Molecular Developmental Biology: Norwegian Marine Research Scientists Solve the Mysteries of Evolution

February 12, 2013

The human nervous system is an infinitely complex network consisting of some 100 billions of neurons. It is the result of many-faceted evolutionary processes spanning millions of years which, like the development of other organ systems, have been little researched so far. In his research team “Comparative Developmental Biology“ at the Sars International Centre for Marine Molecular Biology in Bergen, Norway, zoologist and developmental biologist Dr. Andreas Hejnol is studying marine animals to solve these and many other mysteries of evolution, aided by stereomicroscopes and confocal microscopes. 

Norway is the perfect place for a marine biological research center. The vast coastal regions are populated with Baltic fauna as well as Atlantic species borne by the Gulf Stream, offering immense resource potential that is ideal for the wide developmental biological approach adopted by Andreas Hejnol and his team. “The other research groups at the Sars are working on individual species such as the sea anemone  Nematostella”, Hejnol explains. “My graduate students and post-docs are examining various marine animals to discover how specific organ systems – such as the nervous system – have developed in the course of evolution. They are looking at cellular processes to find out which genes are in involved this development.” 

Marine animals eminently suitable for research in evolutionary biology

The sea holds the secrets of the origins of life – most animal species lived in the sea in an earlier stage of evolution and took millions of years to transfer to a terrestrial  habitat. As the various species underwent this process independently of each other, marine animals are particularly suitable for evolutionary research. “Marine animals tell us a lot about the evolution of animals in general,” Hejnol reports. “We look at what happens between fertilization of an oocyte and the point where it has developed into an adult capable of reproducing. We work on different animal species at the same time and then compare them.”  

Andreas Hejnol and his team use several research ships to collect fauna samples at different places along the Norwegian coast. In the lab, they then use the modern next-generation sequencing method to be able to sequence as many molecules as possible within the shortest possible time. Once they have obtained the necessary amount of sequencing information, they continue their research by applying molecular biological methods.  “We collaborate with research groups all over the world to obtain sequencing information. For example, our partner institute European Molecular Biology Laboratory (EMBL) in Heidelberg supplies us with sequencing information obtained with the Illumina method,” says Hejnol.

Close-up image of a zooid belonging to the colony of the bryozoan <i>Membranipora membranacea</i>. The researchers at Bergen are examining its embryology and are sequencing its genome. Photo: Bruno Vellutini, Sars

Function of molecules changes

Using the sequencing information provided by other research groups, Andreas Hejnol and his team then examine, for instance, the genes involved in the formation of the nervous system of both the fly and the human – two very distantly related creatures. They then compare these observations with the function of these genes in more closely related animals – and also in animals that are even more distantly related.  The scientists use these studies to understand the evolutionary role of molecules. In concrete terms, they are trying to find out how the function of molecules has changed within the course of evolution and the effects this has had on the structure of the nervous system.  The nervous system is a good example of the development process of an organ system. In the course of evolution a neural network, which jellyfish, for example, still have today, has developed into a condensed bundle of nerve fibers in the form of the human spinal cord.

Microscopy plays a key role in the work of the “Comparative Developmental Biology” research group. First, Andreas Hejnol and his team use basic stereomicroscopes like the Leica M60 and M80 to identify the animal species. Under the Leica M165 with fluorescence filters, the scientists can establish transgenic lines, for example. “This process always takes a bit longer for animals taken from their natural habitat, explains Hejnol. “But with the Leica M165 we can also examine gene expression under the stereomicroscope – using markers like GFP.”

Time-lapse recordings of living embryos

Inverted research microscopes like the Leica DMI4000B enable the scientists to make time-lapse recordings of embryogenesis – i.e. the development of an embryo. To do this, they put a living embryo at the single-cell stage under the microscope. A computer-controlled recording of the embryo is then made with normal Nomarski optics. Roughly every 45 seconds, the system takes optical sections and stores them on hard disk. This gives the scientists a three-dimensional time-lapse recording of all the cell divisions inside the embryo. “The advantage is that this is a non-invasive method,” says Hejnol. “Methods like gene expression are often unestablished for the living animal embryos we examine. This way, we can study the cells’ family tree simply by using the optical resources at hand to find out which cell forms which tissue and which organ.”

Hejnol’s team use the Leica TCS SP5 confocal microscope for anatomical studies, staining specific muscle or nervous systems in the animals to obtain a three- dimensional view. They combine the technique with the fluorescence display of gene expression. “This not only gives us a color-coded view of the organ systems in three dimensions, but also a picture of the gene expression. We can then exactly identify the cell types and characterize them on the basis of morphology, position and gene expression. By comparing this data with that of other animals, we get an evolution of cell type generation,” reports Hejnol.

This confocal image shows one scan layer of the acoel worm <i>Isodiametra pulchra</i>. The following parts are stained: cyan: nuclei, green: stem cells, magenta: expression of the genes T-Brain/Eomes (gonads und oocytes). Left: anterior. The worm is about 1 mm of size. Photo: Aina Børve, Sars

Evolution of the digestive system

The “Comparative Developmental Biology” research group is not only investigating the evolution of the nervous system, however. One of the issues the team is currently focusing on is the development of the digestive system. Species that originated earlier in the evolutionary chain such as jellyfish have an intestine with only one opening, i.e. one that serves both as a mouth and anus. The scientists now want to find out how this evolved into a continuous digestive tract with two openings. Marker genes play a significant role here, too. Andreas Hejnol reports: “There is a certain percentage of genes that are specifically expressed in the mouth of animals. Other genes are only expressed in the hindgut. For instance, we have examined worms that have no anus. Instead, they have a posterior orifice to emit sperm – a so-called male gonoporus. We found all the genes expressed by the human anus in this male gonoporus. This suggests that in the course of the evolutionary process a link was formed between the intestine and the gonoporus – a cloaca, which birds still have today. So the cloaca was there first and then the intestine, not the other way round.”

However, this is only one of many projects that Andreas Hejnol and his colleagues are currently exploring – the future holds an inexhaustible supply of unsolved issues. For the zoologist and developmental biologist, this is one of the best aspects of his work: “There are still loads of questions that nobody has been able to answer yet. I find the fact that I can help solve these mysteries extremely exciting. I’m very happy that I’ve been able to turn my hobby into a profession,” says Hejnol.

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