30 January

Thesis defense Annabel Ebbing: Dimorphisms in Nematodes

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Annabel Ebbing, from the group of Rik Korswagen, succesfully defended her thesis “Dimorphisms in Nematodes” on the 30th of January. During her PhD, Ebbing studied dimorphisms. An organism is dimorphic when it has two forms, such as for example the differences between two sexes, left-right differences or behavioral differences between genetically identical organisms. Ebbing investigated these differences using the roundworm Caenorhabditis elegans. In addition to her research, she also designed the beautiful covers for two journals in which her scientific articles were published.

Covers of the two scientific journals designed by Annabel Ebbing, for the journal Development (left) and Developmental Cell (right). © Annabel Ebbing

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.

Dimorphisms in the animal kingdom (A) Sexual dimorphism typified by the different feather cover of male and female birds. (B) Left-right asymmetry in a lobster with the left and right half of the lobster showing a clear color difference. (C) Behavioral difference in identical twins, in which the left twin is happy while the right twin appears angry. © Annabel Ebbing
© Annabel Ebbing

To investigate sexual dimorphism, Ebbing studied differences between the two possible sexes in C. elegans: hermaphrodites (females that can also produce sperm and thus fertilize themselves) and males. Despite the fact that the worms can differ in sex, they are often genetically almost identical (the male only has one X chromosome less). Nevertheless, the differences between the two sexes are visible in both behavior and appearance. Because these differences cannot be fully explained genetically, Ebbing focused on gene expression. During her PhD research, she compared the gene expression patterns of hermaphrodites and males. She identified genes that are specifically expressed in certain places in the body of a male or hermaphrodite. In addition, she has also discovered genes that are only present in the male’s reproductive organ. With follow-up research, Ebbing demonstrated that these specific genes are involved in male fertility. It was previously known that males are more efficient than hermaphrodites in reproduction; eggs are fertilized by the semen of a male rather than a hermaphrodite. The newly discovered genes were found to code for proteins in the seminal fluid. These are therefore probably new components of the seminal fluid that influence the activity and health of the sperm.

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.

Credit: Annabel Ebbing, © Hubrecht Institute

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.

© Annabel Ebbing.



Annabel Ebbing has been working at ttopstart since October, a consultancy for innovation in the life sciences and healthcare. Here she supports companies and researchers in setting up innovative projects and obtaining grants.


Banner image: © Annabel Ebbing.