So a while back I wrote how awesome insect eggs were, with their ability to induce an impressive immune response when encountering bacteria (read it here). Although the red flour beetle is pretty awesome, this little critter is not the only species of insect around. Even at our institute it is not the only one that is kept. After a chat with Professor Daniel Rozen, we decided that it would be very interesting to look at the egg immune response in another beetle species, the burying beetle (Nicrophorus vespilloides). The eggs of the red flour beetle raised the bar high, but how much of an underachiever the egg of Nicrophorus was in the immune response department was a surprise!
The burying beetle
The burying beetle (N. vespilloides) is actually a pretty cool animal! (see the picture below) It is of a decent size, 2-3 cm approximately. Like many insects it is holometabolous, meaning that it has an egg, larva, pupae and then adult stage. When mom and dad find a nice dead animal, preferably about the size of a mouse or small bird, they immediately start to bury it. It isn’t called the burying beetle for nothing. They use this animal carcass to raise their offspring. Contrary to many other insects, the burying beetle shows parental care. Parents clean the carcass and secrete antimicrobials onto the carcass to keep it fresh for their larva. Mom lays her eggs in the soil around the carcass and when they hatch they will move to the carcass to feed. After a while the full-grown larva will leave the carcass to pupate in the soil and hatch into fresh new adults. So far so good!
What is the problem then? Well, finding a fresh carcass isn’t always easy. Previous research showed that females actually prefer a fresh carcass over an old carcass when they have the choice. But they might not always have that choice. The difference between an fresh and an old carcass is basically that the old carcass has been given 7 days to rot at room temperature, which makes working with these a lot of fun! Seeing as the eggs are laid in the soil surrounding the carcass, we hypothesized that this soil might play an important role in egg survival. Soil near an rotting carcass was likely contaminated, this contamination might influence the survival of eggs unless they have a decent immune response. So we performed some experiments to figure out whether eggs were influenced by the presence of a rotting carcass.
Eggs cannot handle adverse conditions
By a series of experiments (for details see the full paper, reference below) we discovered several things. We found that females lay fewer eggs near a rotting carcass and these eggs are less likely to hatch. We also show that this is likely due to the fact that these eggs are incapable of mounting an immune response. They are not only terrible in defending themselves against microbes, we also showed that they cannot survive dry conditions either! So these eggs are pretty defenseless! Because eggs of the red flour beetle are protected by the serosa (more about the serosa here), we used microscopy to find out whether the eggs of Nicrophorus made a serosa at all. And yes, they do make a serosa. However, in this species of insect it lost its defensive capabilities.
Too fast to get hurt
Although it seems strange that eggs which are near a rotting carcasses are completely defenseless, we believe there might be an explanation. Many insect eggs take a while to develop, but the eggs of the burying beetle are super-fast! They only take about 2.5 days to develop. This makes sense as the carcass isn’t going to be there forever. They need to get there before it is gone. As soon as the eggs hatch, the larva are capable of defending themselves. So it might be that they rather spend all their energy on developing as fast as possible so they can make use of the carcass, instead of spending it on defending themselves as eggs. Future experiments will tell us more whether this scenario is real or not.
Jacobs, C. G. C., Wang, Y., Vogel, H., Vilcinskas, A., Van der Zee, M., and Rozen, D. E. (2014). Egg survival is reduced by grave-soil microbes in the carrion beetle, Nicrophorus vespilloides. BMC Evolutionary Biology, 14(1). doi: 10.1186/s12862-014-0208-x