Since the days of Darwin, we have greatly increased our understanding of how organisms have been shaped by evolution. We can uncover the interaction between genes and the environment by studying fossils and the processes by which animals develop. However, the evolution of behaviour is more difficult to study; progress can only be made by trying to understand and compare how different nervous systems produce behaviours, as the fossil record holds very few clues.
Historically, neuroscientists have designed their experiments based on the assumption that a behaviour found in two related species must be produced by the same neural mechanism. But is this really the case? A team of scientists led by Paul Katz at Georgia State University has tackled this question by looking at how the nervous systems of two closely related species of sea slug (Melibe leonina and Dendronotus iris) produce their side-to-side swimming behaviour.
The core of the neural swimming machinery of Melibe consists of two pairs of cells, swim interneuron 1 (Si1) and swim interneuron 2 (Si2), which are interconnected and fire bursts of action potentials in phase with the swimming motion. The first experiment the team performed was to identify the homologues of these cells in Dendronotus, which they did by injecting cells in the homologous area of the brain with a fluorescent dye.
The team found two pairs of cells that fitted the bill. The cells looked similar, had projections to the same areas within the brain, and had the same neighbouring cells as their Melibe counterparts.
After identifying the Si1 and Si2 homologues, the team tried to find out whether they could have the same role in the two species by measuring whether the patterns of electrical activity of the cells were similar.
They found that while the Dendronotus Si2 homologue did have very similar activity patterns to those of Melibe, the Si1 homologue did not. It did not fire action potentials in phase with the swimming movement, which means that it is not part of the core neural swimming machinery and suggests that there are differences in how the two nervous systems produce the swimming behaviour.
To further study the potential differences between the two species, they characterized the pattern of connectivity of Si1 and Si2 in Dendronotus.
The team saw that there are a number of differences in how the cells are wired up. For instance, Dendronotus overall has fewer inhibitory connections, lacking those between Si1 and Si2 altogether. The cells therefore function differently in the production of a similar behaviour.
The team's findings show that similar behaviours in closely related species do not necessarily have the same underlying neural mechanisms. In order to unravel the evolutionary history of the side-to-side swimming behaviour in sea slugs the team will have to look at other closely related species to see which neural mechanism must have been used by the common ancestor. Furthermore, their findings have important practical implications for neuroscientists and should serve as a cautionary tale for those studying the evolution of behaviour.
