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The Electric Code for a Partner – Mormyrids rely on electrical signals to choose partner

Mormyrids in an experimental aquarium. Picture: Karla Fritze

Mormyrids in an experimental aquarium. Picture: Karla Fritze

How do new species develop? This question has plagued researchers for centuries and remains one of the most topical questions in evolutionary and biodiversity research. Potsdam biologists are looking for mechanisms in the behavior and genome of African mormyrids that divide one species into two. The weakly electric fish seems to have taken a special evolutionary path.

It is feeding time. Transparent mosquito larvae float and twitch in the big and small aquaria. The mormyrids in the basin usually stay in their hiding spots - brown plastic tubes. Every once in a while a fish scurries out. “They are nocturnal animals,” explains the biologist Rebecca Nagel, who studies the mormyrids’ behavior in the research group Evolutionary Biology. When it is light, they prefer to stay in hiding. It is immediately evident why these brownish grey animals are also called elephant-nose fish or tapirfish: Their mouth has a trunk-like protrusion that is strikingly bent. Their homes are sub-Saharan African rivers. These animals burrow through the muddy river bottom with their trunks looking for insect larvae. Nagel is less interested in the eye-catching trait but rather in an invisible one. Mormyrids have a special organ at the root of their tail fin that produces weak electrical signals. Similar to bats, they sense their environment through impulses and electroreceptors on their bodies – because they usually live in very murky water and can hardly orientate themselves with their eyes.

These electrical signals also seem important for the love life of these fish. “We hypothesize that the electrical impulses also have a social function,” Nagel explains. It is assumed that the electrical signals allow the fish to recognize each other because different species of mormyrids often live near each other in murky currents of the Congo and Nile. The electrical discharges differ slightly in each species. What sounds like a diffuse crackling and noise to the human ear is possibly a signal to identify a conspecific (that is, a member of the same species) and potential mate. The intriguing thing is “that the closer the species are related, the more different the signals are.” Previous examinations of the research group showed this, as Nagel explains, who is writing her doctoral thesis about mormyrids. “Closely related fish can better differentiate themselves from each other.”

The biologist wants to observe the animals’ behavior to find out if the male mormyrids select females of the same species based on their electrical identifying code. Mormyrids are especially interesting to evolutionary biologists because they are rich in species. Speciation may result from their ability to communicate through electrical signals. Biologists call the interrupted genetic flow between populations once belonging to the same species “reproductive isolation”. Only those individuals will mate that belong to a specific group. Reproductive isolation is considered a crucial factor in the development of new species. Geographic barriers are usually the reason for isolation. Darwin’s finches on the Galapagos Islands are perhaps the best-known example. The specific signals of mormyrids may have a similar barrier effect that once precipitated evolution.

The studies are part of the project “Funktionelle GENomik biologischer ARTbildung” (GENART), funded by the Leibniz Association. A network of molecular geneticists, neuroethologists, bioinformaticians, and evolutionary biologists is researching the genomes of three evolutionarily successful animal groups – mormyrids, locusts, and crickets – and is identifying the genes essential for speciation and their functions. Ralph Tiedemann, Professor of Evolutionary Biology and Systematic Zoology, heads the project in Potsdam. He works in close cooperation with Professor Frank Kirschbaum from Humboldt-Universität in Berlin.

The aquarium for Nagel’s tests is about two meters long. Gratings separate the basin into three areas. The male swims in the middle, the females on the right and left – a conspecific and a non-conspecific. Once the lights go off at 5pm, a camera begins recording the movements of the fish. For 12 hours it captures the male mormyrid’s location. Does he prefer the side of the conspecific female?

Nagel evaluates the test the following morning. The video of the nocturnal activities shows a black fish silhouette against a light background. Broken lines on both sides of the aquarium glass delineate the various areas of the basin. If the fish crosses these lines, it is considered an approach to the female. The biologist studies 20 mormyrid couples belonging to two different species in this way. Special software spares her from having to watch hours of video by analyzing the male’s duration in various parts of the aquarium.

The researcher also carries out a second test to exclude that the male is reacting to other signals like chemical messenger substances or visual stimuli. During this test no females are in the two external basins but only electrodes that imitate typical female electrical signals. If Nagel demonstrates that the male spends more time near the conspecific signal during both tests, her hypothesis would be confirmed and this would explain how different species are able to develop in a habitat without geographical barriers. Nagel will next use artificially generated signals to examine if the animals also recognize slightly altered patterns as conspecific and how large the margin for alterations is.



The biologist will use genetic tests to ultimately find out how the electric organ of mormyrids developed and how exactly it functions, because this has not yet been clarified in detail. The researchers already know that this organ developed out of skeletal muscles. Nagel is now examining gene activities in the cells of the skeletal muscle and the electric organ at the fin tail of the fish. Her assumption is that the genes in the ion channels of the cell walls in these two cell types are differentially active – and are responsible for the signal generation. She expects to find higher gene activity in the electric organ, which would indicate the evolutionary origin of the typical signals. “These fish have a so-called fish-specific gene duplication,” Nagel explains. Some gene groups exist twice in the genome of the fish. “We assume the electric organ developed in connection with this duplication.” Mutations can occur at the doubly present genome sections without creating a selective disadvantage for the organ, because one gene remains intact to fulfill the original function. Gene duplications are regarded as the “motor” of evolution, because the duplicative genes can develop new functions. This mechanism in African mormyrids may have resulted in electrical communication among the animals and delimitation from other groups, which enabled them to develop so many species.

The Researcher

Rebecca Nagel studied biology and molecular biology in Rochester (New York) and Mainz. Since 2014 she has been working on her doctoral thesis at the University of Potsdam.

Contact

Universität Potsdam
Institut für Biochemie und Biologie
Karl-Liebknecht-Str. 24–25, 14476 Potsdam
E-Mail: rnagel@uni-potsdam.nomorespam.de

The Project

GENART: Funktionelle GENomik biologischer ARTbildung
Participating: Museum für Naturkunde - Leibniz Institute for Evolution and Biodiversity Science at Humboldt University Berlin, University of Potsdam, Berlin Centre for Genomics in Biodiversity Research (BeGenDiv), Leibniz Institute for Zoo and Wildlife Research (IZW), Berlin Institute for Medical Systems Biology (BIMSB) of the Max Delbrück Center for Molecular Medicine
Duration: 2012–2015
Funded: Leibniz Association
Website: http://www.naturkundemuseum-berlin.de/forschung/genart

Text: Heike Kampe, Translation: Susanne Voigt
Online-Editing: Agnes Bressa
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