Researchers have reconstructed historical changes in the African landscape using the evolutionary history of the African sharptooth catfish. They were able to date important geological and climatic events, such as periods of severe drought, more precisely than ever before.
If you google images of an 'African landscape', you are guaranteed to find a succession of arid savannas with here and there a solitary Acacia (at sunset). In reality, this continent is extremely diverse, with a complex network of mountain ranges, marshes, rivers and lakes. This is a result of historical changes in climate and geology, which have shaped today's African landscape. But its 'how' and 'when' are still difficult to determine, even for geologists with a detailed knowledge of stratigraphy (the sequence of rock layers).
A new study by Maarten Van Steenberge (ichthyologist at the Royal Belgian Institute of Natural Sciences) shows that the DNA of African sharptooth catfish (Clarias gariepinus), the most widespread freshwater fish on the continent, contains important clues. Researchers from Belgium, Austria, Canada, DR Congo and South Africa collected genetic samples from all over the continent. By investigating the relationship between the populations based on their genetic blueprint, they found out when deserts, lakes and mountain ranges formed and when rivers were connected or separated. "When a population is split up by a change in the landscape or climate, this leaves traces in the genes," says Professor Filip Volckaert, expert in evolutionary biology at the KU Leuven and lead researcher of the study.
The African Great Lakes
The deepest splits in kinship among catfish are found in the area of the East African Great Lakes. "We suspect that the species originated there and spread from this area across the entire continent," says Maarten Van Steenberge. The genetic analysis also revealed two large zones in which the populations are closely related: one in the north and one in the south-west of the continent. Remarkably, these areas currently contain some of the driest areas in Africa: the Sahara and the Sahel in the north, and the Kalahari, Namib and Karoo in the south.
The populations studied live in lakes and rivers at enormous distances from each other, with only desert in between. "You'd expect these inhospitable regions to be barriers to fish migration. And they are. But the close relationship we observed reflects a totally different landscape in a fairly recent past," explains Van Steenberge. During the African wet period, until about 5000 years ago, there were no deserts there, but gigantic lakes! Of these, 'Mega Chad' was the largest, covering an area of around 300,000 km².
Rivers changing course
Something completely different is going on in the Congo River basin. The populations in the centre are closely related, but on the edges, they are as different as can be. "They are much closer related to populations in adjacent river basins," says Professor Auguste Chocha Manda (University of Lumumbashi). "Today's rivers were connected to other rivers at the time, which then split up by geological changes. As a result, they literally reversed direction. Now they flow in the direction of the Congo River, where fish with very different genetic backgrounds come together!”
Not your regular fish
The choice for the African sharptooth catfish was not accidental. "It is the only freshwater fish that is spread across the entire continent and can be found in almost every possible biotope: in lakes, rivers and swamps", Van Steenberge explains. "That’s necessary if you want to get a picture of how the landscape of an entire continent changed."
The fish owes this wide distribution to its extreme adaptability: it is omnivorous, very resistant to drought and even breathes air thanks to a special organ in its gills. This allows it to migrate over land! "Once it has colonized a place, it will probably never leave. And that makes him an ideal witness to geological history."
However, these special characteristics also make the African sharptooth catfish a popular species for aquaculture. It is feared that farmed fish will end up in rivers or lakes, disrupting the natural genetic pattern of the populations present. "Our research shows that the local populations are genetically well adapted to their environment. The introduction of a cultured relative could disrupt this balance," emphasizes Professor Rouvay Roodt-Wilding (Stellenbosch University). In this way, the research contributes to sustainable fisheries management and aquaculture.
This research, published in the journal Journal of Biogeography, is the result of a multidisciplinary and international collaboration between researchers from KU Leuven, the Royal Museum for Central Africa in Tervuren, the Royal Belgian Institute of Natural Sciences, Université de Namur, Hasselt University, Stellenbosch University (South Africa), Université de Lumumbashi (DR Congo), University of the Fraser Valley (Canada) and Karl-Franzens-University Graz (Austria). It was financed by FWO, KU Leuven, the Belgian Development Cooperation and the Belgian Development Agency.
