Re: ORIGINS: multicellularity

David Campbell (
Wed, 19 Feb 1997 16:43:47 -0500

>Some of the simpler animals have a few cells, and they "branch" or divide
>to reproduce. So, do their cells, like people's, have the same DNA but
>are differentiated?

Some animals (sponges) and all plants have the ability to regrow an entire
individual from any cell, thus showing that all the necessary DNA is
present. Most animals manipulate the function of cells to the point that
this is not possible, but still the DNA in different cells is the same
(assuming no mutation).

>If A has a given function, and B has a given function, but they both have
>the same DNA, then my hypothesis is that they could not have evolved. The
>common text-book idea of the origin of multicellularity is that various
>cells developed inter-dependence. Then, they somehow got together.
Another possibility is that a single cell divided into multiple cells.
This doesn't affect your arguement-the question of how they differentiate

> But
>if A & B have the same DNA but a different environment, making a different
>expression of the DNA, then their whole scenario brakes down, I believe.
Many studies of gene regulation have found that DNA (with interacting
proteins) readily behaves differently in different environments (Ptashne, A
Genetic Switch [2nd ed., 1992], is a detailed account of one particularly
well-studied example, with discussion of others). Differences in the
cellular environment are a major control of development in complex
There is more of a "jump" to explain in the question of how did the
organism get an environmental difference between cells (or parts of cells)
to respond to? A developmental example discussed by Ptashne has the
initial protein gradient established by maternal cells. This won't work
for the initial evolution of cellular differentiation. Perhaps after a
group of identical cells has evolved association, differential position in
the colony could develop a gradient. I do not know what studies have been
done on this issue; a likely place to look would be research on the
"volvocine series". A group of green algae includes single-celled species,
species with a few identical cells, etc. up to Volvox, with a few hundred
cells forming a ball. Volvox also has some specialization of cells, with
some specialized for locomotion and others specialized for reproduction.
Any biochemical studies on them would tie in closely to your question,
since the series is considered a model of how multicellularity could

David Campbell

"Old Seashells"
Department of Geology
CB 3315 Mitchell Hall
University of North Carolina at Chapel Hill
Chapel Hill NC 27599-3315

"He had discovered an unknown bivalve, forming a new genus"-E. A. Poe, The
Gold Bug