In Ian Fleming’s novel Dr No, James Bond’s nemesis Julius No survives a murder attempt thanks to a rare medical condition. The shot that is fired into the left side of his body misses its mark because No’s heart is located on the right. Situs inversus is a congenital condition that reverses the location of the internal organs in the body and affects fewer than one person in 10,000. It is strange how, given the symmetry of our external bodies, we are so asymmetrical on the inside. It is one of the intriguing challenges of foetal development how this combination of symmetry and asymmetry evolves as an embryo grows. Recent research, though, into the way a snail’s shell spirals may throw new light on this puzzle.
A new embryo starts as a single cell with perfect symmetry. As it begins its growth it divides into two cells whose combination has mirror symmetry. Two cells double to four cells, which tend to arrange themselves into a square formation. This dividing and doubling continues as an extraordinarily complex organism evolves. Some of that early symmetry has been retained as the mirror symmetry we display externally. But internally our bodies are asymmetrical: our heart is on the left, appendix on the right, the left lung has two lobes while the right has three.
How the simple symmetry and its breaking evolves during the growth of the embryo is intriguing. Clearly our genetic code is an important part of managing the growth — as the cellular structure of the outcome bares a striking resemblance to the people whose DNA combined to begin the process of dividing that first cell.
But recent research from Japan on the growth of snails gives some insight into how the environment can have a big influence on what happens at each stage of development. The previous arrangement of cells at one stage will have an important bearing on how the cells are configured when they next double. Change that arrangement and you can dramatically alter the resulting organism.
If you place a spiralling shell down on the table with the spiral facing up then in most shells the spiral evolves in a clockwise orientation from the centre of the shell out. But this is not always the case. Some species of snail have shells that spiral out in an anticlockwise direction, but it is much rarer. However there is one species of pond snail, Lymnaea stagnalis, which demonstrates examples of both clockwise or anticlockwise spiralling shells. The orientation of the spiral is determined by the genetic make-up of the snail’s mother.
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Professor Reiko Kuroda and her colleagues at Tokyo University were intrigued to find out if they could cause a genetically clockwise shell to reverse its direction during embryonic development. Remarkably, by giving the cells a little push in the right direction, they could overcome the genetic programme. The crucial stage of development in which you can change the direction of the spiral turned out to be surprisingly early on. It seems that when four cells divide into eight the asymmetry sets in and the shell begins to spiral one way or the other. The four new “daughter” cells, which appear at this stage, sit on top of the first four cells arranged in a square. The genetic code decides whether they sit with a slight twist towards clockwise or anticlockwise.
But by giving the four cells sitting on top a little push with two glass rods, the Japanese team found that it could alter the orientation of the cells and later development of the snail would spiral in the opposite direction to the one determined by the genetic code. It seems that the development of the shell from this point is determined more by the previous arrangement of cells as they divide, rather than information coming from the DNA of the cells.
It wasn’t only the spiral of the shell that was altered. As with human beings, the internal organs of the snail are also arranged asymmetrically. If the spiral of the shell is changed at the eight-cell stage then the orientation of the snail’s heart, stomach, liver coiling and gut looping are similarly flipped.
Of course, this prod with a stick has not changed the genetic make-up of the snail so that mother snails that were genetically meant to be clockwise and give birth to clockwise prodigy would still do so, even though they might have been pushed by the Japanese team into developing an anticlockwise shell.
The research on the humble pond snail is likely to have important implications beyond perceiving the spiralling of shells. It could help us to understand the moment when asymmetry sets in during the development of other species similar to ourselves.
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Cool fact
If you take the molecular structure of the drug LSD and manufacture a mirror image of the molecule then the resulting chemical has no hallucinogenic effect on the body.