10 April 2020


Welcome back. This is the story about the importance of loners, or to be precise, learning about the importance of loners. (It’s not a homage to social distancing.)

By loners, we are talking about those individuals that avoid the company of others, that resist following the crowd. Loners are not just human contrarians or misfits. They’re common across the natural world--migrating wildebeest, schooling fish, swarming locust, even microbes.

Researchers affiliated with Princeton University and other institutions sought to determine if loners were simply incidental byproducts of large-scale coordination attempts or if loners could actually be an important part of life-history strategies.

Toward that end, they opted to study the loner behaviors of the cellular slime mold Dictyostelium discoideum. Now, don’t get squeamish. Stay with me. You’ll see--it’s fascinating!

Dictyostelium discoideum life cycle (from
Dictyostelium discoideum Life Cycle
D. discoideum is a species of soil-dwelling amoeba. Invisible to the naked eye, the single-celled organism spends most of its life in a growth phase, preying upon bacteria and dividing by mitosis. When famine threatens, they send out chemical signals which attract other amoebae and they coalesce. Thousands of amoebae rapidly aggregate to form a tight mound, then enter a stage where cells remain motile and differentiate.

This culminates in the formation of a multicellular stalk that supports a ball of encapsulated, dormant, starvation-resistant spores. The stalk grows upward, responding to light, until the spores catch a ride with the wind or a passing insect to locations with better food sources. The cells making up the base and stalk die so the species can disperse and survive.

And the loners that don’t aggregate? Those solitary cells also die of prolonged starvation, though they persist temporarily. If food is replenished, they eat and divide, and their progeny subsequently repeat the multicellular development.

Establishing Loners’ Role
Initial experiments with D. discoideum collected from the wild, found that up to 30% of the cells chose the loner life over collective action. Moreover, the loners were not a constant fraction of the population of starving cells. Their number depended on the density of the population.

Photo of aggregating D. discoideum with image processed to show cells in black. Aggregating cells near center of petri dish are about to enter the stage prior to developing the stalk; loners are denser at border (from journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.3000642).
Through experiment and modeling, the researchers established that loners could be a vital part of the species’ life-history strategy. They showed that natural populations of D. discoideum have heritable variation in loner behavior (i.e., heritability) and that the loner behavior of different strains of the species affects the relative abundances of the strain in their natural environments. In essence, the proportion of loner cells is determined by interactions of the population with the environment.

Wrap Up
The researchers explain that collective actions, such as aggregating, offer huge benefits; yet those actions are accompanied by risk. 

The loners might be hedging the population’s bet, ensuring that damage to the majority doesn't eradicate the entire population or its ability to be social. Even if the loners themselves aren’t social, their progeny retain that ability under the right conditions. 

It appears that the decision to avoid aggregating and be a loner is not made by the cell but is instead a collective decision.

Well? Wasn’t it worth staying to the end? Thanks for stopping by.

Background on Dictyostelium discoideum:
Study of slime mold loners in PLOS Biology: journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.3000642
Article on study on EurekAlert! website: www.eurekalert.org/pub_releases/2020-03/pu-lhs031820.php

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