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Just follow your nose

Sharks can sense a few drops of blood in water. In fact, they are able to smell blood in dilutions of as little as one to 10 billion. Is it just sharks or piranhas that have such an acute sense of smell? The answer is no - all fish have a highly developed sense of smell.

Dr. Jasminca Behrmann-Godel is examining the role that scent plays in the reproduction of perch in Lake Constance. She is looking specifically at two large perch populations, one in the upper lake and one in the lower lake area. It is assumed that the fish populations are genetically optimally adapted to the environmental conditions they live in, in terms of, for example, food availability and pressure from predators.

When seeking a mate, the fish have to choose a partner from their own population because the animals are adapted perfectly to the environmental conditions in which they live. However, if the relationship between the fish is too close, the fish won’t be able to mate. For example, the fish do not choose their own siblings as mating partners as this would lead to genetic defects among their progeny. Fish therefore have to select their partners very carefully and have a clear idea which fish they must not choose.
Perch (Photo: University of Constance)
Dr. Behrmann-Godel is examining how the genetically determined sense of smell influences the choice of a mating partner. In her doctoral thesis, the scientist was able to prove that young fish that did not know their siblings and hence were not familiar with their smell nevertheless were able to distinguish their own siblings from fish that were not related to them. Genes seem to be the decisive factor. The scientist now hopes to find out more about the genes that play a role in the recognition of scent. The most plausible candidates are the MHC (major histocompatibility complex) genes.

The MHC affects the choice of partner

The MHC complex plays a major role in the immune system. The MHC genes are very polymorphic and control immunological self/non-self recognition. The MHC genes carry the information for particular immune system constituents, i.e. the MHC receptor proteins that bind fragments of foreign proteins (peptides). The fragments might for example originate from bacteria, viruses or macroparasites that have entered the organism. The body’s defence cells are presented an “image” of the intruder, at which point specific defence reactions are sparked off. Everything is organised on a key and lock principle and is hence well-designed to rapidly eliminate pathogens,” said Behrmann-Godel who has been interested in fish from early childhood.

But what have the immune system and kin recognition got in common? The answer is another characteristic of the MHC genes, namely their effect on body scent. “Many organisms are able to smell the genes that best suit their own genes, which makes them able to smell suitable partners. Of course, the choice of ideal partner also ensures that the progeny is equipped with effective immunological constituents. This is not only the case for fish but also for humans where the MHC-controlled sense of smell plays an important role in the choice of partners. Many investigations have already shown this,” said the biologist standing in front of a large aquarium, one of many that she owns.

Determining the genotype of fish

The mating of fish leads to the combination of two different immune systems. The objective is that the progeny have as many different MHC complexes as possible in order for them to be able to effectively combat pathogens. However, the MHC complexes must not be too different. Too great a difference in the MHC loci means that the potential partner is no longer a member of the same “clan”. The partners are able to smell whether they have chosen the correct genotypes or not. For Behrmann-Godel the decisive question to be resolved is whether each population has specific MHC genes that send a signal to the fish telling them whether they belong to the same population or not.

Behrmann-Godel and her team of researchers determine the genotype of the fish. At present, the scientist is establishing a genetic laboratory at the Institute of Limnology. “We have already determined the genotype of more than 150 fish. We use a tiny piece of the fin. Each fish has a typical MHC composition which differs from the MHC of other individuals,” said Behrmann-Godel. The genotyped fish are put into an aquarium which is divided in three. The fish under investigation is put into the central section. “The fish has to chose its preferred group,” said the scientist. Initial attraction is induced by stimulant fish in the other two sections; however these fish are only “optical stimuli”.

The fish in the central section must make a choice based on the most attractive olfactory cues. Is it the fish on the right or the one on the left that is most attractive in terms of scent? The “fish perfume” is created by letting the fish distribute their odour in the water in big tanks. This water will then carry the individual odour of the fish and can be used to test different odour combinations in order to investigate how the MHC genes affect the behaviour of perch.

The biologist will need to carry out many investigations in order to clarify this phenomenon and she also faces the difficult task of breeding “test fish families” that can later be used to carry out experiments to elucidate their behaviour. “We have already started with the initial investigations. It will continue to be an exciting area of research,” said Behrmann-Godel.

Source: Uni’kon – issue 30/08 (CL)
Website address: https://www.gesundheitsindustrie-bw.de/en/article/news/just-follow-your-nose