|MadSci Network: General Biology|
Dear Zoechan, You’ve asked a very interesting question. I can give you an answer for mammals and take a try at crustaceans and plants. Last year, a group from Yale University published a very important paper on this issue in the scientific jounral, Cell: McGrath J, Somlo S, Makova S, Tian X, Brueckner M. Two populations of node monocilia initiate left-right asymmetry in the mouse. Cell. 2003 Jul 11;114(1):61-73. What this group showed was that the sweeping action of cilia that are present on the surface of cells of the early embryo (and later, too) determines the left-right body axis. This was unexpected by most people who had been interested in the process. They showed that mutations in a certain protein, classified as a “motor” protein, called left-right dynein, caused a loss of the preferred left-right patterning in mice. Motor proteins are present in all cells and they normally help move proteins and organelles along the microtubules inside cells. Microtubules are one of type of cytoskeleton filament and they are present in all cells with nuclei, from yeast to humans. Special bundles of microtubules form what is called the “axoneme”, which forms the core of flagella in algae and cilia in animal cells. The cilia, under the influence of their associated motor proteins, make a continuous, waving movement. It is this waving movement during embryo formation that establishes the left-right polarity of the early embryo, apparently by circulating growth factors and other molecules in a pattern. When the left-right dynein protein doesn’t function in its usual way, left-right patterning becomes random, i.e., half the resulting mice have the normal, left-right body pattern and half have the opposite. So it’s not really that genes have a left-right “preference,” it’s more that cell growth and gene expression patterns get set very early by cues in the embryo itself. There seems to be no real advantage to having the left-right axis one way or the other, so this protein almost certainly doesn’t exist simply to establish left-right asymmetry. People sometimes have the opposite left- right pattern from the usual, a sort of mirror image of the normal one. There doesn’t seem to be any disadvantage to this, as such people live perfectly normal lives. One very strange exception was the famous American magician, Harry Houdini, who had the opposite left-right asymmetry from normal. He died of a burst appendix. Doctors looked for his appendix to remove it, but it was on the left, not the usual right, side. Now, to go outside my expertise, I’ll take a stab at the crustaceans. Crabs or lobsters usually need to have two different types of claw, one heavy, crushing claw, and one lighter, pinching claw. I believe this happens because their bodies only permit one claw to be the heavy one, and the other is inhibited from growing more once one becomes established as the heavy one. This in turn determines which side they favor as they walk. And in plants, one may guess that the same kind of early patterning gets established as in animals, perhaps using the same microtubule-based motions, since all the components of the microtubules and their associated motors are all present in plants, too. Hope this answers your question. Paul Odgren, Ph.D. Dept. of Cell Biology University of Massachusetts Medical School Worcester
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