by Katie Galloway
Designing routes to connect people to resources has been an on-going endeavor since humans started exploring the earth. Over time the fastest and safest routes tend to become more popular, while poorer routes die away. This is called adaptive network design and it works without any centralized control. Rather, individuals process and make decisions about what path to choose; the more people choose a particular path the more popular and “worn” it becomes.
To envision adaptive network design in action, think of cows in a grassy field. In one corner of the field stands a grain trough and in the opposite corner a water trough. The cows can take any path they please between the two troughs. Initially, there may be as many paths in the grass as cows in the field. However, over time the cows want to limit how far they have to go between the troughs. The path carving a straight line between troughs becomes more worn and the less frequently used paths fade. Now throw in a ferocious dog tethered to a pole in the middle of the straight path. The cows will adapt their paths to skirt the dog. As they adapt their route, the old path will fade as new ones emerge. This is adaptive design at work.
Researchers in Japan recently showed that there are rules governing adaptive network design. They studied the slime mold Physarum polycephalum as a model of how transport networks evolve around fixed locations to connect the organism to valuable food sources. (For a recent review of how the slime’s adaptive network designs parallel human transportation systems, please see this post by Jeff Zweerink.)
P. polycephalum (a.k.a. the “many-headed slime”)is a yellow fungus that spends most of its life as a plasmodium foraging for food. A plasmodium is a large multinucleated cell that forms when two myxamoebae (fungal amoeba) mate. Think the Blob! Just like the Blob, the plasmodium grows by consuming organic materials like bacteria through the secretion of digestive enzymes and phagocytosis (engulfing its food).
As the plasmodium grows out, it advances like a fan, with “many heads” spanning out in search of food. As this happens, a mycelia network forms in the plasmodium’s wake to supply food to the advancing cell. This critical transport system is the cell’s supply chain.
For humans, eating healthy can be tough enough on an average day, but getting all those fruit and veggie servings is nearly impossible when traveling. Yet this lowly and literally brainless slime manages to balance its diet while constructing an impressive transport network. Or rather it builds an impressive network in order to eat a balanced diet. Actually, both scenarios appear to be true.
When given a choice of what to eat, plasmodia have been observed to choose the food patch with a higher concentration of nutrients. A research team from Australia and France then asked if the slime mold could “solve complex nutrient balancing problems by altering its growth form and movement to maintain an optimal ratio of macronutrients in the face of variation in the nutritional environment”. Basically, when faced with many options, can the slime find the best food source to optimize growth or will it get lost and settle for Milk Duds, an over-baked hotdog, and a soda at the nearest Kwik-E-Mart? (Ok, the Kwik-E-Mart was not really an option for the slime mold.)
First, the team established the optimal diet composition of carbohydrates and proteins (the two macronutrients of interest in this study) by observing the growth (area and mass) of slime mold on 35 food patches of varying concentrations and ratios. Not surprisingly, the slime grew densely on higher concentrations of nutrients and spread (increasing its area in order to increase nutrient uptake) when seeded on diluted food sources. The slime mold showed a preference for protein, growing most densely on diets with two times as much protein as carbohydrates. However, if the amount of carbohydrates in the food patch exceeded a certain level, then slime growth rates plummeted.
To determine if the many-headed slime could navigate its way through a maze of nutritional choices, the team arranged 11 different food patches of constant carbohydrate concentration and varying ratios of protein in a circle. By seeding the slime mold in the middle of these various diets, they observed that the slime grew to cover the patches of food that offered the optimal diet. In other words, the slime mold did not settle for the “Milk Duds.”
While social animals, such as bees, have been shown to optimize colony foraging resources using decentralized control, the slime mold seeks the best food sources without the aid of individual foragers. While the exact mechanism of how nutritional cues are tied into plasmodial locomotion are not yet known, the process may be driven by the same rules of adaptive design shown by the Japanese research team’s work.
Finding a balanced diet can be a life and death decision for the slime mold and yet God has endowed it with the ability to make choices that allow it to thrive. While the slime’s decision-making could be seen as a necessary objective in its evolution generated by simple rules of adaptive design, the logical functions and genetic regulatory networks that underlie a rational decision are not trivial. Cellular movement in the direction of a chemical attractant, know as chemotaxis, requires the integration of multiple layers of control in a cell. Furthermore, that the plasmodium makes a rational choice in a multi-attractant environment displays design even in the simple rules that appear to direct its decisions.
The plasmodium’s “brainless behavior” may demonstrate how simple rules govern a seemingly complex process, but when looking at the detail of those rules you can see a Creator’s handiwork that’s marked by elegant design from humans to the lowliest slime.