Re-Envisioning Cell Theory (Part 3): Sublime Slime Molds

It’s the last part of our latest blog series written by Sherrie L. Lyons, author of From Cells to Organisms. The book delves into the nature of scientific practice, showing that results are interpreted not only through the lens of a microscope, but also through the lens of particular ideas and prior philosophical convictions. In this final part, Lyons discusses the fascinating world of slime molds, the unlikely superhero within the ecosystem.

Read Part 1 and Part 2.

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Sublime Slime Molds

“Look up. In the sky! It’s a bird. It’s a plane. It’s Superman,” introduces the iconic 1950s TV show. It also begins my discussion of slime molds in From Cells to Organisms. But instead, I ask the reader to: “Look down. It’s a plant. It’s a fungus. It’s an animal. No, it’s a slime mold.”

This blob-like creature seems an unlikely superhero, but the more we are learning about it, the more fascinating it has become. Slime molds in many ways epitomize everything we still do not fully comprehend about this amazing phenomenon called life. Their complex lifecycle highlight the limitations of cell theory in trying to understand how a cell grows into an organism that is communicating and functioning within an ecosystem.

Cellular Slime Molds

Over nine hundred species of slime molds have been identified and are divided into two major groups. Cellular slime molds are truly multicellular with different cell types, while plasmodial slime molds are not. Both groups exhibit enormous variability. They come in all kinds of colors. They range in size from microscopic to several square meters. One is most likely to encounter slime molds in the woods clinging to rotting logs.

The cellular slime mold Dictyostelium discoides is happy to spend much of its life cycle as a haploid amoeba creeping along, feeding on decaying matter. But once food becomes scarce, one amoeba sends out a message, and thousands of them come join it to form a giant slug. Inside the slug, a small number of the cells take on the role of the white cells of our immune system. They move through the slug eating pathogenic bacteria. The slug inches along towards the light, and if conditions are right something quite amazing happens. It stops, upends itself, and turns into a true multi-celled organism as individual cells differentiate.

The bottom cells anchor the whole organism while the middle cells grow into a stiff stalk, turning their insides into bundles of cellulose. Cells at the tip grow into a fruiting body with a cap containing thousands of spores. Wind or rain knock off the cap, dispersing the spores. When conditions are right, the spores germinate, turning into amoebas, and the cycle continues.

Meanwhile, the cells that make up the stalk die.

Many species of plants such as wheat, produce a stalk that withers and dies after its seeds are dispersed. However, wheat cells have spent their entire life as part of a multicellular plant. In the case of the slime mold, the individual ameboid cells probably originated from many different fruiting bodies that come together. The cells of the stalk die, essentially sacrificing themselves for the good of the group. This suggests a kind of altruism, something that was thought to only exist in more highly developed organisms. The evolution of altruism has and continues to be a topic of contentious debate among evolutionary biologists ever since Darwin. Without getting into the details of that debate, at the very least, the slime mold’s behavior is a type of cooperation, and shows that cooperation evolved very early in the history of life.

Plasmodial Slime Molds

DNA sequencing suggests that cellular slime molds evolved around six hundred million years ago. The common ancestor of all slime molds may have evolved over a billion years ago. It was one of the earliest pioneers on land, arriving hundreds of millions of years before the first plants and animals. Cooperation then has been an important strategy. Just like the kinds of symbiosis that were discussed in my second post, the slime mold contradicts the prevailing “nature red in tooth and claw” characterization of evolution. Instead, “survival of the fittest” may mean survival of the cooperators. Something our own species would be well to heed.

Slime molds in the second major group are even making us rethink how we define intelligence. In 2000, Japanese researcher Toshiyuki Nakagaki and colleagues wanted to study how Physarum polycephalum moved. They cut one slime mold into lots of little pieces and scattered them in a plastic maze. The pieces grew, finding each other as they traversed the entire maze. The researchers then put blocks of agar with nutrients at either end of the maze. After four hours, the slime mold had retracted all its branches that led to dead ends. Canadian researchers placed rolled oats (slime molds really like oats!) on the major population centers of a map of Canada and the slime mold was placed on Toronto. It worked its way across the entire map sprouting its tendrils that mimicked the Canadian highway system.

This experiment has been repeated using maps of the United States, Tokyo, and London. The slime mold moves in a pattern that resembles the highway system of the United States, the railroad system of Tokyo, and the London underground. Quite impressive for an organism that doesn’t even have a neuron, much less a brain! In 2010, Andrew Adamatzky and colleagues placed a slime mold in the middle of a map of Spain and Portugal, with pieces of food on the largest cities. The slime mold grew a network of tentacles that was nearly identical to the actual highway system on the Iberian Peninsula. Should a slime mold be enlisted to help design highway systems in developing countries by following routes that it makes on a map of the country that has food placed on the population centers?

As our knowledge of the natural world increases, specialization inevitably happens. Heredity, development, and evolution became distinct disciplines and we have learned a great deal. At the same time deeper and more difficult questions were often avoided. From Cells to Organisms explores the ways cell theory hampered the reintegration of these fields. Slime molds are opening up new areas of inquiry as we break down these disciplinary boundaries and “re-envision the cell.”

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Interested in finding out more about From Cells to Organisms? Click here to read an excerpt from the book.

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