In 2005, Denis Fournier, Arnaud Estoup, Jérôme Orivel, Julien Foucaud, Hervé Jourdan, Julien Le Breton and Laurent Keller published a paper in Nature describing their discovery of the unique reproductive system of the little fire ant (Wasmannia auropunctata): males and queens reproduce clonally, while the workers are produced by normal sexual reproduction. While clonal reproduction in queens has been reported before, from other ant species, this is the first report of male clonal reproduction. When Fournier and colleagues genotyped the sperm in the spermatheca of queens of the little fire ant, he found it to be identical to that of males. Based on this, they proposed that, in fertilised eggs that develop into males, the paternal genome has somehow eliminated the maternal genome after fertilisation. What is particularly interesting about this study is that it wasn’t planned – the authors stumbled upon this discovery during the course of a conventional population genetics investigation.
Eleven years after the paper was published, I spoke to Laurent Keller about the origins of this study, the story behind the discovery and the impact this paper has had on our understanding of reproductive systems in social insects.
Hari Sridhar: You say “While conducting a genetic population study of this species, we discovered a new genetic system in which females and males both reproduce clonally”. What was your motivation to study this species in the first place?
Laurent Keller: We were interested in the little fire ant because it has been introduced in many parts of the world. We wanted to conduct a population genetic study to see from where they were introduced and how they moved, to check if we could track from which population a new population originated. To do this we wanted to look at quite a few populations worldwide.
HS: When did you first realise that there was something unusual in the genetics of this species?
LK: We obtained very unusual microsatellite data for the first population we looked at. There were fixed genotypes which were not in Hardy-Weinberg distribution. That’s when we realised that something was special.
HS: Who did the genetic analysis?
LK: Denis Fournier, first author of the paper, who was a post-doc. at that time.
HS: When you noticed this unusual pattern in the data, can you give us a sense of the discussions that happened among the authors? What did you make of it? What were the explanations you considered?
LK: That was a bit strange – initially, we had some discussions, but we didn’t know what to do. The people I was working with, in the university in southern France – Fournier was working there at that time –, thought that maybe the pattern could be explained by one male reproducing with all the females in the population. Because I know the biology of ants I knew this was not possible, but I didn’t have an alternate explanation. So I took all the genotypic data and spent a few hours to think about what could be the possible mechanism. Then I went there, and we had a meeting, and I talked about my idea that males may be reproducing clonally. That’s when we decided to test this further, by genotyping the sperm in the spermatheca of queens and comparing it to the genotype of the males.
HS: The paper has seven authors. Can you tell me how this group together?
LK: Arnaud Estoup and I started the project. Arnaud wrote up the proposal and it was funded by a granting agency in France. Arnaud was in charge of organising the collection of all the samples and the lab work and I mainly dealt with understanding the data that emerged from the study. Denis Fournier did most of the collection and all the lab work. He was a post-doc. co-supervised by Arnaud and me. The others on the paper, I am not really sure now. Some helped with the collection of samples.
HS: In the paper you also use some data from New Caledonia – was that collected for a different project?
LK: No, that was collected for the same project. We couldn’t get enough males and queens from the French Guiana site. That’s why we also included information from another site.
HS: Where did all the lab work happen?
LK: In Arnaud Estoup’s lab in France.
HS: Did this paper have a smooth ride through peer-review?
LK: It went through quite easily. The reviewers were all very positive. They made a few good suggestions which we followed and it went quite smoothly. I think it was only a single revision.
HS: In the paper you say that this is the first report of clonal reproduction by males in ants, and probably only the second report in any animal. Have other cases been discovered since the paper was published?
LK: Yes, there have been two more species – two other ant species -which have exactly the same mode of reproduction. One was discovered just after our study actually. That’s an interesting story. There was a Japanese scientist who already had data that was quite interesting – he had microsatellite data of queens, males and workers of an ant species called Vollenhovia emeyri, in which the males never had alleles of the mother. He sent me a manuscript in which he had suggested this pattern arose because every time males were produced there were mutations at those microsatellites. I felt that that was not very likely, and told him about our own similar findings and my explanation for it. I asked him to send me the data, and after looking at it, I told him that he most likely had the same type of social organisation that we discovered. I sent him a copy of our manuscript and asked him to do a few more analyses to confirm what he had, which he did and then published his study.
The other case was also interesting. I saw a poster of a master’s student reporting data on 12 microsatellites for the longhorn crazy ant (Paratrechina longicornis). She told me her Master’s project had failed because none of her microsatellites worked – she got very different genotypes for males, workers and queens. I told her that maybe it’s interesting and asked her to send me the data. After looking at it, I told her that her species most likely had the same type of reproduction we had just uncovered. I discussed with her and her boss and proposed that they continue the study. However, the student finally decided not to continue in science and so we hired a post-doc. – Morgan Pearcy – to continue the work. He collected more data, confirmed that it was the same mode of reproduction as ours, and we published that together.
HS: Do you think this finding opened up a new line of research – a new way of thinking about reproduction in social insects?
LK: Yes, I think people are much more open-minded now, to the idea that there may be a lot more diversity in reproductive systems in social insects, than was thought before. My guess is that maybe as many as 10-20% of ants, and probable many other social insects, show unusual mode of reproduction. A similar case has also been discovered in termites.
HS: Has there been more research on the little fire ant after this paper?
LK: Yes, the group from France has done more, to see if they can find a population where there is sexual reproduction; which they did find. They also conducted several other studies on the biology of this ant later on.
HS: But has the main finding from this study – clonal reproduction of males and queens –been contradicted by later studies?
LK: No. In fact, it has been confirmed by later work.
HS: In the 11 years since this paper was published, have you ever read the paper again?
LK: Not really, except, occasionally, when we need to cite it somewhere else. Then, I check it to see exactly what we said.
HS: Did the paper create a buzz when it was published?
LK: There was some stuff but I didn’t follow it too much. There was a “News and Views” piece in Nature, and many other journals mentioned it – Current Biology, Nature Genetics maybe. I don’t remember exactly which one, but there were quite a few which mentioned our study.
HS: When you did the work, did you anticipate that it would attract a lot of attention?
LK: Yes, we did think that people will be interested by this.
HS: Did this paper have an impact on your future research?
LK: It’s difficult to say, I was already interested in unusual modes of reproduction at this time, and so it, probably, just strengthened my interest in this type of research.
HS: What would you say to a student or researcher who is about to read this paper today? What should he or she take away from it?
LK: First, of course, the data itself. But also that, when you do a study, you should be open-minded to strange results. As I mentioned earlier, the Japanese scientist also had similar data to ours, but because he never thought that a son could have no genes from the mother, he had to come up with some strange and unlikely mechanism to account for the data. So, I think what’s really important is to be open-minded enough to expect strange results. Then, to think carefully about what it could mean. And finally to take into account not only what’s known, but also what’s unknown.