Transgenerational developmental effects of immune priming in the red flour beetle Tribolium castaneum

Abstract

14Immune priming, the increased chance to survive a secondary encounter with a pathogen, has been 15 described for many invertebrate species, which lack the classical adaptive immune system of 16vertebrates. Priming can be specific even for closely related bacterial strains, last up to the entire 17 lifespan of an individual, and in some species, it can also be transferred to the offspring and is then 18 called transgenerational immune priming (TGIP). In the red flour beetle Tribolium castaneum, a pest 19 of stored grains, TGIP has even been shown to be transferred paternally after injection of adult 20 beetles with heat-killed Bacillus thuringiensis. 21

Paternal knockdown of tRNA (cytosine-5-)-methyltransferase (Dnmt2) increases offspring susceptibility to infection in red flour beetles

“…Furthermore, we did not observe any upregulation in the previously described marker genes for immune and stress responses (Eggert et al, 2014) nor any changes in the expression of transposable elements. This surprising lack of any priming effect (in contrast to previous studies: (Eggert et al, 2014; Roth et al, 2010; Schulz et al, 2019) might have been caused by the wounding associated with the injection of dsRNA into all fathers for the pupal RNAi treatment, which took place before the injection of the heat-killed bacteria (note that we included an RNAi control, but no fully naïve control for the RNAi treatment, Figure 2 Overview of the experimental design combining paternal RNAi-mediated knockdown of Dnmt2 with paternal TGIP. The entire experiment was conducted in three replicates ( asnA =RNAi control; PBS=injection control; naïve=handling control).).…”

“…We were able to show that the Dnmt2 knockdown extended to the testes, where Dnmt2 transcripts were strongly reduced, which is a likely prerequisite for manipulating the transfer of information between fathers and offspring. The offspring of Dnmt2 RNAi-treated fathers needed longer to reach pupation and thus remained longer in the vulnerable larval stage, e.g., the stage susceptible to oral bacterial infection (Milutinović et al, 2015; Schulz et al, 2019). They also died at a higher rate from a B. thuringiensis infection, which may point towards a generally higher stress sensitivity.…”

“…While maternal transfer appears to be a relatively common phenomenon, reports about paternal transgenerational immune priming (TGIP) are scarce 25,29 . The red flour beetle, Tribolium castaneum is the first example where paternal TGIP against a variety of bacterial pathogens has been demonstrated [29][30][31] . However, the mechanisms underlying TGIP remain elusive.…”

“…While maternal transfer of immunity appears to be a relatively common phenomenon and recent studies are beginning to shed some light on how it functions (Mondotte et al, 2020; Tetreau et al, 2019), reports about paternal transgenerational immune priming (TGIP) are scarce (Eggert et al, 2014; Roth et al, 2010) and its underlying mechanisms remain elusive. However, one thing we know about paternally transferred immune priming is that it can be not only an intergenerational but rather a transgenerational phenomenon, i.e., the protective effect of the immune priming is passed on to more than one offspring generation (Schulz et al, 2019). The paternal route of priming narrows down the possibilities by which effectors or information could be transferred from father to offspring and thus makes the involvement of epigenetic modifications, such as methylation of sperm RNA, especially likely (Roth et al, 2018).…”

Innate immunological memory: from plants to animals

“…For example, queen Apis mellifera honeybees injected with heatkilled Paenibacillus larvae give rise to offspring that contain up to three times as many differentiated hemocytes, and survive better against P. larvae infection [39]. Furthermore, infection of the adult red flour beetle (Tribolium castaneum, a pest of stored grain) with heat-killed Bacillus thuringiensis leads to paternal priming, protecting not only the first offspring generation, but also the second [40]. Finally, infection of the crustacean Daphnia magna with pathogenic bacteria Pasteuria romosa can maternally transfer strain-specific immunity to their offspring [41].…”