Precise conservation of the dynamics of embryonic regulatory circuits plays key role in maintaining morphological similarity between species

The Mediterranean sea urchin and the Pacific Ocean sea urchin are distant relatives that parted from a common ancestor about fifty millions years ago. In addition to the long period of time that has elapsed since then, the two species also live in profoundly different habitats: the Mediterranean differs significantly from the Pacific Ocean in terms of salinity, temperature, and numerous other characteristics. Accordingly, the two species show a similarity of just 85 percent in their protein-coding DNA -- a level similar to that between humans and mice.

However, Dr. Ben-Tabou de-Leon continues, in terms of morphological similarity, the two species have a very similar external appearance. This similarity is reflected not only in their final form -- both species are sea urchins with very similar shape, spines, and so forth -- but also in terms of embryonic development. The embryonic cells in both species undergo similar changes at similar stages in their development: the same cells in both species "choose" to become the skeleton, while others "choose" to become pigment cells, intestinal cells, and so on.

In the present study, which Dr. de-Leon conducted together with Dr. Tsvia Gildor, the researchers attempted to locate the biological mechanism that maintains this high level of similarity between two species that could have looked very different given their genetic differences. They chose to concentrate on the regulatory circuits of the embryonic cells. Genetic regulatory circuits consist of control genes the mutual relations between which determine the size, location, and function of the cell within the embryo. The findings showed that the dynamics of the regulatory circuits are very similar in both species of sea urchin. In other words, the temporal expression of the control genes is almost identical in the two different species of sea urchins once the developmental rates are scaled. "We have found the 'system core' in terms of the similarity between the species. We have effectively found that the developing embryo can make numerous environmental adjustments altering its DNA profile without changing this 'core' in the developmental regulatory circuits," says Dr. Ben-Tabou de-Leon.

Dr. de-Leon is now attempting to examine what happens when changes do occur to the core of the morphological system. In order to do this, she is going a few years further back, or rather a few million years. Sea cucumbers, starfish, and sea urchins all share a common parent dating back 500 million years. In other words, the three species are morphologically similar in their embryonic stage, yet develop into completely distinct adults. One of the key differences between these organisms is the skeletal rods that develop in the sea urchin but not in the sea star or the sea cucumber. Dr. Ben-Tabou de-Leon will seek to investigate the timing of the expression of the control genes on each species, and then to examine what happens when this timing is altered. The hypothesis is that changing the timing of the expression on the control genes in sea cucumbers will lead them to grow a skeleton similar to that of the sea urchin.

"The late Professor Steven Gold explained 40 years ago that the main source of evolutionary change is the changes in the developmental rate of species. In our research we have recognized that this is connected to the change in the developmental rate of certain embryonic genes. What I'm searching for is this rate and this change," she concluded.