Loud, here is your answer to why a single cell decided to split .... the first part does not address your question but nevertheless is still interesting .....i hope you will read it although it is quite lengthy
MadSci Network: Evolution
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Re: How does a cell evolve from a single-cell to a multicellular organism??
Date: Thu May 10 14:10:20 2001
Posted By: Joseph E. Armstrong, Faculty, Botany, Illinois State University
Area of science: Evolution
ID: 989341859.Ev
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Message:
You've asked an interesting question, actually two questions, one about
the origin of multicellularity and another about differentiation during
development. Your basic information is correct. Prior to cell division,
genetic material is copied. As you probably know, the DNA molecule has
two complementary halves; each half acts as a template to form a new
complementary half. Mitosis is a process that separates the two identical
sets of chromosomes. Usually mitosis is coupled with cytokinesis, a
process that divides the cytoplasm producing two separate cells. Since
multicellular organisms arise from a single cell, every cell has exactly
the same genetic information.
It seems most logical to answer the last question first. First
multicellularity exists in a range of different forms. In some
multicellular organisms all cells are identical and therefore perform
similar functions. In other multicellular organisms cells may
differentiate into specialized forms for specialized functions. In simple
organisms, only occasional cells may have specialized functions. In
others virtually all cells may become specialized to form familiar tissues
and organs. But while all these specialized cells are genetically
identical, different sets of genes, different portions of these genetic
instructions, are being followed. The developmental process is how cells
obtain information about their "location" and what set of instructions to
follow. Developmental biology studies how genes are regulated, turned on
and turned off, as an organism develops from a single cell. It's sort of
like a map to a city. All maps are identical, but different people follow
different routes to arrive at different destinations.
Very simple organisms have much simpler genetic programs. Consider a very
simple organism, a filamentous algae. A filament is a chain of identical
cells, although some filamentous algae do produce specialized cells. The
developmental program, in descriptive language, says to a cell, divide at
right angles to your long axis. This set of instructions is simply
repeated to produce a filament. If a second set of instructions was
added, a more complex form could be produced. For example, every 10th
time, divide parallel to your long axis then return to dividing at right
angles. This will produce a branching filament. Obviously more complex
forms develop from the addition of new sets of instructions, which must be
derived from the instructions that already exist. In other words, you
wouldn't expect a whole new set of instructions to just magically appear;
you might however expect a new set of instructions that are just a little
different from some set of instructions the cell already has. And we know
how such differences happen, mutations during DNA duplication.
Now let's address your first question. How did multicellularity arise?
Obviously from a small change in the instructions on how to divide into
two separate cells, the most common form of asexual reproduction. If for
example you were to watch a large number of Chlamydomonas cell divide, you
will occasionally see a double cell resulting from a failure of
cytokinesis. Perhaps this happens because of some mutation that causes a
failure to separate. Most likely the failure will result in the early
death of the double cell. This is a form of natural selection and an
example of how it works, in this case by weeding out mutations. But
suppose the mutation only causes separation failure, say once every 20
divisions or so. Since it works OK 19 out of 20 times, the mutated
instructions survive in the population even if the one of 20 dies. But
how could this become the regular case? If for some reason the double
cell found itself at an advantage, where it thrived instead of died, then
natural selection would be reversed to favor the mutation. Now I have no
idea about what might cause selection for a double cell that could lead to
multiple celled colonies and finally a magnificent organism like Volvox.
However, let's consider another example, a very simple seaweed, a single
celled algae anchored to a rock. Space is very limited on something like
a rock, so a larger size would allow this simple seaweed to compete better
for space and therefore light. So maybe it spreads out broadly, and this
works fine, until a different simple seaweed moves into the adjacent
space. Rather than have a short, broad cell, the new seaweed produces a
tall, slender cell that shades the short, broad cell, so the tall slender
cell is a better competitor for light under these conditions. Maybe a
third even taller seaweed arrives, but this competitive race will end
because there is a size limitation on single cells for both functional
reasons (cells must maintain a functional volume to surface area) and
structural reasons (big water balloons break easier than small water
balloons). If one of these single celled seaweeds had a mutation to its
genetic program for dividing and failing separate every 20th time, similar
to the one described above, then the production of a filament would be of
real competive advantage allowing an even bigger size via
multicellularity. So that's the basic answer. What was originally a
mistake in the genetic instructions to divide and separate became
instructions for making a simple multicellular organism (divide the cell,
but don't separate), and for some reason, in this case compeition for
light, the multicellular condition was an advantage.
This is how evolution works. A random process, mutation, causes variants,
many of which aren't functional at all. But some variants exist.
Changing environmental conditions select among variants, favoring some and
not others. Favored sets of instructions, even altered ones, become a
part of the organism's genetic instructions. Complex sets of
developmental instructions represent an accumulation of successful
mistakes.
Again think of the map analogy. You learn a new route to a new location
by retracing part of an old route and adding some new instructions to it.
Where's the new Thai restaurant (I'm always hoping)? Well, you know how
to get to the grease-burger palace? Well, rather than turning right by
the school, just go one block further and turn right. I found it by
accidentally missing the turn. And then you discover you like Thai food
so much, you never return to the burger palace. So now the instructions
have become the route to phad thai not to fries. Where's the bicycle shop
where you got the cool recumbent bike that replaced your gas guzzler
(still always hoping)? Well, you know how to get to Bangkok Palace? See
how things develop?
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