Bruno in Columbus via wikimedia commons. Dictyostelium Discoideum form stems and fruits to reproduce during bad times.

Amoebas may be some of the simplest living things on Earth, but their interactions are complex enough to serve as models of far more widespread issues. A new study investigates why some amoebas cooperate, while others look after themselves to the detriment of the species.

Social species exist because when everyone works together the overall outcome is usually enhanced. Nevertheless, it is often to the benefit of a single individual to “cheat” on the collective. An individual who fails to warn the mob of a predator gains a benefit at the expense of those acting in the common interest, leading biologists to ponder the question of what prevents cheaters from always winning the evolutionary race.

Amoebas make easier study subjects than wild animals. Dictyostelium Discoideum normally live singly in soil, feeding on bacteria. During times of crisis, however, tens of thousands clump together to form a slime mold “slug” made up of a stalk and “fruit.” The fruit releases spores that, when better conditions return, become new amoebas, while the amoebas of the stalk sacrifice themselves.

A paper in Current Biology argues, “If successful exploitation of social partners during cooperative interactions increases relative fitness, then we expect selection to lead to the emergence of a single optimal winning strategy in which individuals maximize their gain from cooperation while minimizing their associated costs.”

Professor Chris Thompson of the University of Manchester notes that to make the fruiting body, the usually solitary amoebas need to cooperate. However, all amoebas do not benefit equally. Under the “selfish gene” hypothesis, it makes sense to make the sacrifice for their genetic twins, but the slugs usually contain a genetic mixture, with some genotypes over-represented in the fruit. Moreover, those that are concentrated in the fruit have been shown to produce more spores, raising the question of why these traits have not come to dominate.

Thompson has spent years trying to explain how “altruistic” amoebas that tend to be consigned to the stalk, and produce fewer spores, survive in the presence of cheats. "If the cheats always win, then according to Darwin, altruism shouldn't exist,” Thompson says. “To study this we looked at why the single fittest strategy in the amoeba community doesn't dominate."

He argues that the genes that regulate cooperative behavior provide the answer, “since some genetic changes might result in gains that are offset by other fitness costs.”

Credit: Hideshi via Wikimedia Commons. The lifecycle of Dictyostelium Discoideum.

The paper reveals flaws in previous measures of success. While some amoebas produce more spores, others produce fewer but larger spores with greater chances of survival. The production of larger spores is not only associated with reduced numbers, but seems to hamper this type of amoeba in the quest to form the fruit.

Lead author Professor Jason Wolf of Bath University says, “Our study shows that whilst there are definitely winners and losers in social cooperation, you can’t measure social success just by counting the number of spores these molds produce. Those that produce lots of spores often make inferior ones that don’t have any overall advantage over their competitors.”

Thompson says, “When we look at systems through just one aspect then that system can appear to be unbalanced and shouldn’t work, but in reality we are a collection of many features that together determine our success, and so our variety helps to make us more equal.”

Credit: Wolf et al. Despite more often being consigned to the stalk, and producing fewer spores when they do reach the fruit, some types of amoeba survive through quality over quantity.

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