Using life’s natural symmetry to study biological phenomena

By Sarah Ruttenberg. Mentored and edited by William Schulz.

Scientists have looked to gene sequencing to understand the role of symmetry in the evolution of life. This kind of data provides insight into how environmental factors affect physical characteristics of different species over time. However, their method is limited by the availability of sequencing information, and they cannot always account for changes in DNA over a lifespan.

Recently, however, Punit Gandhi, Veronica Ciocanel, Karl Niklas, and Adriana Dawes supplemented this hole in the data by applying concepts from information theory. The group presented their method at the 2022 AAAS annual meeting.

Symmetry is both a visual and a mathematical concept. It can be defined as a correspondence in size, shape, and relative position of parts on opposite sides of a dividing line or axis. The symmetry found in nature can provide information about the evolution, genetics, and regulatory mechanisms of a species.

Most animals with two ears, two eyes, two arms or legs, or even two fins are called “bilaterians.” They have bilateral symmetry as embryos, however, as development continues, there are regular breaks in their symmetry. They have one dimple instead of two, a mole on one knee but not the other, or they might be animals with different patterns or coloring on either side of their bodies.

Symmetry, then, is a quality that can range in “symmetricalness.” It can be difficult to quantify and is therefore difficult to evaluate it as a source of biological information.

Dawes and her team tackled this problem by applying the concept of relative entropy. In information theory, entropy signifies the amount of uncertainty in a probability distribution. For instance, rolling a die with six sides has more entropy than tossing a coin with only two sides. That is, the die has more possible outcomes, so there is more uncertainty involved in rolling it than in tossing a coin.

Relative entropy is the difference in the amount of uncertainty between two probability distributions. So, by measuring quantifiable properties of two forms, or two halves of one form, a measure of their similarity can be produced. This creates a definitive measure of the symmetry between them.

Take your ears for example. If you measured the length and width of each ear, their different angles and curvatures, and their distance away from your nose, that information could be compared and combined into a measure of the symmetry of your ears.

Once there is a standard for measuring how symmetrical a living being is, you can compare the “symmetricalness” of one organism to another. This property can then be evaluated in relation to other factors like environmental stimuli, genetic mutations, mating or feeding habits, and more to determine their effects on the physical characteristics exhibited by the organism.

Sarah Ruttenberg is a chemistry doctoral candidate at the University of California, Irvine.

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