Mechanisms of transgenerational immune priming in insects

Parents invest in their offspring by preparing them for defense against pathogens and parasites that only the parents have encountered, a phenomenon known as transgenerational immune priming (TGIP). The priming effect can be passed maternally or paternally to the next generation, thus increasing the survival of offspring exposed to the same pathogen. The scope of the resulting immune response can be narrow (primarily targeting the triggering pathogen) or much more general, depending on the underlying mechanism. Maternal TGIP is often narrowly focused because the major mechanism is the transfer of microbes or fragments thereof, encountered by mothers at the larval stage, to the developing eggs along with the uptake of lipophorins and vitellogenins. This induces the expression of zygotic defense genes, including those encoding antimicrobial peptides (AMPs), comparable to the defenses observed in the larvae and adults. Maternal TGIP does not appear to involve the direct vertical transmission of immunity-related effectors such as AMPs (or the corresponding mRNAs) to the eggs. Parental investment in offspring is also mediated by epigenetic mechanisms such as DNA methylation, histone acetylation and microRNA expression, which can be imprinted on the gametes by either parent without changes in the DNA sequence. Epigenetic inheritance is the only known mechanism of paternal TGIP, and results in a more general fortification of the immune response. This review considers the mechanistic basis of TGIP, its role in evolutionary processes such as the establishment of resistance against pathogens, and the impact of pathogens and parasites on the epigenetic machinery of host insects.