In 1974, Sydney Brenner wrote his classic paper on mutagenesis of Caenorhabditis elegans, and the rest is history. With a single publication, he elevated the tiny worm (1 mm long) to a hallowed place in science's Hall of Fame. Despite its diminutive stature, it has been dissected and scrutinised to ever-increasing resolution, culminating last year in 1998 in the determination of the DNA sequence of its entire genome.
A great deal is known about the way the worm’s nervous system regulates the elegant sinusoidal path it leads through life, thanks to members of the species that are less coordinated. However, very little little is understood about the way that these animals respond to external stimuli, and even less about their ability to learn. Yuichi Iino explains that learning can be defined as 'a phenomenon where the behaviour of an animal changes depending on its previous experience'. So how do you test a worm's ability to learn? Sounds like a tricky test. Over the years some of the worm's behavioural traits have formed the bases of 'learning' assays, and now Iino and his team have come up with another reliable method based on two strong incentives: starvation and salt.
Most of us will recognise the craving for a certain salty flavour, and C. elegans isn't much different. Under most circumstances, worms placed on a Petri dish covered in nourishing E.coli will migrate up a salt gradient, attracted by the salty source. However, when the worms are starved in the presence of salt, they adapt their behaviour so that a fresh salt gradient holds no attraction, and the worms move randomly across the plate. In other words, the worm has learned. Iino points out that the worms need both stimuli to 'learn' this response, and he believes that this is a form of associative learning. Armed with this knowledge, the team of three(?) set out to design a chemotaxis assay that can be used to sort out A-grade students from their less adaptable classmates.
In the first stage of the assay, well-fed adult hermaphrodite worms are transferred to a training plate, where they are conditioned with NaCl under starvation conditions for 4 h. After training, the animal’s reaction to a fresh salt gradient is tested, and the worms that have adapted show no interest in the tempting salty spot. They then test how the trained worms respond to volatile chemoattractants. In this case, the worms don’t alter their response, finding isoamylalcohol equally tempting before and after training. In other words, the behaviour is specific.
Having established that the new assay worked, they decided to use it to screen mutagenized worms with the hope of identifying individuals that didn’t catch on as quickly as their classmates. And they did: in fact they isolated three mutants that failed to learn and continued to migrate along a salt gradient, despite starvation training.
In the same way that the genome has opened up many new avenues of research, Iino's new simple learning system has expanded the range of techniques available to anyone interested in adaptive behaviour in worms. The next challenges are to expand the screen to identify more mutants with a learning deficit, and to begin unravelling the complex relationships between inputs that lead the starving worms to learn. Ultimately, Iino would like to clone and characterise the mutated genes and delve into the mechanisms of learning at the molecular level. Who knows, we might even be using a few of those learning genes ourselves, but that's a lesson for another day.
