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The coral body plan
Written by Tim Wijgerde   

Did you know that the genes which build a fly, a mouse, or even a human embryo are pretty much the same as those which create a coral? Nature has not changed much of its key laws of animal development in over 500 million years!

Since the dawn of early life, many key developmental processes have evolved to build complex animals. Processes like the evolution of the three embryonic germ layers, the main body axes and gastrulation have allowed highly complex life forms to arise. Much is known about both vertebrate and invertebrate developmental processes, however, their evolutionary origin remains unclear. By studying the Cnidaria, one of the oldest living groups of life on earth, scientists have found important clues about how life became more and more complex. 

All organisms develop one or more body axes. Having these axes is very important in the process of locomotion and separation of bodily functions. By splitting  up different functions by body section, animals can move forward in their environment much more easily, while the most important sense organs are pointed to the direction of travel to warn them of danger or potential food. It also allows organisms to create asymmetrical or bilaterally symmetrical structures that would not work very efficiently if they were forced to exist in a radially symmetrical body.

The animal body axes

Figure 1, right: The three animal body axes. Shown are the anterior-posterior axis, the dorsoventral axis and the left-right axis. Many Cnidarians such as corals, hydras and jellyfish only have one clear body axis. This axis runs from their mouth to their base, such as the polyp base of a coral where its skeleton is secreted. This means that Cnidarians are much simpler animals than flies, fish, mice or humans, when regarding their morphology. Some Cnidarians, however, including corals, have two body axes (image: Wikipedia).

Biologists distinguish between three major animal body axes; the anterior-posterior axis, the dorso-ventral axis, and the left-right axis (fig.1). The anterior end is the one where the head grows and where the mouth, brain, and major sensory organs develop. The posterior end is where wastes are discharged. The dorsal side is the top or back part of the animal, and the ventral side is the bottom or belly-side (fig. 1).

Coral development

Figure 2: Many genes regulate animal development, ranging from corals to humans. (A): A coral planula larva, in this case from the stony coral species Acropora millepora. Genes are abbreviated with letters and visualized with different colors. The left side of the larvae is where the mouth develops, indicated with a blue stripe. Other genes pattern the lateral and aboral (competely right) sides of the embryo, shown in red and purple. (B): A fly pupa with the larva inside (top picture of B) and a mouse embryo (lower picture of B), with the anterior, or head part, on the left side. Genes such as Emx and Otx control head development. Another very important group of genes, called Hox genes, pattern the fly and mouse body  all the way from "neck to tail". (C): A cross-section of a fly larva (on the left) and a mouse embryo (on the right). The dorsal, or back side, of the animals is the top side of the cross-section. The ventral part, or belly-side, is the lower side of the cross-section. Shown are different genes, in different colors, which control development of the nervous systems (from Danielle de Jong et al, Developmental Biology, 2006).

Some Cnidarians, including corals, have two body axes, making them bilaterally symmetrical. This is proven by the existence of the siphonoglyph, a ciliated groove which is located close to the polyp mouth. It allows the polyp to effectively take in water and food particles. Water intake allows Cnidarians to build up tissue pressure, allowing a colony to expand. Corals have also been shown to express genes in a bilateral fashion. The presence of both bilateral and radial body structures has led scientists to refer to Cnidarians as "biradial" animals.  The current prevailing theory is that Cnidarians and Bilaterians, including humans, have evolved from a common ancestor which had already evolved a bilateral body plan. To make matters even more interesting, biologists have also found that corals express mesodermal genes, which pattern a third embryonic tissue layer. This layer is called the mesoderm, and does not even exist in corals! This indicates that corals have somehow halted their morphological development during evolution.

In the end, to build any animal, it all comes down to the same sets of genes, regulating animal development in both time (from fertilization to adult) and space (head part, body parts, and dorso-ventral, anterior-posterior, and left-right axes). Genes exist for developing the front, middle and behind parts of the animal. Genes also overlap, programming development of specific structures like limbs, eyes, or the brain.

Isn't it wonderful to realize that humans share many genes with even corals, and that these have not changed that much over millions of years? This shows how unique the process of evolution is, connecting all life on earth. 

References:

De Jong, D. et al, 2006. Components of both major axial patterning systems of the Bilateria are differentially expressed along the primary axis of a ‘radiate’ animal, the anthozoan cnidarian Acropora millepora, Developmental Biology 298:632-643