What exactly is altruism? By definition, altruists behave selflessly in the interest of others. The ability to set aside all self-interest may seem like a distinctly human phenomenon, wrapped up more with ideas of morality or philosophy than biology. Yet professors David Queller, the Spencer T. Olin Professor of Biology, and Joan Strassmann, professor of biology, study the presence and function of altruism even at the microscopic level of single-celled amoebae.
According to Strassmann and Queller, altruism is one of a suite of behaviors that have evolved in Dictyostelium discoideum. The altruistic actions of these single-celled creatures are dramatic; when threatened by starvation, large numbers of amoebae will die so others have a better chance at survival.
The sacrificial behavior of D. discoideum prompts a range of evolutionary conundrums. How do genes responsible for altruism get passed on from generation to generation, if the amoebae that carry that gene die for the benefit of others? How do ‘cheater genes’—genes that make an amoeba more likely to survive than sacrifice itself—not rampantly spread through natural selection? Queller and Strassmann have spent years investigating these and related questions.
Though scientists refer to D. discoideum as ‘social amoebae,’ like many organisms, their social behaviors only emerge in times of need. The majority of their lives, these amoebae are “just sort of like what people think of as amoebae,” Queller explains. They consume bacteria in the soil, and when they get big enough, they divide into two. The two clones then go their separate, bacteria-eating ways. But when an amoeba exhausts its nearby supply of food, the hungry cell emits chemical signals to others in the surrounding area. In essence, the signal says: “Hey, I’m starving, let’s get together,” according to Queller.
The amoebae answer this distress call by aggregating to form a group referred to as a ‘slug.’ The millimeter-long slug, which might include some ten to a hundred thousand individual cells, makes its way toward the surface of the soil as if it were a single organism. There the slug begins its transformation into a ‘fruiting body.’
About twenty percent of the total group produces stiff cellulose, together creating a miniscule stalk rising out of the dirt—killing themselves in the process. The surviving amoebae then slide up through the hollow stalk, forming a round translucent globule at the top. There they undertake their own transformation into spores and wait to be transferred to a new home, perhaps by a passing insect, or the splash of a raindrop.
In order to give the majority of the group a chance to move on to greener (or at least more bacteria-rich) pastures, the amoebae within the stalk perform what appears to be the ultimate evolutionary sacrifice—dying so that others might live.
The role of relatedness
The idea of relatedness is important to understanding cooperation and altruism in social amoebae, because unlike within a multicellular organism like a human being, the cells that aggregate to form a slug and fruiting body can carry different genes. As Strassmann notes, “You are made up of a whole bunch of cells all cooperating to allow you to survive and reproduce. You don’t have a lot of conflict within your body because the cells are all genetically identical. Your liver isn’t fighting to get into your gonads; it’s trying to be the best liver that your drinking habits will let it be.”
It makes evolutionary sense for an amoeba to cooperate with—and even sacrifice itself for—a clonemate or a relative, because clonemates or relatives share genes and will presumably live on to spread those genes. But with genetic difference arises the possibility for competition and cheating.
In order to better understand the role of relatedness in the social behaviors of D. discoideum, Strassmann and Queller collected fruiting bodies from the wild. At the time, this was an unusual and innovative move in a scientific community that focuses nearly exclusively on a single established lab strain. After successfully finding the miniscule spores and stalks in a forest in Virginia—something that Strassmann recalls being told “would be impossible to do”—the team discovered that fruiting bodies in nature have a very high level of relatedness, higher than full sisters. This led them back to the lab to figure out how, and why.
Among their most exciting finds in this area, Queller and Strassmann discovered that even unicellular organisms have methods of recognizing their kin. “It’s not a perfect system by any means, but it’s part of the answer, and it’s a very cool to think that amoebae can discriminate levels of kinship at all,” Queller explains.
When possible, these social amoebae tend to stick with their own. But there are some advantages to grouping with genetically different cells. For example, it’s better to form a large slug by joining with non-relatives than to form a smaller slug with kin alone. “It’s good to be in a bigger aggregation because a bigger slug can move farther and make a taller stalk,” according to Strassmann.
Over generations, genes mutate, and Queller and Strassmann have found that some genetic mutations increase an amoeba’s chance of survival in a mixed grouping. When allowed to evolve under low-relatedness conditions in the lab, the mutants with so–called ‘cheater genes’ spread through natural selection. These cells take advantage of patterns that in part determine which amoebae in a slug will behave altruistically and which will live on as spore.
One such pattern concerns placement within a slug. The front section of the slug differentiates as stalk, so finding ways to slip to the back of the group increases an amoeba’s chance for survival. Those that first send starvation signals to the group also survive. “Our interpretation of that, though we need additional evidence, is that the first ones to start into the process are turning on their competitive strategies first—whatever genes are good at making them possible cheaters or resisting against other possible cheaters. They’re getting a head start in the process,” Queller notes.
High relatedness is one method of controlling potential cheaters, but Queller and Strassmann theorize about other strategies as well. For example, a single gene can be responsible for more than one trait. In one experiment, Queller and Strassmann reduced amoebae’s ability to adhere together by knocking out the gene csaA. In the lab, these were the cheaters that avoided sticking to the front of a slug and becoming stalk. But in nature, cells without csaA lose their ability aggregate at all. Rather than gaming the system, they end up left out of it entirely.
The big message
Strassmann and Queller are reluctant to transfer any lessons from single-celled amoebae to the messy, cultural world of human behavior. But Queller recognizes at least an analogous relationship between the interactions of altruists and cheaters in the very different lives of D. discoideum and humans.
“When you do something cooperative, whether it’s altruism or just two individuals coming together and producing some public good, that good they’re producing is something that might be exploited by somebody else that’s not really part of the game,” Queller explains. “And if the only difference is that you benefit but the altruist pays the cost, the ones who are not paying the cost are going to do better and spread through natural selection.”
Yet despite these possibilities for cheating the system, evolution has determined that cooperative behaviors live on in the world of D. discoideum. Social amoebae continue to eat bacteria, divide into more amoebae, and, in some cases, die so that others may do the same. In the infinitely larger and more complex human world, a similar balance between competition and cooperation plays out on an individual and global scale.
“That’s one of our big messages,” Strassmann notes. “Any time you cooperate with others there are costs that need to be controlled, but there are also benefits. And that’s why it all happens.”
- In addition to her editorial role, Claire Navarro produces and hosts the A&S podcast series Hold That Thought.
All images courtesy of Alex Wild Photography.