Naturally Selected Infections - Symbiotic Bacteria and Evolution
Ben - Also in the news this week, researchers in America have identified a novel mechanism for evolution. In the wild, a gene that produces an organism's chances of surviving increases their odds of reproducing. It's usually passed down to the next generation and so, that gene will become more common in the population. But what if instead of a gene giving an advantage, it's actually an infection with a symbiotic bacteria? Professor John Jaenike is a biologist at Rochester University in New York and he joins us now. Thank you for joining us John, what was it that made you look into this in the first place?
John - Well actually, I'd been studying Drosophila and their interactions with nematode parasites in the 1980s and 90s, and back then there's one particular species, Drosophila neotestacea that was really getting clobbered by nematodes which I've studied quite a bit back then. For the past 10 years or so, I've been studying a different sort of infection which are endosymbiotic bacteria that are passed on from mothers to offspring. We had recently discovered a Spiroplasma bacteria in Drosophila neotestacea, and it didn't seem to be doing anything to enhance its own transmission. So, I wanted to see whether or not maybe they provided some sort of benefit for flies that are parasitized by nematodes and lo and behold, we found in the lab that they had a whopping effect. When we look at this out in the field, it was equally strong or even stronger. So there was more than a tenfold increase in the fertility of female flies if they carried the Spiroplasma.
Ben - So they obviously give an enormous advantage in fertility, but is there a trade off? Do the bacteria affect the flies? Do they need to eat more? Is it harder to fly? There must be something - they must be paying something for that advantage.
John - There probably is some cost. We haven't seen anything though. We're just wrapping up right now a population case experiment to look at the dynamics of the Spiroplasma infection in the absence of nematode parasites. We don't see any obvious fitness cost. Fertility is unaffected, the dynamics of the infection, it just seems to act like a neutral trait in the absence of the nematodes. So there may be a cost but it's not big enough for us to detect. The main problem is that the transmission rate is less than perfect. An infected female passes the Spiroplasma onto about 97% of her offspring. So in the absence of any selective benefit, that actually would be lost from the population very quickly.
Ben - And how have we seen the rates actually changing in the population and where have you got your flies from in order to look back historically and see the relationship?
John - Well, in the 1980s, virtually every single nematode parasitized fly that I collected was completely sterile. Recently, the situation has changed dramatically. Now, the vast majority of parasitized flies have some level of fertility - which is actually astounding. So based on that sort of evidence, I've been able to infer that the infection rate by Spiroplasma increased from about 10% in the 1980s to about 70 or 80% today - this is around Rochester. Also, I was able to get some museum specimens that actually, a former student of mine, Dave Grimaldi and I had collected in the 1980s, and we developed PCR primers to look for Spiroplasma in these museum specimens and it turns out that none of the flies, none of the 20, were infected with Spiroplasma from the 1980s. So, the confidence limits on that are around 0 to 15%, so our best guess is that the infection frequency was around 10 to 15% in the 1980s, and that's increased dramatically in the last 20 years.
Ben - That's definitely a significant change. Is this the only example that we know of, of an infection like this offering a selective advantage?
John - There's actually a handful of cases now that have been published. So there's a very nice example of a bacterium called Hamiltonella which provides aphids resistance against parasitoid wasps. There have been more recent studies of Wolbachia which is another endosymbiotic bacteria, conferring resistance to RNA viruses in Drosophila and in mosquitoes. So there are few examples. The previous studies have all been done in the laboratory so we've actually been able to show that this works out in the wild, and also, we've been able to estimate the relevant parameters that govern the dynamics, and those parameters are consistent with this very rapid increase in the infection in the last 20 years.
Ben - So, how can this tell us a bit about evolution? As I said in the introduction, the mechanism is very similar, but instead of a gene, we have this symbiotic infection. What can we learn about the way that the flies have evolved from this mechanism?
John - Well I think this may be the tip of the iceberg, these few cases that have been found so far. People have been surveying insects now for the last 10 years or so for infection by endosymbionts. There are a number of species of endosymbionts that infect insects and it turns out that the majority of insect species are infected by one or more species of endosymbionts. In the vast majority of cases, we have no idea what they're doing and I wouldn't be surprised if in many cases, the endosymbionts actually provide some kind of protection against some kind of natural enemy that they encounter in the wild. It's exactly analogous to adaptation by spread of a beneficial mutation. So all the standard criteria for evolution by natural selection are met in this case, but it just doesn't involve a gene.
Ben - This brings me on to my final question that you said most insects we think of probably got a relationship like this. Can this tell us anything about tackling bugs that are a problem, those that carry disease?
John - There are a couple of human diseases - river blindness and lymphatic filariasis, also known as elephantiasis. These diseases are caused by nematodes that are carried from person to another by insect vectors, black flies and mosquitoes. So it's occurred to us that if Spiroplasma adversely affects nematodes in these insect vectors, the same way it does in Drosophila, one might be able to use Spiroplasma as a means to actually control the spread of these particular diseases which infect tens of millions of people in tropical regions especially Africa. So it's a potentially novel means of controlling these particular diseases.
Ben - Well this is a wonderful finding. A very elegant paper and it's nice that there's so many different angles to take from it, but thank you ever so much for joining us. That was Professor John Jaenike from Rochester University.