Aphids live pretty simple lives, suck up some plant juices, find a mate, and make some babies…sounds quite chilled, right? Wrong. In fact, the life of a pea aphid or Acyrthosiphon pisum, is a constant battle against predators and parasites. In fact, if you were to compare aphid life to anything, it would be Ridley Scott’s Alien movies. These films were pretty gory (one of the reasons I illustrated this post with the Shrek reference), and I always wondered where he came up with the idea of baby aliens bursting out of people’s chests. Well I can I quit my wondering, because parasitoid wasps or Aphidius ervi, did it first.
Now before you join team aphid, you have to realise that aphids are actually pests of multiple crops, and cause problems by spreading devastating plant viruses. And those parasitoid wasps you thought were the bad guys? Farmers actually use them as biological control agents to keep down aphid populations. Isn’t that the plot twist of the 21st century!
Now for those of you who are still on team aphid (who likes vegetables anyway), fear not, these little guys have someone defending them. Some lucky aphids harbour a symbiont, the Enterobacteriaceae, Hamiltonella defensa. However, unlike the Captain America that I have portrayed them as, these bacteria require something in return (so much for superhero). The aphid provides the bacteria with nutrients, and the bacteria provides toxins which act to prevent the development of parasitoid wasp larvae. Or specifically, the bacteriophage H. defensa harbours produces the toxins. Is three a crowd? Obviously not to the pea aphid!
Now the question that is addressed in the study I’m reviewing for you here is, is it all really worth it? Does harbouring this symbiont really save these pea aphids from the doom of being eaten alive by wasp larvae? Think I’m just being pessimistic? You may just change your mind after reading what they discovered in this article.
In this study, the first experiment they performed determined what level of defence H. defensa provided to the aphids. This involved growing parasitoid wasps on two aphid populations, one harbouring the symbiont, the other not, and counting how many stung aphids survived (ie. didn’t hatching any wasp larvae). They found that:
143/200 aphids harbouring H. defensa survived
33/400 aphids not harbouring H. defensa survived
So that symbiont bacteria really does make a difference to aphid survival! What we can also see from this data is that aphids also have an innate (or natural) immunity, evident from the fact that while 143 of the non-symbiont-harbouring aphids hatched wasp larvae and died, there were still 57 that survived.
The second experiment they performed was an evolution experiment that monitored components of fitness from parasitoids grown on symbiont-harbouring or non-symbiont-harbouring aphids over 10 generations.
Now fitness in the science world doesn’t mean the most muscular or the fastest organism, instead it means ability to survive (guess you can tell your friends you’re fit now). In this experiment, the fitness components measured were; effective parasitism rate (of the number of aphid stung, how many hatched wasp larvae), and the size of the left tibia of the wasps that emerge (used as a determinate of wasp size).
It was found that after just 4 generations, the parasitism rate of wasps grown on the symbiont-harbouring population had increased to equal that of the wasps grown on the non-symbiont-harbouring population. Did H. defensa take a holiday? Nope, that’s the power of evolution kids. Just like you evolved to eating cereal out of a pot because your flat mates never clean the dishes, these parasitoid wasps can evolve against the symbiont-mediated resistance in their aphid prey. Basically, there is massive selection pressure on these wasps, either their young survive, or they don’t. Therefore, only the wasps best adapted that carry the most useful genes (possibly acting to de-toxify that symbiont toxin), will survive to contribute to the next generation.
The other fitness component measured was the left tibia of surviving wasps, which estimates the body size. The researchers wanted to know if the wasps that emerged from the aphids harbouring the symbiont got smaller, suggesting that there had been a trade-off. Essentially, some of the nutrients wasps would normally use for growth are siphoned off (or traded) for the production of proteins that protect the wasp larvae from toxins. And indeed, these results were found in the experiment. After 10 generations, the wasps grown on the symbiont-harbouring population got smaller. Therefore, while these parasitoid wasps are getting the better of the only defence these aphids have, at least they’re looking a little bit starved in the process. Totally makes up for their babies hatching out of aphid larvae and eating them alive…
Interestingly, a further study by these researchers found that a trade-off occurs in the aphids as well, from their symbiont-mediated resistance. Aphids that harboured the symbiont suffered higher predation by ladybugs because they didn’t use as many defensive behaviours (Polin, Simon, Outreman, 2014). The researchers called this an “evolutionary cost”, and suggested that symbiosis might not be as beneficial as it’s talked up to be. Guess that’s even more bad news for the pea aphid!
The last experiment they performed really hits hard the shortcomings of H. defensa. For this experiment, they used the same wasp populations that were used in experiment 2 and exposed them to two different H. defensa-harbouring aphid clones (aphids can reproduce asexually essentially cloning themselves, these aphids are the same species but different clones). They then measured the effective parasitism rate (how often a stung aphid hatches wasp larvae), of the two different wasp populations and compared this to the effective parasitism rate they calculated from experiment 1, before evolution. This experiment will determine if selection of symbiont-mediated resistance will help these wasps survive on new symbiont-harbouring aphids.
And would you look at that, in both clones the wasps that had been grown on a symbiont-harbouring aphid population had a higher effective parasitism rate after evolution. Therefore, was it all worth it? Well, carrying the symbiont H. defensa did provide protection to the aphids, however it doesn’t last for long in these laboratory conditions. I did tell you this story doesn’t have a happy ending! However, this study does illustrate the power of evolution and that in nature, there will always be predators and prey, that’s life. No symbiont will offer complete protection, which is good news for farmers (and you if you like vegetables)!
Now you might be wondering what the practical uses of this study are. As I mentioned earlier, farmers are using these wasps to manage aphid populations. When they were first introduced in North America, the native parasitoid wasp populations were displaced and only one remained to compete with A. ervi (Schellhorn, Kuhman, Olson, Ives, 2002). This shows how important it is that we understand the biological systems we are manipulating and the implications that could occur. The use of parasitoid wasps as biological control agents disrupts the ecological system in crops, and the evolution and adaption of these organisms must be studied to anticipate and manage any changes that could occur.
It is becoming ever more prevalent that nature is a complex system and when meddled with, can result in disaster. I’m sure you’ve all heard about “super bugs” that are becoming resistant to every antibiotic, how is this different to pests becoming resistant to their parasitoids? There are also stories about messing with the food chain, such as this interesting story about shark finning. Science and evolution is a way to understand how to manipulate nature to our advantage, while preventing any catastrophes. Essentially, we need to be aware about what could happen if we introduce something new into the ecological system or take something away. Therefore this study is just one small part of the bigger picture, which is to understand and safely manipulate this biological system.
However, all this still doesn’t help the poor pea aphids. At least they helped invent a block buster movie enterprise! Someone should really let them know they aren’t suffering for nothing…
Dion, E., Zele, F., Simon, J., & Outreman, Y. (2011). Rapid evolution of parasitoids when faced with the symbiont-mediated resistance of their hosts. Journal of Evolutionary Biology 24, 741-750. doi: 10.1111/j.1420-9101.2010.02207.x
Polin, S., Simon, J., Outreman, Y. (2014). An ecological cost associated with protective symbionts of aphids. Ecology and Evolution 4(6), 836-840. doi: 10.1002/ece3.991
Schellhorn, N. A., Kuhman, T. R., Olson, A. C., & Ives, A. R. (2002). Competition between Native and Introduced Parasitoids of Aphids: Nontarget Effects and Biological Control. Ecology, 83(10), 2745-2757. doi:10.2307/3072012