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Almost all animals have symmetrical bodies. Bilateral symmetry is almost universal in all animals and is only very rarely broken—with exceptions like the five-armed starfish or crab species that have one large and one small claw.

One example of broken bilateral symmetry is the cichlid fish Perissodus microlepis, which is native to Lake Tanganyika in Africa. Its head and especially its mouth are shifted sideways in one direction, depending on the individual, either to the left or to the right.

The fish have a permanently "crooked head," so to speak, which they use to their advantage when hunting.

Axel Meyer, an evolutionary biologist from Konstanz, and his team conducted extensive analyses of the genome to find out why this is the case and which genes are responsible. The research results have been in Science Advances.

The cichlid fish P. microlepis feeds almost exclusively on the scales of other fish: it bites them off the living animals from their sides by attacking torpedo-like from behind and below and scrapes off the scales with backward-curved teeth.

The favored direction of the cichlid's attack corresponds to its laterally asymmetric head shape: it prefers to attack the prey fish either from the right or from the left side, depending on the direction in which its head is bent. In the cichlid population, the numerical ratio between "left-headed" and "right-headed" fluctuates around every four to five years, but in the long term it remains very consistent at 50:50.

First author Xiaomeng Tian has a plausible explanation for this: "If, for example, the proportion of left-headers grows, the prey that survives these attacks pays more attention to the side that is more frequently attacked, so that right-headers, in this case, are at an advantage—and vice versa."

This is why none of the variants is permanently more successful than the other and the ratio stabilizes at 50:50. "This is a prime example of rare frequency-dependent selection in which the more common form is at a disadvantage," explains the biologist.

Genetic tracing

But what is the genetic basis of this very unusual asymmetrical head shape? In a comprehensive study, Axel Meyer's research team has now investigated the genetic puzzle of the broken symmetry.

They examined 102 cichlids, carried out extensive genetic analyses and performed around 1,000 micro-computed tomography analyses per fish to analyze its morphology in 3D. They were able to identify 72 regions in the genome that are associated with the development of the asymmetrical head shape.

"Previous studies assumed that the polymorphism in the cichlid Perissodus microlepis was based on a simple Mendelian trait with a bimodal distribution. However, our investigations show that the situation is actually more complex: Not one single gene is responsible for the asymmetry, but many genes that are spread throughout the entire genome of this species," explains Axel Meyer.

When deciding what came first—the shifted head shape or the one-sided preference in hunting behavior—it's like the question of the hen and the egg.

First author Xiaomeng Tian provides an answer: "The two probably evolved together and reinforced each other," says the evolutionary biologist from Konstanz.

The preferred direction of attack is therefore a result of both genetic causes and behavior-related experiences. Further studies of the fishes' brains in the Konstanz laboratory show that the preferred direction in hunting is partly the result of an asymmetrical activation of genes (gene expression) on the left or right side of the brain.

"Our results suggest that both morphological and behavioral asymmetries have a measurable genetic component and have a common—or linked—genetic basis," confirms Axel Meyer.

"Based on our study, we have come to the following conclusion," summarizes Xiaomeng Tian.

"Firstly, the asymmetry of the head shape is not caused by a single gene, but we identified 72 regions in the genome that each contribute a measurable effect. Secondly, the unilateral preference in the cichlids' hunting behavior affects the asymmetrical morphology of its head through developmental plasticity—but the genes in turn influence the fishes' behavior as well. There is thus an interaction between the asymmetry of head shape and behavior, both of which have strong genetic components."