19 August 2010

Asymmetry in snails (and people...)

I once tried to make a pair of earrings from two snail shells, each a mirror-image of the other. The mollusc I have spent (or wasted) my career on comes, almost always, with the opening on the right hand side of the shell. Very occasionally – perhaps once in 10,000 times – a reversed, or "sinistral", individual turns up, giving the opportunity for earrings...

Why should a mollusc – or anything else – veer to the left (or for that matter to the right)? Now the culprit has been identified. It is a gene called nodal, which is active on the right side of the embryo of right-coiling snails, and the other side in the leftists...

In plants, the framework of every cell is built of a certain protein whose units slot together into long hollow cylinders. In a mustard much used in genetics, a mutation called "lefty" enlarges one end of each building block and causes the normally upright stem to spiral to the left, while another version does the opposite...

And what about us? We have plenty of asymmetries of our own: in almost everyone, the heart is on the left side and the liver on the right. And, as at least half of the population knows, there is also an interesting lopsidedness in the male genitalia. Sometimes, though, those patterns disappear. In around one person in 10,000, the heart is on the right; and in even fewer the whole contents of the abdomen are a mirror image of normal...

Another unexpected discovery is that identical twins are often mirror images of each other, at least above the neck. The hair whorl on the back of the head often twists one way in one member of the pair, and another in the twin. In the same way, if one has a single wisdom tooth on the left, the other tends to have theirs on the right...
I had never realized that "sinistral" snails were so rare.

Photo credit to Pet Snails.


  1. Are ALL sinistral shells rare? Or the species s/he studies?

    I heard somewhere also about people who have reverse symmetry - heart on the right, various ducts emptying to the "wrong" side, etc.

  2. Hmm, did you intend for all four of your links in this text to be identical?

  3. Yes, I did. You'll find I do the same thing on many posts with extended text.

    The reason is that it serves as a type of "highlighting" for the many visitors who don't have time (or don't want to take time) to read a lengthy piece.

    I've tried bolding text, but this seems to be more effective.

  4. I am much more interested in the lopsidedness of male genitalia than snails, so I was kind of misled when that link went to the same place.

  5. Noumenon - Here you go:



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