Expectant mothers have a long list of dietary dos and don'ts to worry about to give their babies the best start in life. Sushi, alcohol and raw eggs are all off the menu, and folic acid supplements suddenly become a necessity. The same appears to be true for the red flour beetle (Tribolium castaneum). Beetles that eat a diet contaminated with different species of bacteria produce eggs that are better primed to fight off bacterial pathogens, and recent research from Andreas Vilcinskas’ laboratory at the Fraunhofer Institute for Molecular Biology and Applied Ecology in Giessen, Germany, shows us why.
Despite the earlier belief that invertebrates cannot mount an immune response, recent research has shown that invertebrates that encounter a pathogen such as a bacterium or fungus can subsequently defend themselves against that type of pathogen. Even more surprisingly, this priming can be passed on to subsequent generations in a process called transgenerational immune priming (TGIP). Using the red flour beetle, which is a commonly used model organism, Vilcinskas and his team set out to test two possible mechanisms for transgenerational immunes priming: one in which different genes are activated or inactivated by changes in DNA methylation in the eggs, and a second in which the eggs are directly exposed to the bacteria in the mother's body.
The team first looked at changes in the expression of stress- and immune-related genes in eggs laid by parents that were fed three different types of bacteria, each with different cell walls: Escherichia coli, Micrococcus luteus and Pseudomonas entomophila. They found upregulation of these genes, which turned out to be specific to the bacteria that the parents had eaten, with the eggs responding most strongly to the diet that included M. luteus.
Next, the researchers decided to look at whether there were wholesale changes in how many genes were turned ‘on’ or ‘off’ in the eggs by measuring the amount of DNA methylation in the eggs of parents fed the different bacterial species. However, they found no differences among the eggs laid by moms with different diets, so they moved on to the second hypothesis: that the eggs themselves became contaminated with the bacteria that their parents consumed.
To test this theory, Vilcinskas and colleagues fed beetles a very special meal – E. coli attached to a fluorescing particle that the team could use to identify where bacteria ended up in the bodies of the mothers that had consumed the meal. Using fluorescent microscopy to track the location of the bacteria, they found that not only did E. coli translocate from the gut of the mother beetles to the fat body, but also that E. coli could be detected between the wall of the developing egg and the mother, as well as on the eggs in the oviduct. The researchers concluded that the eggs were primed for an immune response because they were exposed to bacterial pathogens from their mother's diet well before they were laid and exposed to the external world.
So it turns out that expectant red flour beetle mothers can arm their offspring to deal with pathogens long before they are laid by eating a diet that primes an immune response when bacteria that have been consumed reach unlaid eggs in the mother's body. And fathers can also influence their offspring's immune function, but how this occurs remains a mystery that the researchers hope to tackle sometime soon.