Revisiting Strassmann et al. 2000

 In a paper published in Nature in 2000, Joan Strassmann, Yong Zhu and David Queller showed, using a mixing experiment and analysing microsatellite markers, that different clones of Dictyostelium discoideum from a field population readily form multi-clonal slugs and that approximately half of these slugs are “not fair”, i.e. one clone produces more spore cells and exploits the other clone’s stalk cells. Based on these results, Strassmann and colleagues questioned the use of D. discoideum as a model system for  studying multicellular development, and instead suggested that it could be a useful model to understand social evolution. Sixteen years after the paper was published I asked Joan Strassmann about how she got interested in social amoeba, the making of this study and what we have learnt since about the social life of D. discoideum.

Citation: Strassmann, J. E., Zhu, Y., & Queller, D. C. (2000). Altruism and social cheating in the social amoeba Dictyostelium discoideum. Nature, 408(6815), 965.

Date of interview: Questions sent by email on 29th November 2016; responses received by email on 9th February 2017.

 

Hari Sridhar: I would like to start by asking you a about your motivation to do the work presented in this paper. By looking at your publication profile, I came to know that this is your first paper on social amoeba. Could you share with us why you decided to start working on this new system at this point in your career?

 Joan Strassmann: One of the challenges in science is to find new systems, where your fondest theories might be proven wrong. After all, if we were always right, we would not have to actually do experimental work. One of the things many people don’t realize is how often we are wrong. In fact, it is only by being wrong that we can discover new theories – new ways – that explain what is going on. The social amoebae were tantalizing to any student of social evolution since they form what looks like a real body from an aggregation. Some in this body die to help others. How does this evolve? Are they clone-mates? Do they have a system that makes it fair, with each participating clone contributing an equal amount to spore and stalk?

This might seem like an obscure system but in fact it is not. John Bonner‘s popular books brought this system to many. The real question is what made us go ahead when so many others studying social insects did not. The answers to this would take a lot of time, but here is a summary. First, David Queller and I work together. We like big questions. We are not afraid of taking risks. We decided to explore this system and had a grad student who needed another project. But what really made it happen was a very collegial Dictyostelium community. I started posting questions on the Dictybase listserv and got answers. The most important answer was that Dennis Welker offered us a collection of wild clones with which we could work. We got them, got more help from the community to grow them, and away we went!

 

 HS: Stepping back a bit, could you tell us how you got interested in the topic of social evolution?

JS: I was an undergraduate in the early 1970s, at a time when sociobiology was blooming. Hamilton‘s kin selection papers were becoming widely noticed. Robert Trivers was busy extending them in various ways. But most important of all was I had a wonderful undergraduate mentor passionate about social evolution and about taking time with undergraduates, Richard D. Alexander. I owe that early passion for the subject to him. I hope I have extended my interests, but what I most love is about sociality, trade-offs, costs, and benefits, relatedness, and conflicts of interest. I still visit him at his farm regularly.

 

 HS: This paper has three authors. How did this group come together and what did each person contribute to this paper?

JS: David Queller and I are a life team. We are married and discuss everything, day and night. We started this project. We had known about the system and its potential for so long, we finally decided to take the plunge. I remember thinking we would feel like fools if someone else initiated something we had thought about for so long. Yong Zhu was a graduate student who had already done a couple of excellent projects and was looking for a third to round out his graduate career and this was it. So we were already together as a group and working on wasps. Yong is now a professor at Yale University.

 

 HS: Was there a reason why you chose the type locality to collect clonal isolates? You say you also used the original type clone NC4 – where is this type housed currently? 

JS: We were given these clones collected at the type location by John Eisenberg. As I said, we got them from Dennis Welker. We have since collected over 700 clones of Dictyostelium discoideum, but at the time we had no idea how to go about this. So, the answer is happenstance for the location. The original type clone NC4 and genetic modifications that can be grown in liquid can be obtained from the Dictybase stock center.

 

HS: Could you tell us what this area – Little Butts Gap, North Carolina – is like? Do you continue to work in this type locality? When was the last time you visited this place? Has it changed in any way in the last 16 years?

JS: We have visited the general area, not too far from Ashville, North Carolina, a number of times. Little Butt’s Gap is off the Blue Ridge Road, though we accessed it from the back, from another road. We did not initially visit it, as I said above. We did visit the exact spot, as far as we could tell, a few years ago during the Dictyostelium meetings in 2013. Most of our field work in that general area has been a few hours northwest, at Mountain Lake Biological Station of the University of Virginia. The area is a really rich, diverse, wet forest of pines, oaks, beeches, magnolias, birches and other trees. D. discoideum seems most common high up. We have also worked collecting in Texas and Missouri and several other states. We even have a clone or two from Japan. The distribution is eastern USA into the neotropics, and eastern Asia.

 

 HS: Do you continue to use descendants of the clonal isolates you collected for this study? How did you transport them when you moved universities?

JS: Yes, we use the original clones and the ones in the first study. We are careful not to let them evolve in the lab. We keep them as spores in a glycerine solution in an ultra cold -80 degrees F freezer. When we moved universities, the freezers went on a special truck and were plugged in. They even had an empty back up freezer in case ours broke. The truck was so expensive; we put all our samples in it, so we were nervous until they arrived in St. Louis where Washington University received our material.

 

 HS: Would you remember how long it took you to do these experiments? Since this was the first time you were working on this system, how difficult was this to do?

JS: We spent a couple of years doing the experiments. Every step was hard, both for us since we had not worked with microbes before, and generally, since others had not done this sort of analysis before. How to grow them, how to mix them, how to assess which clone was where were all things we had to work out. It was very challenging but also exciting.

 

HS: Do you remember how long it took you to write the paper, and when and where you did most of the writing?

 JS: The paper took a couple of months to write. We wrote it mostly in our offices and it went back and forth among the three of us a lot. One of the referees wanted more work so that required a new experiment and more work and more writing.

 

HS: If you don’t mind, I would like to go over the names of people you acknowledge to find out a little more about who they were, how you knew them and how they helped:

JS: Richard Gomer works on Dictyostelium and at the time was right upstairs from us at Rice University. He was very helpful in answering tons of Dicty culturing and biology questions.

Dennis Welker provided the wild collected clones. Who knows if or when we would even have switched to this system.

June Keay was an undergrad who helped in various ways in the lab. In particular she helped with the assay with determining the prespore/stalk divide in slugs.

Wendy Castle was the lab technician at the time and helped in all the general ways of keeping things running.

Simha Reddy was an undergrad in the lab.

Jesse Damon was an undergrad in the lab.

Danny Rozen gave helpful comments on the project at a number of times.

Rich Kessin is a Dicty expert who has been a tireless supporter, always willing to take time out to help us in various ways, usually over the phone with questions about the system.

Greg [Velicer] was a referee of the paper. He was very helpful though exacting and had us do some extra experiments.

John Bonner was helpful with a few important questions.

 

HS: Did this paper have a relatively smooth ride through peer-review? Was Nature the first place you submitted this to?

JS: We first sent it to Science where it was rejected, probably without review. At Nature, one referee wanted a lot of extra work. It was challenging.

 

HS: Do you remember how this paper was received when it was published? Did it attract a lot of attention and discussion?

JS: I don’t particularly remember, but it did get some attention.

 

HS: This paper has been cited over 300 times. At the time you did the study did you have any inkling how important it would turn out to be? Do you have a sense of what the paper mostly gets cited for?

JS: Yes, we figured this would be viewed as an important paper. We expected it would be cited by students of social behavior as an example of microbial sociality and altruism. What we could not predict is that the zeitgeist of the times meant that suddenly many people were studying microbial sociality in protists, in bacteria, in viruses, so this study fit right in with a trend, something that has not particularly happened before in my research. Wasps never were all that popular, even among social insect biologists.

 

HS: Did this paper have any kind of direct impact on your career? In what way did this paper influence the future course of your research?

JS: This paper completely changed our research. It was the first one switching us, our group, and our interests, from social wasps and stingless bees to microbes. No other paper has had a larger impact on my career.

 

HS: Today, 16 years after it was published, would you say the main conclusions still hold true, more-or-less?

JS: Yes, the conclusions have been verified many times.

 

HS: If you were to redo these experiments today, would you change anything about them?

JS: We redo aspects of these experiments all the time, as do others. The phosphorimager technique for measuring mixtures was fairly crude and involved radiation, so we are glad we don’t have to use it any more. We do things differently according to what we want to show.

 

HS: You say “The ability to cheat appears to be common in this natural population, something that has not been demonstrated for other examples of developmental cheating in a social bacterium, a colonial ascidian, and Dictyostelium. Most previous examples come from laboratory mutants whose significance in natural populations is unknown. In examples from natural populations, the incidence of cheating appears low….How cheating is maintained at such high levels as D. discoideum is not yet known. Factors that may be important are the frequency of between-clone encounters in the field, whether a clone’s ability to cheat is general or specific to certain victim clones, and costs of being a cheater.”

Today, do we have more examples of cheating in natural populations in other social species? And, do we know more about why cheating is high in natural populations of D. discoideum?

JS: Whether cheating is high or low depends on population structure. It is clear that there are genes for avoiding being cheated and cheating, so this has a long evolutionary history in the species. What cheating is can be complicated. Melanie Ghoul wrote a review on it. How prevalent it is in natural populations is a topic too long for here.

 

HS: You say “The prevalence of cheating in D. discoideum complicates its use as a model system for development of organisms like vertebrates, which are clonal and essentially free from such conflicts”

Did this study have an impact on the use of D. discoideum as a model system for vertebrate development?

JS: I could not really answer that. I would say that Dicty as a model for development has declined, mostly because newer techniques make animals easier, particularly worms like Caenorhabditis elegans and even zebra fish. There are other things Dicty is really good for, like chemo taxis, actin-based movement, and phagocytosis.

 

HS: You say that D. discoideum is “an invaluable model system for evolutionary and mechanistic studies of altruism and cheating” because of its short generation time, the imminent completion of the genome sequence, and decades of careful work on its cell biology and molecular biology”. Today, 16 years later, could you reflect on how popular social amoebae have become as model systems for social evolution?

JS: We should not take full credit for the emergence of this system as one for social evolution and mutualism, but we get some of the credit. There are now maybe ten groups working on this system from this perspective. This is incredibly gratifying. New discoveries are being made by others all the time. The system has proven worthy of its early promise.

 

HS: In the 16 years since this paper was published, have you ever read it again? If yes, in what context? What strikes you most about it when you read it now?

JS: Yes, I read it when I assign it to students. I read it when I give talks based on it and how we know what we know in science. I read it thoroughly before preparing the talk I gave in Bangalore, India: Validating laboratory experiments on microbial social behavior: evidence from genomics and field work. National Centre for Biological Sciences, Bangalore India, 19 October 2016.

What strikes me most is that we did it right. We used methods that worked well at the time. We started with spores and compared relative abundance in spore and stalk of two different clones. Thus we avoided all the challenges of having healthy cells (not spores) in the same stages. It also strikes me that nearly all the methods have changed since then.

 

HS: Would you count this as one of your favourites, among all the papers you have published?

JS: Yes, because it started us on a whole new track that has taken over our lab now. It has been really fun.

 

HS: What would you say to a student who is about to read this paper today? What should he or she take away from it? Would you add any caveats?

JS: The paper is a good one. It might be fun for a new student to see how our approach to the system has changed, what others have found, and to think about what they might do. We used a phosphorimager to measure relative concentration of the two clones, for example. We don’t use that now. What are the pros and cons of different measurement systems?

 

 

 

 

 

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