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Only real partners need apply – the evolution of chemical communication

Life would not only be terribly lonely without communication, our planet would never have been populated. Mating and reproduction are only possible through the exchange of information between different individuals. The biologist Dr. Thomas Schmitt is investigating how chemical communication, an ancient form of dialogue, developed during the evolution of insects. Although at first sight his investigations may appear to be basic research, some of Schmitt’s findings also have the potential for practical application – for example in combating pests in vineyards.

Dr. Thomas Schmitt (Photo: private)
Thomas Schmitt has always been interested in evolution. His doctoral thesis at the University of Würzburg dealt with the evolution of pheromone communication in the European bee beetle (solitary sphecoid wasps). “I wanted to gain insights into why certain components of the sex pheromone mixture are used whilst others are not,” said the biologist who became an assistant in the department of Prof. Klaus Peschke at the Faculty of Biology at the University of Freiburg after his PhD.

Choosing the wrong partner is a very expensive mistake

At present, one of Thomas Schmitt’s projects involves the study of parasitic wasps of the genus Nasonia, which attack fly nymphs. Three Nasonia species are known and they have most likely developed from one single species. It is assumed that bacterial Wolbachia infections have led to the separation of the wasp species. The infestation with Wolbachia has created a post-zygotic hybridisation barrier between the three species. This means that females and males can pair and produce zygotes, but these zygotes do not develop further into vital progeny. “The selection of a wrong partner is an extremely expensive mistake,” Thomas Schmitt explains. That is why it is hugely important to choose a mating partner from one’s own species. “This is the only way that vital Nasonia progeny can be produced,” said Thomas Schmitt explaining that the selection pressure to recognise a mating partner from one’s own species is particularly high.
They have found the right partner – mating Nasonia vitripennis specimens. (Photo: Christopher Geim)
In a similar way to the bee beetle, chemical communication, which involves pheromones as well as surface hydrocarbons, determines the mating behaviour of Nasonia. “We are interested in the role of a broad range of chemical signals. How does a wasp recognise the right mate?” asks the evolutionary researcher who is investigating how, under pressure to produce as much healthy progeny as possible, the phenotype alters with the development of new species. Working with researchers from Jürgen Gadau’s team at Arizona State University, Schmitt is investigating how the genotype changes when new pheromone components are generated. Schmitt and Gadau were able to show that the modification of a single gene locus leads to modifications in the sex pheromone of the three species.

Help in the search for the “right” partner

Thomas Schmitt is also fascinated by the question as to whether chemical communication by way of surface hydrocarbons contributes to finding the “right” partner. Wasps of the species Nasonia vitripennis use these chemical profiles on the body surface to differentiate between males and females. Might these surface molecules also prevent the wasps from choosing the wrong partner? The scientists’ research shows that several genes affect this trait. That is why male and female wasps differ in their surface hydrocarbon pattern, but also why the three Nasonia species have typical chemical profiles. There are clear differences between the three wasp species. However, it still remains to be clarified whether these patterns are also used to select the right partner.
Female Cerceris sabulosa specimen in front of its breeding burrow (Photo: Sven Kurz)
The scientists from Freiburg are also interested in the evolution of the surface hydrocarbons for other reasons. The surface hydrocarbons are found on all the insects that have been investigated so far. It is assumed that they originally served as a protection against evaporation. However, the chemical profiles differ between different genera, families and species. Some solitary hymenoptera species only have between 20 and 40 components, of which virtually none are used for communication; other insects, for example eusocial ants, possess up to 300 substances, which are used to exchange a broad range of information. But how are these different profiles generated? What kind of selection pressure affects the relatively simply structured solitary hymenoptera? Is it their prey, is it the climate? What is the effect of parasite pressure? Thomas Schmitt’s research group is investigating these questions using a group of sphecoid wasps (Cerceris sabulosa, a genus of digger wasps). Gold wasps intrude into the breeding holes of Cerceris wasps and by camouflaging themselves with a chemical profile that is almost identical to that of the Cerceris wasps. Schmitt is interested in finding out more about the variability of the chemical surface profile of these wasps and whether the chemical mimicry of the gold wasps leads to the modification of the surface hydrocarbon pattern in the host species.

Fundamental evolutionary mechanism

“With our research on communication we are working on a fundamental evolutionary mechanism. This is mainly basic research,” said the scientist. However, the findings of Schmitt and his team over the last few years also have the potential for practical application. In a joint project with the State Viticulture Institute in Freiburg, the researchers are trying to combat the European grape berry moth by using egg parasitoids instead of the already known pheromone confusion method.

“In some areas, the pheromone confusion method does not work,” said Thomas Schmitt explaining that in such cases the European grape berry moth is prevented from reproducing by the use of Trichogramma wasps. Trichogramma wasps parasitise the eggs of the European grape berry moth. But how do Trichogramma wasps find and identify the moth eggs? “The eggs release chemical substances which are detected by the Trichogramma wasps,” explains the biologist. However, this only happens during a very restricted time period after the eggs have been laid. “This means that effective monitoring is required in order to find out when the moths fly out to lay their eggs. We have to find the best time to release the Trichogramma.” The researchers have two vineyards at their disposal in order to identify the decisive period of time during which the release of Trichogramma is the most effective and whether the use of this wasp species is at all suitable for practical application. The researchers are supported by the company AMW Nützlinge that provides the wasps for the research project. Thomas Schmitt is convinced that the practical project will provide them with useful information. “The project is extremely interesting in that a more or less academic question has led us to a practical possibility to biologically combat vine pests as well as provided us with an excellent means for studying the complex process of chemical communication in insects.”

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