Immune competence in insect eggs depends on the extraembryonic serosa

• Eggs of the beetle Tribolium can mount an immune response comparable to adults.
• Antimicrobial peptides (AMPs) are weakly upregulated in challenged eggs of Drosophila.
• Tribolium eggs possess an extraembryonic serosa in contrast to Drosophila.
• Serosa-less Tribolium eggs reveal little upregulation of AMPs upon infection.
• The serosa plays a crucial role in the immune response of Tribolium castaneum eggs.

Innate immunity is common to all metazoans and serves as a first line of defense against pathogens. Although the immune response of adult and larval insects has been well characterized, it remains unknown whether the insect egg is able to mount an immune response. Contrary to Drosophila, Tribolium eggs develop an extraembryonic epithelium, the serosa. Epithelia are well known for their ability to fight infection, so the serosa has the potential to protect the embryo against pathogens. To test this hypothesis we created serosa-less eggs by Tc-zen1 parental RNAi. We found that the Tribolium egg upregulates several immune genes to comparable levels as adults in response to infection. Drosophila eggs and serosa-less Tribolium eggs, however, show little to no upregulation of any of the tested immune genes. We conclude that the extraembryonic serosa is crucial for the early immune competence of the Tribolium egg.