Re: Why do some creatures have preferance for left or right?

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