Natural Asymmetries

When imbalances gain the upper hand

I have written about symmetry once and had pointed out that many animals (and humans) apparently exhibit an innate preference for symmetric over asymmetric patterns. And yet, if we examine Nature and her organisms more closely we find numerous examples of asymmetries. It starts already with the “Big Bang” and the atomic organization of matter and antimatter; you’ll find it with L (left) and D (right) forms of polarized light rotating sugars like glucose (the latter chirality very common, the former very rare). And you’d wonder why cells use only the L-forms of amino acids to build left-handed proteins, when the D-form is nothing but the right-handed mirror image of the L form. But we can move on to whole organisms and although most are at least outwardly symmetric, internal organs (as a look into the human body confirms) are frequently not.

What is really weird is that some animals, not at all symmetric as adults, start their lives as perfectly symmetric creatures. At some point in their development their genetic code then leads them to construct asymmetries. Crustaceans with asymmetric claw sizes, carried to an extreme amongst the fiddler crabs, come to mind, but snails are an even better example. To find a symmetry axis that neatly divides a snail into identical halves is impossible and yet, embryologically snails start their lives as symmetric organisms. The information about the future shell “twist” (whether left or right), in which the anus ends up near the head, is actually not present until the third cleavage when 4 embryonic cells become 8.

In flatfish (like the sole and plaice) processes leading to asymmetries also take place, but instead of the whole body undergoing twisting, it is the eyes that gradually shift to one side of the body and the fish acquiring an increasingly lopsided stance and swimming orientation – until the fish has turned the larval left and right body sides into ventral and dorsal surfaces of an adult complete with their characteristic lighter and darker skin colorations. Deep sea Histioteuthidae squid don’t go as far as the flatfish with their eye development, but swimming sideways, one of their two eyes (presumably the one peering upward) is always larger than the other eye. When during development this asymmetry starts and whether it is influenced by the direction of the light are interesting but on account of the rarity of these squid (and the difficulties in keeping them alive) scientifically hard to tackle questions. Off the coast of Namibia I have once come across a fish termed Neoharriotta sp., which like some of its holocephalian relatives have asymmetric faces, but nobody knows why.

However, what prompted me to write this essay was a chat I had with the Swedish botanist Magnus Lidén when visiting the Himalayas: he showed me examples of asymmetric flowers. Many species of the orchid genus Pedicularis possess highly asymmetric inflorescences, which I found intriguing, because most flowers I knew either exhibited bilateral or radial symmetries (something that pollinators like bees seem to recognize – and like). Whom the asymmetric Pedicularis flowers entice to pollinate them, how these pollinators “handle” the asymmetric flowers and whether such pollinators also possess an innate preference for asymmetry would therefore be an intriguing question to follow up – and of course a great excuse to visit the Himalayas again!

dog japan best friend human origin

If you want to read the story of the origin of our best friend. Click here for “Dog Genetics and japanese origins”

© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2017.
Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com with appropriate and specific direction to the original content.

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