Water Fleas and Evolution's Secrets

The mythical figure of Daphne inspired Richard Strauss in 1934 to compose his “Bucolic Tragedy in One Act.” As early as 1785 the Danish naturalist Otto Friedrich Müller had in fact dedicated an entire genus to the legendary nymph. And forty years later the French zoologist Hercule Eugène Straus wrote his Mémoire sur les Daphnia in which he was the first to describe the Daphnia magna, a species to which the German-Swiss evolutionary biologist Dieter Ebert has devoted intensive research. Generally known as “water fleas,” they are actually tiny crustaceans and in Ebert’s laboratory at the University of Basel they teem in the thousands.

The Daphniidae have held Ebert in thrall but his interest is not to be imagined as having any kind of mythological basis. And you will also be disabused if you expect natural ponds in the Vesalgasse, where the university is located and where he and his co-workers breed these pond dwellers which are only a few millimeters in size. Dozens of open screw-top jars are arrayed on the shelves, filled with water and meticulously labeled with laboratory codes, so not aquariums but rather “honey jars, they are particularly stable” explains Ebert on our tour through his faunal realm. Even the layperson can see that there is something moving in these mini-pools, and he or she would perhaps deduce plankton rather than the class Crustacea and designate the dark structures on the floor as resting stages.

Daphnia like having an environment that is neither too cold nor for that matter especially warm. They can be found most everywhere in the northern hemisphere but not in tropical freshwater. They are groomed for experimenting, and their existence in the famous little waterdrop makes them regulars in microscopy 101. Being transparent and of extremely graceful form they are also favorite models of limnology – the study of inland aquatic ecosystems. With their rudder arms and their cyclopean eye these water fleas are less reminiscent of nymphs than they are of hobgoblins, and let others draw parallels with the tragic love story of Daphne and Apollo! Dieter Ebert is focusing on other dramas in the lives of these pipsqueaks – sex, illness, death and epidemics. His projects endeavor to seize on the biological principles of infectious diseases and parasite infestation. He concocts suitable experiments so as to develop and test evolutionary concepts particularly in the realm of co-evolution. Quite helpful in this respect is the water flea’s ability to reproduce both sexually and by parthenogenesis. Genetic factors can thus be distinguished experimentally from environmental influences, just as has been attempted for humans in those classic studies of twins.

If other scientists wish to investigate just how influenza viruses or malaria parasites impact their respective host organisms, they cannot solely concentrate on the intricacies of their complex research but must also deal with ethical questions pertaining to animal experimentation. Or they are compelled to prepare clinical studies. This makes the research slow and arduous and the accompanying experiments very expensive or even impossible. So Ebert has no wish to involve mice or apes or humans in his basic research – not even yeast fungi or fruit flies. But why exactly then water fleas? Ebert says that they stand up especially well to the test of a model system when it comes to researching infectious diseases. As a schoolboy in Trier he enthused for ornamental fish and had not just one but eventually five aquariums – though now the tall slender biologist is not interested so much in fish as in their food. And he has indeed developed an infallible sense for it: “I immediately know if I am dealing with female or male animals, I see when they are sick, I can diagnose the symptoms and distinguish among the various parasites.” An exact knowledge of this model system now allows him and his team to find answers to very complex questions. He makes no distinction between practice and the theoretical framework because he tests the evolutionary concepts in elaborate experiments and then rethinks them according to any deviant findings. He still regularly performs work in the laboratory. And twice a year, to get away from his writing desk, he will spend extended periods in the great outdoors, doing fieldwork. But more on that shortly.

Ebert originally went to Basel to get his doctorate. And this at a time when evolutionary biology in Germany was seemingly doomed to extinction, judged as an old-fashioned niche discipline, and his advisor Jürgen Jacobs at the Ludwig-Maximilians-Universität in Munich was regarded as one of the last of a dying breed. Ebert was counseled to enter the field of neurobiology, but he was untempted by that up-and-coming discipline, so he first went to study with Stephen Stearns in Switzerland where he got his doctorate and then spent his postdoc years in Panama and Oxford, where he worked with William D. Hamilton. In 1995 he returned as an assistant professor to Basel’s Zoological Institute. Apart from a short period at the University of Fribourg, Ebert has remained faithful to the city on the Rhine. He has been a professor of zoology and evolutionary biology in Basel since 2004 and witnessed his children benefiting from the family’s proximity to the water by becoming passionate competitive rowers.

An upper floor in the time-honored Vesalianum was refurbished for the particular use of his research team. Great researchers have worked here, their names festoon the stairwell, names like Friedrich Miescher who discovered nucleic acid. Once devoted to anatomy and physiology, the building radiates the charm of the late nineteenth century. Still flanking the corridors are mighty cabinets, only now they are modern and bright white. The technology is also new, and Ebert’s floor accommodates his unique collection of water fleas. Swimming around in the honey jars are representatives of what will soon be a total of forty nations from four continents, among those nations Kyrgyzstan, Mongolia and Iran. In other words, taking part in this “diversity panel” are 211 populations from a wide variety of ponds, tarns, lakes, swamplands, rocky islands, salt marshes and even giant puddles. They are a reflection of natural distribution, they give information about the spread of parasite resistance, they even give testimony to the ice age in Europe – in short they provide “spectacular data.” And every month there is more. The respective phenotype can be correlated with the environmental data and the genetic endowment, and it is through a comparison of the various traits that conclusions can then be drawn regarding evolutionary processes and not only those concerning local adaptation. That is the true treasure which is to be discovered.

These water fleas arrive in Basel in sometimes adventurous ways. Many of them are brought back by Ebert from his trips, most recently from Spain and Norway. Others are sent to him by mail or are collected by his associates. One of them traveled thousands of kilometers by car through Russia in order to expand the variety of this collection. For the Basel scientists the fact that these animals reproduce through parthenogenesis has proven immensely useful because they are in fact clones whose genome is now being gradually decoded. Moreover the species D. magna is large enough to characterize the respective phenotype, and the ecological nexus is also of importance. “Smartphones now make our documentation easier,” says Ebert, “for instance photos of the countryside and measurement instruments can forestall mixups since the GPS data are always included.” We are talking in his office. In one corner there is still an old chart that depicts branchiopods, of which water fleas are one.

The yellowed drawings are decorative but have proven to be incorrect. Ebert’s sphere of expertise has greatly benefited from technological advances. These advances were first initiated with the PCR method, which enables the in vitro replication of DNA, and have been accelerated by today’s new and faster sequencing methods. Ebert’s research team is still pursuing classic fieldwork, but the core questions cannot be answered without genetics and genomics: “I have to go to the genetic level if I want to know how various traits evolve.” Monocultures are in fact more delicate and susceptible to vermin than are mixed cultures. Scientists have long been familiar with this phenomenon, but it was only through experiments performed by Ebert’s research group on the aquatic dwarves that it could first be clearly shown that the phenomenon can be traced back to the genetic structure of a population and indeed to its lack of diversity; interference factors were obviated through experimentation. And environmental influences such as temperature and salinity apparently have an impact which is completely different from that of a coevolution with parasites, for the latter occurs with much greater alacrity, and Ebert wants to find the genes involved. The water fleas are merely a model system, but they help him to better understand the relationships between organisms and their symbionts and parasites: the interplay between virulence and resistance in theory and empirically speaking. In 2018 his team was able to solve a sixty-year-old riddle and instead of a bacterium it was an Iridovirus which could be traced as the pathogen of the mysterious White Bacterial Disease, the infected daphnia being imbued with a green-white luster.

Even though the genetic work with water fleas proved difficult at first, Ebert very much appreciates the possibilities that these pipsqueaks afford him. Daphnia are surprisingly well suited for the testing of concepts and for investigating their interaction with all manner of microbes. How do they harm the water fleas? What companions are the hosts reliant on? Experiments have shown that without them, in any event, they do not thrive and will eventually wither away. According to Ebert: “We can not only investigate sick individuals but can also research the impact of parasites on entire populations. And then replicate the experiments.” That which is comparatively easy to attain with daphnia is much more difficult or almost impossible with other animal models, for instance mammals, since reliable numbers are necessary for deriving any meaningful statistics. If an infected water flea dies it must not necessarily mean the end of a pond community; but on the other hand, it might in fact signal its demise. Ebert is interested in evolutionary patterns in time and space, and these can be best observed in those numerous populations existing in the tide pools of southwestern Finland.

The University of Helsinki maintains a research station in the Tvärminne Archipelago where Dieter Ebert sojourns for a few weeks every year to research populations in this island world. Here he can put his various evolutionary-biological theories to the test under natural conditions. Though it must be said that it is difficult to imagine Ebert in one of the gazillion rock pools up there. And not because he is a typical laboratory biologist who assiduously avoids hiking boots or those of the rubber variety. To the contrary. But Ebert is always very calm and chooses his words carefully, whether he is talking about the randomness of nature, parasites and co-evolution, the red-queen hypothesis, or the origins of Pilates (he is familiar with this topic as well). A childhood disease which destroyed a facial nerve has robbed him of half his smile but never his sense of humor. And it caused him to embrace books at a very young age. Biking and basketball only came later. At his present age he even gives off a pronouncedly athletic vibe. Not to mention the impression he gives of a sober-minded and determined scientist who remains unflappable in the face of sometimes vehemently hostile criticism from creationists, adherents of intelligent design or other fanatics who think little of evolutionary theory.

Yet this evolutionary biologist is not exclusively fixated on his own specialty field but also enjoys the interchange among a variety of other disciplines. As of 2016 he has been a Permanent Fellow at the Wissenschaftskolleg zu Berlin where at any given moment he can find himself lost in discussion, for example over breakfast, as Ebert attests: “You stay sitting and just keep talking, for one or two hours.” He is excited by the fact that he can help shape this institution, for he appreciates the stimulating atmosphere and places a high value on scholarly excellence irrespective of age, sex or origin. That holds true for his own department, for Ebert meticulously edits every manuscript before publishing it. His acute visual perception allows him very often to locate the smallest of errors in the graphics. Which might also be owing to his weakness for patterns and concepts. Which doesn’t mean that he’s a control freak. Taking a page from Carl Jung, he permits his students “the freedom to run wild.” He can patiently await their results. Perhaps not until the very end when a master thesis has been completed, but indeed for a number of weeks. And sometimes a person whom you judged a bit of an unimaginative bore will completely surprise you, whereas the valedictorian might crash and burn and fall into a pit of despair because his or her data doesn’t fit within the conceptual framework. Not everyone can stand the challenges of this discipline. As Ebert reemphasizes: “Nature is very messy and only unwillingly discloses its secrets.”