Low Society

It’s not plant or animal—or bacteria or fungus, for that matter—but it’s living. Take a walk outside, and it’s all around you. The cellular slime mold Dictyostelium discoideum is a social amoeba belonging to the neglected sixth kingdom of life, the Mycetozoa. In the labs of Rice University’s Department of Ecology and Evolutionary Biology, this unlikely little creature is fast becoming to social and evolutionary biology what the fruit fly has long been to developmental biology—a model organism with which scientists can pry open nature’s secrets.

Richard Gomer, professor of biochemistry, monitors the activity of Dictyostelium discoideum.

Kingdom Mycetozoa is perched in the tree of life somewhere between the Animalia/Fungi group and plants, although it has long been assumed to be more closely related to toadstools than toads. Recent genetic sequencing of Dictyostelium in England and Germany and at Northwestern University and Baylor College of Medicine show, however, that while slime mold is no animal, it shares a greater number of genes with animals than it does with fungi like yeast.

This genetic overlap with ourselves, coupled with slime mold’s relatively primitive simplicity, makes it an attractive model organism for studying the molecular processes that build us all. It is ideal for discovering the genetic mechanisms in complex life forms like us, it turns out, because we’ve been borrowing pages from the slime mold playbook from the beginning.

Take cell migration. Long before the first animals wriggled through primordial seas, hungry slime mold were using amoebic mobility to hunt down bacteria. And that process continues in higher forms of life. “As you’re reading this, immune system cells called neutrophils in your lungs are ‘smelling’ bacteria that you’ve inhaled and are crawling toward them to kill them,” says Richard Gomer, professor of biochemistry and cell biology and Howard Hughes Medical Institute investigator. “This same sort of movement also is used extensively during embryonic development to get cells from one place to another to build structures.” By studying how slime mold cells move, biologists will be able to learn how our own cells migrate during development. This is important territory since subtle errors in embryonic cell migration are believed to contribute to neurodevelopmental diseases like schizophrenia.

The Single Slime Mold
When all is well, slime mold cells are invisibly dispersed throughout damp forest soils, hunting hapless bacteria and proliferating, like their prey, via simple cell division. “Solitary cells are highly agile predators that live in the soil all around us,” explains Kevin Foster, Huxley Research Instructor in the Department of Ecology and Evolutionary Biology. “They spend their lives in pursuit of bacterial quarry.”

But slime molds aren’t quite so simply single. “As they feed,” Foster says, “they multiply until they have depleted their food and begin to starve, which starts them on their developmental transformation into a multicellular organism.” As starvation looms, hungry amoeba send out a chemical alarm. They converge by the hundreds of thousands into streaming, saffron-yellow-hued aggregations and stick together to form multicellular migratory blobs, called slugs, that move as a single super-
organism.

Once a suitably sunny spot is found, a brutal division of labor ensues as the slug is transformed into a spore-launching pad called the fruiting body. The cells up front—the leading 20 percent of the slug—sacrifice themselves, piling upon one another to become a reproductive scaffolding called a stalk. The lucky majority will perch atop the stalk at the dizzying height of two millimeters and become dispersing spores—wayfarers poised to catch a ride to better hunting grounds, either on the breeze or on soil invertebrates.

“ Altruistic stalk cells give up on reproduction in order to benefit the spore cells by lifting them above the hazards of the soil or increasing their chances of dispersal to a more favorable environment,” explains Joan Strassmann, professor of ecology and evolutionary biology.

Intriguingly, this carnage does not look like a crudely Darwinian, tooth-and-claw scramble for dispersal status. Instead, the process appears to be intricately choreographed, with a fifth of the migrants altruistically dying that the others might sail away and survive. The architecture of fruiting bodies is surprisingly consistent, says David Queller, also a professor of ecology and evolutionary biology. “The overall size may change, but the shape is pretty uniform.”

The process is undeniably social—the coming together of interacting individuals—but it also bears remarkable similarities to precisely orchestrated embryonic development, in which cells follow a genetic program of differentiation and culling, producing specialized tissues and structures. This overlap of social and developmental processes makes slime mold a useful window into the evolutionary rules of cooperation, as well as the molecular rules of development.

Waste Not, Want Not
The chemicals that regulate slime mold aggregation and the formation of special structures probably developed from tools previously used for foraging by their solitary-celled ancestors. The complex molecule that controls aggregation size, for example, is partly composed of lysozymes that originally served as chemical claws for breaching the protective walls of bacteria cells. The call that triggers aggregation is a chemical called cyclic AMP (cAMP), which remains a cellular signal used by our own cells during development and metabolism.

Metazoans—multicellular creatures like us that develop from a single fertilized egg cell—have in turn co-opted many of these molecules for yet other uses. Those bacteria-skinning lysozymes that slime mold used first for hunting and later to help regulate slug growth, for instance, are an important natural antibiotic in our own blood and tears. “Evolution does not reinvent things from scratch,” explains Strassmann, “It builds on what has been selected before.”

Some scientists, Gomer included, suspect that metazoans themselves evolved from a slime mold or a slime mold-like creature. Strassmann disagrees. “It’s very unlikely,” she says, “because metazoans go though a single cell en route to multicellularity, and slime molds don’t.” And wondering whether metazoans have recycled molecular tools available to slime mold raises another fundamental issue. “Social amoebae appear to possess most of the kinds of molecular mechanisms required to evolve more complex forms of multicellularity,” says Strassmann, “but they haven’t done so.”

In the same vein, although slime mold was aggregating—“going multicellular”—long before the first metazoan arrived on the Darwinian stage, it has stuck with its parlor trick of building simple stalks and spores, rather than going on to the spectacular structural diversity seen among metazoans like plants and animals. “Slime molds are interesting,” says Queller, “because it’s the road not taken. Sometimes you learn about one system by studying its alternatives.”

Queller and Strassmann believe that some of what they learn by studying slime mold may get to the heart not only of the evolution of multicellularity but of cooperation and even altruism.

All in the Family
Because all of the cells in metazoan individuals descend from that single fertilized egg, they are clones—100 percent related to one another. This means that a liver cell, for example, does not need to compete with sperm or ova to survive. So long as they function properly long enough for ova to become fertilized, liver cells can rest assured that copies of their genes will survive into the next generation, albeit indirectly.

So complete is this confluence of self-interests among your body’s cells, that a cell suffering irreparable damage will commit a programmed bit of hara-kiri called “apoptosis,” to avoid harming the whole. When slime mold stalk cells are closely related to spore cells, their selflessness is akin to apoptosis. But there’s a catch: Slime mold’s multicellular aggregations are not always family reunions.

The Rules of Attraction
In the wild, unrelated strains or lineages of slime mold co-inhabit the same soils, and most readily aggregate with one another, forming genetic “chimeras”—named for the mythical fire-breathing beast of old that sported a serpent for a tail and the heads of both lion and goat. Cooperation between genetically identical cells is one thing, but for evolutionary biologists like Strassmann and Queller, altruism amongst strangers is a much more interesting beast. “Slime molds may be more analogous to the societies of social insects,” reasons Strassmann, “which have sterile castes but also have genetic conflicts of interests.”

Suspecting that natural selection could favor “cheaters that contribute less than their fair share to the sterile stalk,” Strassmann and Queller joined forces a few years ago with colleague Yong Zhu, a graduate student in ecology and evolutionary biology, to study whether seemingly harmonious slime mold societies are in fact shaped by conflict as well as by cooperation.

The researchers collected over two-dozen strains of D. discoideum from the forests of North Carolina and mixed them together. Pairing up different strains into 12 chimeric mixtures of two strains each, the team used DNA fingerprinting techniques to identify each strain’s relative contributions to the sections of migrating slugs destined to become stalk and spore cells, respectively.

In fully half of the mixtures, one strain played the other for chumps, contributing fewer cells to stalk formation and selfishly grabbing a disproportionate share of the spores. The Strassmann and Queller team had discovered one of the most primitive examples of social cheating known to science. Not all strains cheated, and those that did cheated to varying degrees. But one freeloading strain contributed nearly none of its cells to stalk formation.

The team’s results offer the first proof that slime mold aggregations can be societies of competing individuals with distinct genetic self-interests rather than paragons of selflessness.

On Your Mark
To see how internal conflicts might affect an entire slime mold society, Foster teamed up with Strassmann, Queller, and Angelo Fortunato, a graduate student in ecology and evolutionary biology, to see how well migrating chimeras perform compared to slugs made up of closely related cells.

Professors of ecology and evolutionary biology David Queller and Joan Strassmann

Slime molds were housed in petri dishes, either alone or in mixtures of two, five, or 10 strains mixed together to yield aggregations with equal total numbers of cells. The petri dishes were blacked out except for a tiny slit on one side that served as a light source—the “sunny spot” wild aggregations would seek out on the forest floor. After a week, all of the slugs had stopped migrating and had built fruiting bodies; however, the clonal slugs had migrated nearly twice as far as had chimeras of the same size.

What’s more, chimeras made up of two strains outperformed those with five or 10 strains, suggesting that the more interacting members of a chimera, the higher the costs to the society at large. This could be evidence of fierce co-evolutionary arms races between cheating strategies and countermeasures. Alternatively, chimeric aggregations could be cellular Towers of Babel, with incompatibilities in different strains and molecular “dialects” complicating cooperation. Foster and Strassmann suspect that active conflict is to blame, and Foster is investigating whether unrelated cells poison or even eat one another within migration aggregations.

Whatever the case, it appeared chimeras are at a distinct disadvantage compared to migrating slugs made up of closely related cells. So why would different strains ever join together? They could avoid it if they wished to; Queller and Strassmann discovered earlier this year that slime mold can use their adhesion molecules to recognize relatives.

There’s clearly an advantage, or chimerism would be rare. The researchers have an inkling of what that advantage may be: size. In nature, chimeras are larger than single-strain slugs. In their lab experiment, the team purposefully kept all slugs the same size. But in nature, two strains with equal cell numbers joining up would double the number of cells in the resulting chimera, compared to aggregations of either strain alone. Perhaps, Foster hypothesized, in the case of chimeras, natural size advantage overcomes the costs of cooperation between nonrelatives.

They repeated their experiment, without artificially controlling the total number of cells. Instead, they simply made sure that the number of cells from each strain was equal. Lo and behold, under these more natural circumstances, the chimeras outperformed the clonal migrations. “The benefits of size strongly outweigh the intrinsic costs of chimerism,” concludes Foster. Despite the backstabbing intrigue—or mutual incomprehension—of life in mixed slime mold societies, it appears that alliances with nonrelatives still pay.

The take-home message, according to Foster, is that collaboration by unrelated cells is costly business. Strassmann suspects that active conflict within the developing structures is responsible for that cost, and that such costs are what made slime mold an evolutionary cul-de-sac, while metazoan species like us, which develop from a single egg, have evolved spectacular structural diversity. Conflict between cells within a tissue or between tissues would disfavor structural specialization, Strassmann believes. “You may not be able to get complex morphological differentiation along with competition,” she says.

“ The cost to chimerism is confirmation of the general rule that relatedness is central to sociality,” Foster says. “The presence of a cost helps us understand why the vast majority of multicellular organisms, including humans, are clonal.”

In chimeras, less-related cells are less able to cooperate, Foster notes. The benefits of large slug size, however, show that sometimes the ecological benefits of cooperation with genetic strangers can override the costs. “Both relatedness and ecological benefits are also important in social insects and vertebrates,” says Foster, “which suggests that there are general rules of social evolution.”

Future studies of slime mold are likely not only to provide new insights into sociality, evolution, development, and molecular biology but to foster a new interdisciplinary synergy between these fields. “The real advances in understanding,” Strassmann says, “will come from interdisciplinary work using evolutionary theory and social theory in a molecular framework.”

“ Sociobiology has largely been practiced in the absence of knowledge of the underlying genes,” adds Queller. That’s about to change. He and Strassmann have received funding from the National Science Foundation that will allow them to team with Baylor researchers and, together, hunt down the specific genes underlying slime mold strains’ cooperative and cheating behaviors—with an eye toward learning, among other things, how evolution limits the proliferation of cheating genes.

By Bryant Furlow
Photography by Tommy LaVergne
Microphotography by Kevin Foster
and Richard Gomer