Wednesday, April 04, 2007

Right-left -- Part 1

If there's a national food of the Bahamas, it's conch, the hefty sea snail that thrives in Bahamian waters.

Like many snails of land and sea, the conch secretes an ever-enlarging cone-shaped shell as it grows. The shell has a whorl, and the whorl is almost always right-handed; that is, it curls clockwise when viewed from the point of the spire. Rarely, a conch has a left-handed whorl. Such oddities are called "sports," and they are valued as collector's items.

Only a very few species of snail -- the Perverse Whelk, for instance -- commonly curl in a left-handed fashion. The handedness of the whorl is genetically determined, but there would appear to be no other reason why a snail's shell should curl one way rather than the other. Both sorts of snails exist in nature, mirror images of one another.

Honeysuckle takes on a left-handed twist as it grows; some other vines spiral to the right. Most vertebrates, including humans, are bilaterally symmetric, externally at least. Inside it's a different story. Our hearts are off-center and have a twist. So do our intestines.

The handedness of a vertebrate's internal organs is constant for a given species. Like the "sport" conch, a mirror image arrangement of internal anatomy sometimes happens, even for humans. Normal organ placement is called situs solitus; the mirror image is called situs inversus. Generally, an individual with all organs reversed suffers no ill effects, but partial reversals can be damaging.

In recent years, scientists have been figuring out the subtle genetic signals that cause a developing embryo to twist one way rather than the other. A gene called Pitx2 seems to play a crucial role in the internal handedness of vertebrates.

The twisted fabric of life goes much deeper than spiral shells or lopsided hearts. Even the very molecules of life have a handedness, or chirality. Proteins and DNA have a helical twist, which is constant across all domains of life. More basic molecules, such as sugars and amino acids (of which proteins are made), come in both right- and left-handed forms, but all life on Earth uses only right-handed sugars (dextrose) and left-handed amino acids.

There would appear to be no reason in principle why creatures made of right-handed amino acids or left-handed sugars couldn't exist. The exclusive use of one chiral form of molecules rather than the other is evidence for the relatedness of all terrestrial life by common descent, and the conservation of handedness over time.

(More tomorrow.)