MadSci Network: Development |
Symmetry is one of those underlying phenomena of nature that has intrigued scientists, philosophers, and theologians for millenia. From our human perspective, we see our two hands that appear as mirror images of each other and grow and develop equally, prompting the question: how does the left hand know what the right hand is doing? It has taken all of those millenia for biologists to discover that symmetry is actually the simplest pattern to develop, and that generating asymmetries is by far more costly and complicated. To better understand the origins of symmetry, let's look at the evolutionary progression of developmental pattern formation in the metazoa.
First, let's
look at the case of nonsymmetry. Growth can be characterized as
nonsymmetric if
there are no intrinsic factors that differentiate the speed and extent of growth
of one part of the organism from any other. Sometimes, this results in an
amorphous blob, but usually, nonsymmetric growth results in something
nearly
spherical, like the sponge pictured here. If you think of a mass of cells
growing equally in all directions, the most likely result would be a sphere;
however, often extrinsic factors like heat, light, food, or disease can cause one
part of the organism to grow more or less than neighboring parts, resulting in a
more amorphous shape. Notice that even in this sponge, the surface features are
unique for any given spot, such that on closer inspection it can almost be
considered amorphous. So then, let's add a single axis of polarity
, and see what happens to the organism's symmetry.
The simplest way to do this is to use a gene to
define a simple structure - like "mouth" - and create a gradient of the product
of the gene across the organism so that the area that expresses the most "mouth"
gene will become the head (rostral: toward the nose), and the area that
expresses the least will become the tail (caudal: toward the tail). In
reality, the "one gene per axis" idea does not hold. Most body axes throughout
the metazoa are defined by at least two genes: in this case, a head gene and a
tail gene. Anyway, the result of introducing this axis is radial symmetry, as
demonstrated by the hydra shown here or by its jellyfish and sea anemone
relatives. This same pattern is seen in the earliest nematode worms, which are
essentially feeding tubes. Most nematodes, however, are not completely
symmetrical - each has a single ventral pore through which they lay eggs. The
location of this pore is determined by "ventral genes" that organize the worm's
reproductive tract to empty into the pore.
With a little more modification, these "ventral
determinants" can be used to generate another axis of polarity: the dorsal
(toward the back) - ventral (toward the belly) axis. The next step up
from the nematode is the platyhelminth or "flatworm" (the nudibranch shown
here is actually a mollusk, but the axes are the same), which has a
head and a tail as well as a top and a bottom. The dorsoventral axis is defined
genetically very similarly to the rostrocaudal axis. There are genes for
"dorsal" and genes for "ventral", and cells which encounter more of the dorsal
signal than the ventral will differentiate to form dorsal structures, and vice
versa. With these two axes we get the bilateral symmetry that has been part of
the basic
coelomate body plan for over 600 million years. In fact, it was during the
Cambrian Era (540 - 500
mya) that this bilaterally symmetric body plan was adapted and modified into all of
the forms seen today, including earthworms, slugs, beetles, and humans.
Actually, humans bring up another point: what happens if you add a third
axis of polarity to your bilaterally symmetric animal?
The answer is: asymmetry. Humans, in fact all
vertebrates, also develop a left - right axis of polarity which is
essential for keeping the heart, stomach, and spleen on the left side of the body
cavity, and the appendix and liver on the right side. However, this isn't the
random asymmetry seen in the sponges, but a very ordered asymmetry involving
several genes controlling each axis, with
congenital
defects resulting from misexpression of these genes. To add to the
confusion, there is one more polarity that I haven't mentioned yet, but that has
been obvious in each of the organisms mentioned so far. In each, the cells/tissues closest (proximal: "close") to the rostrocaudal axis, or center in
the case of the sponge, are different from those farthest away (distal:
"distant") from this axis. As with the other polarities, there are proximal
genes and distal genes which tell the cells where they are in the body, and what
to become. Worse still, not all cells are responsive to all of these axes, e.g.
while the heart is always on the left side of the chest, and the lungs are
asymmetric to leave it some room, the ribs and muscles which form the thoracic
cavity are completely symmetric. So the external symmetry seen in the human
body, or any vertebrate body, occurs independently of the left-right asymmetry
important to the internal organs.
So then, to answer your question (finally), the symmetry seen throughout the animal kingdom requires only a handful of genes. These genes establish a simple three-dimensional field that gives sufficient locational information to the developing cells to guide their proper differentiation. Now, let's return to the example of left and right hands. From the perspective of the body axes, the hands are identical: the thumb is rostral to the fingers; the fingers are distal to the palm; and the palm is ventral to the knuckles. So for the cells forming each hand, these are the cues that determine their general orientation. The more specific determinants, like for the number or length of the fingers, are controlled by a single set of genes that are used identically for both hands. So to get longer fingers on the left hand would require longer fingers on the right hand; in fact, it would probably also result in longer toes - while the rostrocaudal determinants (and several homeotic genes) differentiate forelimbs from hindlimbs, the dorsoventral and proximodistal determinants are the same. As you point out, this allows for evolution to act symmetrically on a bilateral organism, or radially on an organism like a starfish. (I'll leave plants for another discussion, except to point out that "fractal" growth would by definition require a very simple genetic program being iterated throughout the growth of the organism.)
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