this species responds to bacterial infection in general, if the two sexes would differ in their immune response and if the responses would differ depending on the way the pathogens entered their system. I measured immune gene expression levels and encapsulation rate to measure immune response and further was interested in individuals’ lifespan.

Direct exposure to bacteria via injection as well as oral exposure both had an effect on the phenotype. Lifespan was reduced for both sexes and exposure to bacteria resulted in an increase in immune gene expression. However, more immune genes responded to haemocoelic bacterial infection compared to oral exposure. Moreover, females did show higher expression levels for some immune genes, indicating that females invested more in immunity than males supporting the commonly observed susceptible male hypothesis. One explanation for the observed sex difference in immune response might be due to different strategies for reproduction. Females of this species in general live longer and deposit their eggs in several clutches throughout their lifespan, whereas males are able to increase fitness via increased number of matings. Thus they do not need to live as long as females and therefore decreased survival due to bacterial infection does not necessarily reduce their fitness. 

Further studies are needed to investigate the effect of bacterial infection on reproductive success, to investigate why sexes respond differently to infections. Infections potentially play a crucial role in this system, as infected individuals could transfer pathogens to different habitat patches, spreading a disease and thus affecting population dynamics. It will be interesting to further look into effects of other pathogens, like fungi or viral infections, to better understand immunity in this butterfly and insects in general.

Luisa Woestmann is a PhD student at the University of Helsinki and a 2016 LBAYS grant recipient