Thesis defense Annabel Ebbing: Dimorphisms in Nematodes

Caenorhabditis elegans (C. elegans) is a small roundworm, about 1 mm in size, of which the origin of all cells and their location in the body is precisely known. This makes it a good model to study development, function of the nervous system and the functioning of the body. Because the cells are always in the same place, the differences in the activity of genes (gene expression) can be compared in detail between different animals. In addition, the roundworm also shows differences between genders, differences between the left and right side of the body, and differences in behavior between genetically identical organisms. These properties made C. elegans the ideal model organism for the study of dimorphisms in Ebbing’s research.

Left and right neurons
Ebbing also studied left-right dimorphism in C. elegans. During the development of C. elegans, two nerve cells are formed, on the left and right side of the worm. Despite the similar functions of these nerve cells, they move in the opposite direction (left cells migrate to the rear, right cells migrate to the front of the worm). During her Phd, Ebbing has investigated this left-right asymmetric process using different approaches. At the gene expression level, the left and right cells seem very similar. A different process therefore seems to be responsible for the direction difference between the cells. Ebbing demonstrated that transmembrane proteins play an important role in determining the direction of movement. These proteins most likely help the cells to respond appropriately to differences in the environment. In addition, certain molecular signaling pathways, such as Wnt signaling and Hox transcription factor mechanisms, are involved in the asymmetrical direction of movement. Many of these proteins and mechanisms are conserved between a variety of animal species. Moreover, even the function in regulating movement has often been preserved; for example, in many metastatic tumors, such genes are often disrupted.

Salt concentrations
Although roundworms are often described as simple animals, they are nevertheless able to perform reasonably complex behavior. For example, they are able pick up a variety of flavorings and/or fragrances from their environment and respond to them. For this perception behavior they have certain nerve cells in their body that act as taste receptors, such as two nerve cells that can measure the salt concentration in the environment. With the help of these nerve cells, the worm generally searches for an area with a higher salt concentration. You would expect that genetically identical offspring all have exactly the same preference with regard to salt concentration. Remarkably, there are always some worms that prefer a lower salt concentration. Ebbing showed that the same worms do not always have the same preference, which indicates a temporary difference of choice instead of a permanent personality difference. She then studied the gene expression of the worms that behave differently. She discovered a number of genes that are expressed differently in worms that may or may not have a high salt concentration in the environment. The results show that there is a possible relationship between the random expression levels of certain genes and the preference for salt concentration.

Genetic basis
In her dissertation Ebbing shows the existence of dimorphisms (differences) in roundworms at different levels. Moreover, it is clear that these differences do not always have a genetic basis. After all, it is not a difference in the genes themselves, but a difference in the use of genes, the gene expression, that can lead to these results. From an evolutionary perspective, this means that there are differences that can influence the chances of survival and reproduction of certain organisms that have a non-genetic basis.

Banner image: © Annabel Ebbing.