A well-functioning immune system is a crucial factor in the life of any species as infections by parasites may have a negative effect on the host’s fitness. It seems that vertebrates and invertebrates have evolved different strategies for their protection against parasitic infections. Whereas vertebrates have developed a highly effective and adaptive immune system against host-specific parasites, the invertebrate immune system seems to be simpler. 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. The risk of a parasitic infection might 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 phenotype of an individual may not only be affected by its genotype and the environmental conditions experienced during its development or in the current state, but also by the genotype and the environmental conditions experienced by its parents. Past and current environmental conditions experienced by the parents are potentially important factors that can shape the offspring phenotype and are known as trans-generational effects. They represent transmitted non-genetic phenotypic variance e.g by transferred nutrients from mother to embryo that can enhance offspring fitness. Poor environmental conditions experienced by the parents can have a negative effect on the offspring via low provision of resources to the eggs which can result in poor quality offspring with reduced fitness.
It has been shown that environmental conditions experienced by mothers can have an effect on the offspring immune system. Moreover, previous results indicate, that mothers stressed during development produce offspring that are able to cope in a better way with similar type of stress during their own development in comparison to control individuals. In the study that I conducted in spring 2014 in the Lammi Biological Station, I wanted to evaluate the role of direct as well as indirect genetic effects on the variation of the immune system. I was especially interested in the correlation between individual conditions and the plastic adjustment of offspring immune defence.
For this study, I used the Glanville fritillary butterfly (Melitaea cinxia) that is in Finland only present in the Åland Islands where it occurs in a classical metapopulation. Field collected larvae were brought to the Biological Station to rear them in either optimal or food stress conditions. The same conditions were given for the offspring generation that was produced in the lab, resulting in four different groups depending on the food condition for mother and offspring. In the parent generation a direct effect of food stress was observed, resulting in longer developmental time to pupa which allowed them to compensate the food loss and gain the same pupal weight as the control group. Moreover, food stressed mothers laid less eggs per clutch in comparison to control individuals. Having less food available seems to result further in a less active immune system, indicating a trade-off for resource allocation, as immunity is a costly trait. This effect vanished in the offspring generation, resulting in food stressed and control individuals having the same immune activity. The same situation was observed for the number of eggs laid by the females. There was no difference between stressed or control offspring. However, the effect of prolonging the developmental time was also visible in the offspring generation, showing - contrarily to the parent generation – reduced pupal weight for stressed individuals. The offspring therefore was not able to compensate the food loss with longer developmental time. In addition, no maternal effect was observed on any of the measured traits, indicating that food restriction in the parent generation does not transfer any information to the offspring, may it be condition dependent or predictive adaptive.
Further studies are needed to investigate the role of trans-generational effects in natural populations. An additional experiment has already been started with the Glanville fritillary butterfly, investigating the role of food deprivation during the reproductive stage and its possible trans-generational effects. In this experiment I could show that food deprivation of mothers resulted in a direct effect on female reproduction leading to reduced lifetime reproductive success, whereas stressed fathers sired more offspring, possibly due to higher investment into their spermatophores. However, effects on the offspring generation will be assessed after the obligatory diapause ending in spring 2015.