@BugQuestions I was asked today about bacterial symbionts in insects, very disappointed I could only think of half a dozen. Any good ones?
— Jimmy Conway (@InsectophileJim) January 22, 2015
This is a question we received a very long time ago, and I’ve been looking for a way to work it into a current event. I think the recent discovery of plastic eating symbiotes in two insects gives a good way to introduce this topic. The papers are very complicated, but the implications are pretty simple…some insects may be able to digest plastic with the aid of the bacteria which live in their guts.
It’s a question that’s actually a lot more important than it looks, because Jim is asking about a group of animals that are often overlooked because their effects on the biology of their hosts are not always obvious.
Usually when we write about insects, we write about them as a single organisms. This, however, is not necessarily accurate because any bug you pick up is actually a composite of dozens (maybe hundreds) of organisms. Some of these are commensals, others parasites, but quite a few of them are working with the insects towards a common interest. These are termed symbiotes.
The most common symbiotes are bacteria, but a lot of critters work together in surprising ways. Fungi, other animals…even viruses are all really common symbiotes of insects. Although Jimmy was asking about bacteria specifically, I’m not sure a discussion of insect symbionts would really be complete without talking about these examples.
So let’s discuss what symbiotes do in this post, and in the next post we’ll talk about why they’re important.
What do symbiotes do for the insect?
I think if I were to describe the roles of symbiotes in insect biology, I would very broadly say that they’re involved in either nutrition or protection. Most of the symbiotes I’m familiar with are used to either produce nutrients for the insect, or defend them from some threat the insect can’t fight off itself. As with all rules, there are some which don’t follow either of these…and I’ll be going through a couple examples of that. The relationship between symbiote and host can be extremely complicated. Typically symbiosis is portrayed as a mutually beneficial relationship, but this isn’t always the case.
The examples symbiosis used in most textbooks are the role of bees in pollination, and the role of aphids recruiting ants as bodyguards. They’re also a good example of how these sorts of relationships can be antagonistic. In both of these examples, the symbiotes manipulate their hosts. Or the hosts manipulate their symbionts, depending on whose perspective you take.
Plants cram their sperm cells into pollen, which is transported between plants by bees. The bees eat the pollen, but also receive nectar as a reward. This nectar sometimes contains psychoactive compounds, typically caffeine or nicotine, which can manipulate the behavior of bees. Specifically, these compounds reduce the amount of nectar taken while maximizing the number of pollinators which visit the flowers.
Another example is the ant-aphid mutualism. Ants will tend aphids because the aphids reward them with their sugary poop for protecting them against predators and parasitoids. It works out well for both species, except for the fact that some aphids will actually feed on ant larvae when they’re brought into the nest.
In this post we’ll be talking about mutualisms as mutually beneficial relationships…but they can quite easily grade into parasitism. Many times, the relationship is very complicated.
What sorts of symbionts are there?
There are hundreds, maybe even thousands of symbionts that we know at least a little bit about. Every insect species probably has a completely unique suite of symbiotes, so there are probably millions of different organisms which help the bugs out.
I’m not going to go through every example. Instead, I’m going to go through some of the better characterized symbionts which are studied.
Many animals live on diets that are kind of crappy. We’ve talked about cicadas in the past, but there’s a couple articles which are going around the popular media sphere which are showcasing some rather interesting bacterial symbiotes which appear to degrade plastic.
Plastic is not a normal part of the environment, which makes it difficult to break down. However, nature is very adaptive and will eventually learn to process most things. Other materials (notably nylon and iron) which aren’t a normal part of the environment have had bacteria evolve to use them as a nutrient source.
A series of three papers, involving the grain pests Plodia interpunctella and Tenebrio molitor (Indian mealmoth, and mealworms specifically…the papers don’t use the correct common names) provide good evidence that gut bacteria in insects are able to use plastics as a nutrient source. By feeding these insects on styrafoam, the researchers found that the insects didn’t gain weight normally, but also that they didn’t starve. So the insects can use styrafoam as a nutrient source, but can’t develop on styrafoam alone because it doesn’t contain nutrients like proteins which they also need. Plastic isn’t a normal part of the environment, but it’s pretty similar to stuff that’s found in the environment. The particulars of what’s happening isn’t well understood yet, but it looks like the plastic is shuttled into fatty acids which the insects use for energy.
Aphids and oil companies have a lot of stuff in common, in that they commonly outsource some stuff that most do themselves. For example, the job of preventing heat stroke in aphids is outsourced to a bacteria called Buchnera.
Heat stroke is caused by proteins not folding correctly when they’re made. So your body ends up making useless proteins, which gum up all the systems and makes your body not work very well. To keep this from happening, your body produces a group of proteins called heat shock proteins (HSPs, for short) which help the body fold proteins while they’re being made.
Most animals encode HSPs into their genomes, but aphids have a different system. One of their symbiotes, Buchnera, produces HSPs that work better than the aphid HSPs. So Buchnera helps aphids survive high temperatures.
Although nematodes aren’t insects, they’re pretty closely related. Entomologists also study nematodes, particularly the ones parasitic in insects, so a discussion of nematodes is definitely warranted.
Many nematodes which are parasitic on insects do get help from bacteria. The nematode Heterohabditis gets assistance from a bacteria called Photorhabdus which is very pathogenic. The bacteria is introduced when the nematode pukes into it’s new host, and quickly multiples…killing the insect and any other bacteria present. The nematode then swims through the bacterial soup, eating and digesting the bacteria for food.
Many ants, particularly leafcutter ants, use fungus as a food source. They collect leaves from trees, and use it to grow fungus which they then eat.
The wasp Sirex noctilio, which has devastated millions of acres of woodlands, uses a symbiotic fungus to pull this trick off. The fungus grows inside the tree, overwhelming the tree’s defenses. The fungus eats the tree’s wood, and the wasp eats the fungus. So this symbiote has two functions. First, it helps knock out the tree’s immune system. Second, it converts the tree to food for the growing wasp.
The life of an aphid is ruled by a weird combination of plants, viruses, bacteria, and other insects. They look like little handbags, but they’re really quite odd.
Aphids, like most insects, have a whole bunch of wasps which attack and kill them. They’ve got an immune system, but it’s weird in ways that we won’t get into here. It works, but it’s missing components…and nobody knows what’s up with it.
So they’ve oursourced a chunk of their immune system to a bacterial symbiote called Hamiltonella. Hamiltonella has outsourced this job to a bacteriophage, which produces a toxin that kills the wasp but not the aphid.
Aphids are weird.
Parasitoid wasps have a really tough job because they live in an environment that is actively trying to kill them. Some insects live in water that’s close to boiling, but this environment doesn’t produce cells which seek them out an kill them. So in many ways, parasitoids live in the most inhospitable environment possible.
If you’re living inside another animal, you have a very hefty to-do list. You can’t have your host pupating while you develop. You can’t have your host investing in things which don’t benefit you, like reproductive organs. The immune system has to go, and if that goes you have to keep your host from getting sick. You just need to make things generally comfy.
So to get these things done, many wasps outsource this job to viruses which are injected into the wasp’s host with the eggs. Many Braconid and Ichneumonid wasps have independently evolved viruses called Polydnaviruses which act like gene vectors. They’re passed on from mother to offspring, and never reproduce inside the wasp’s host. Instead, they force the wasp’s host to produce proteins encoded in the wasp’s genome.
The viruses found in different wasps do different things. Many knock out the immune system (although some don’t), and change the behavior of the host so it doesn’t eat and expose itself to pathogens. They castrate the host and arrest it’s development to keep it from investing resources in metabolically expensive processes. In many cases, they also change the behavior of the animal to become extremely aggressive after the wasp pupates. In essence, the virus forces the host to act like a bodyguard to its parasite.
Life as a wasp is tough. Pesticides, parasites, and mind-controlling plants are a hefty list of things you’ve got to worry about. Solitary wasps, unfortunately, can’t rely on their sisters.
Wasps get a little bit of help from mites, though. Like all insects, wasps have parasitoids. A lot of these guys have to break into the nest, and lay their eggs in the wasp‘s larva. The larva can’t really defend itself, so they’re easy targets.
Fortunately, many wasps have tiny bodyguards. These bodyguards hide inside a little structure called an acarinarium on the wasp and pop out when the wasp lays her eggs. Depending on the species they eat the wasp’s hemolymph, or debris in the nest, but don’t cause a whole lot of damage to the wasp.
When the wasp’s nest is attacked by a parasitoid, the mites will gang up on the parasitoid and attack it. Sometimes, they even kill the invader.
The Bottom Line
Insect symbiotes have a lot of neat jobs to do. They nourish and defend their hosts, and even help them bring down their prey. They’re hard to study, and not a lot is known about them. However, it’s becoming more and more apparent that they have a central role in insect biology.
This post is the first part of a two-part post. In this post, I focused mainly on why symbiotes are important to the insect. In the second part, I will talk about why insect symbiotes are important to people.
So stay tuned for part 2!
Yang, Jun, et al. “Evidence of polyethylene biodegradation by bacterial strains from the guts of plastic-eating waxworms.” Environmental science & technology 48.23 (2014): 13776-13784.