High temperatures and drought are not necessarily the preferred environmental conditions of snails. However, some snail species are able to survive in desserts. Professor Dr. Heinz-R. Köhler and Professor Dr. Rita Triebskorn, two zoologists from the University of Tübingen, are working with research teams from Avignon, Esslingen, Gießen and Le Havre to investigate how snails are able to adapt to extreme climatic conditions on the molecular, cellular and physiological level. Their “Hot Snail” project is funded by the German Research Foundation (DFG) and its particular objective is to find out whether reactions to heat stress have an effect on the phenotypic diversity of the snails.
Snails originate from the sea, and the majority of the 60,000 snail species known today are marine and freshwater snails. When snails moved from the water to the land, they had to develop a broad range of strategies in order to adapt to terrestrial conditions. “Heat is still a big problem for snails,” said Prof. Dr. Heinz-R. Köhler from the Institute of Evolution and Ecology at the University of Tübingen, explaining that snails have a muscular foot that is lubricated with mucus and covered with epithelial celia, which does not provide sufficient protection against dehydration. Nevertheless, there are some snail species that have adapted to life in hot climates.The biologists from Tübingen are concentrating on the physiological and biochemical mechanisms involved in the heat tolerance of snails as part of the Hot Snail project funded by the German Research Foundation (DFG). They have found that so-called heat shock proteins (HSPs) play a key role in these mechanisms. HSPs were discovered in Drosophila melanogaster and were identified as the fruitflies’ cellular response to high temperatures. “HSPs are now known to be present in all organisms, including bacteria and mammalian cells. It is also known that the proteins have remained largely unchanged during evolution,” said Köhler. The conservation of the proteins suggests that HSPs play a key role in the cellular metabolism.
“Snails also produce heat shock proteins when exposed to high temperatures,” said Prof. Köhler. HSP70 and HSP60, which are two of the most widely studied HSP families, function as intracellular chaperones for other proteins and play an important role in protein folding and in the establishment of proper protein conformation and function. “Heat shock proteins have the outstanding ability of being able to help heat-damaged or otherwise affected proteins to return to their natural state, at least to a certain degree,” said Köhler. This mechanism safeguards the survival of cells, and hence the survival of the entire organism.However, the buffering capacity of heat shock proteins might quickly be depleted in situations of extreme stress, which is the reason why snails possess additional mechanisms to help them to counteract heat stress. “Some snail species are able to keep their body temperature low by increasing the evaporation rate,” said Köhler. Working in cooperation with engineers from the Esslingen University of Applied Sciences, the zoologists from Tübingen are focusing on the thermodynamic relationship of the ambient temperature and the temperature inside the shell. “We have found that certain snail species express a series of heat-protective stress proteins and that the snails’ shell colour varies considerably. One of the issues we now want to clarify is whether snails with dark shells heat up more quickly than snails with light shells.”
In addition to providing protection against extreme temperatures, heat shock proteins have other abilities that are a source of fascination for the researchers from Tübingen. HSPs appear to have the ability, or partial ability, to mask the occurrence of genetic mutations. “A DNA mutation that leads to an amino acid exchange will normally also affect the structure of the protein product,” said Köhler. “However, it is believed that heat shock proteins are able to recognize small deviations from the ideal structure and nevertheless ensure that the proteins are folded correctly.” This might be a huge advantage when it comes to preventing individual genetic mutations from having an immediate effect on an organism’s phenotype. “This possibly leads to the accumulation of mutations without any individual mutation having been tested for its potential evolutionary success,” said the scientist, explaining that this only occurs when a critical number of mutations has built up. “It can be assumed that several different mutations of a protein, each of which would have unfavourable consequences on its own, might, when taken together, improve protein function, and thus be successful in evolutionary terms,” said Köhler. This assumption is currently a subject of debate amongst scientists around the world and is referred to as the ‘capacitor hypothesis’, in which an evolutionary capacity allows developmental variation to accumulate as neutral alleles under normal conditions and manifest morphological differences in situations of environmental stress.
In theory this means that the phenotypic variation of animals should be much lower in populations with large quantities of HSPs. The reason for this is that larger numbers of HSPs are more effective in masking larger numbers of mutations. “The findings we have obtained from the snail populations investigated so far provide clear evidence for the capacitor hypothesis,” said Köhler who assumes the capacitor system might break down in situations of extreme stress, for example when environmental conditions change abruptly. This could be down to the fact that stressful situations require cells to use heat shock proteins for repair rather than for capacitating mutations. “As a consequence, the phenotypic variability of a population increases considerably after a situation involving stress, as the pool of genetic variation that has accumulated over time suddenly has an effect,” said Köhler pointing out that heat shock proteins have already been identified as buffers to the expression of phenotypic morphological variation in Drosophila and Arabidopsis. “Evolution is thus able to act on a larger pool of genetic variations and lead to new phenotypes that are far better adapted to altered environmental conditions than the old ones,” said the zoologist. As part of the Hot Snail project, Köhler and Triebskorn’s team will also study natural Mediterranean Xeropicta derbentina snails in order to investigate the interaction between heat-induced HSP regulation and phenotypic variability and to collect further evidence for the capacitor hypothesis.
Further information:Professor Dr. Heinz-R. KöhlerAnimal Physiological EcologyInstitute of Evolution and EcologyUniversity of TübingenKonrad-Adenauer-Straße 20D-72072 Tübingen