For animals that regulate their body temperature, 30°C can be the perfect temperature to relax and sit back in, but for pond snails (Lymnaea stagnalis), the pressure is on. Their immunity suffers and the snails begin to reproduce frantically as mortality beckons. However, there does seem to be an up side to a blast of heat for the diminutive molluscs. Ken Lukowiak from the University of Calgary, Canada, explains that snails that have experienced a heatwave form long-term memories under circumstances where chilled-out snails wouldn't recall a thing. In addition, a whole suite of genes that produce protective ‘heat shock’ proteins are activated when animals experience heat stress, and these proteins help the molluscs to consolidate long-term memories. Lukowiak and his colleagues wondered whether these memory-promoting changes in gene expression are caused by DNA changes that occur when the temperature rises.
To test their theory, Lukowiak and his colleagues injected the snails with a drug (5-AZA) that prevents a specific type of DNA modification (methylation) that is known to alter gene expression patterns. Then, the team warmed the snails up to 30°C for 1 h before training them to remember to keep their breathing tubes (pneumostomes) closed when they were placed in de-oxygenated water 24 h later.
Snails that had not experienced the heatwave had no memory of the lesson and extended their breathing tube to the surface to gulp air, whereas snails that had experienced the heatwave remembered to keep their breathing tubes closed in the hypoxic water. However, when the team tested the memories of the snails that had received an injection of 5-AZA before the heatwave, they had no recollection of the lesson that they had been taught the day before. So, changes in the methylation of the genes that are activated when pond snails get warm are essential if pond snails are to recall memories formed when they get hot under the collar.
