[...]
Among the most obvious features of cell differentiation is that less
complex organisms possess fewer cell types than more complex organisms.
For example, yeast has three cell types, the coelenterate hydra has 15 to
17, annelid worms have about 60, and humans about 250. A plot of the
number of cell types in an organism as a function of the estimated number
of genes in that organism shows a profound and simple relation: The
number of cell types increases as about a square-root function of the
number of the genes [...]. Why should such a remarkably simple relation
exist across such a wide range of phyla? Yeast, hydra, annelides, and
vertebrates diverged around 600 million years ago. It is hard to believe
that such a relationship is a heritable remnant of a property derived from
a primitive archaen ancestor in the Cambrian or Precambrian as a result
of propinquity of descent. Perhaps instead selection has directly opted
for this property? Perhaps not. I shall argue that this strong
corellation across many phyla is an indirect and virtually inevitable
property of the kinds of cybernetic gene-regulatory systems which
control cell differentiation and ontogeny. [...]
Simple organisms have fewer cell types than complex organisms. For
example, yeast has three, as distinguished above. Bacteria may have two:
vegetative and spore forms. The mold Neurospera has about five [...]. By
similiar criteria, the hydra, a simple coelenterate, has 13 to 15 [...],
as do sponges [...]. Jellyfish are more complex, with perhaps 20 to 30
cell types [...]. Annelids have about 60 cell types [...], and adult
humans about 254 [...]. Therefore, there must be some simple relation
between the number of genes in an organism, and the number of its cell
types. If it is hard to count numbers of cell types, it is also hard to
count the number of genes. The simplest way to count genes is to count
total DNA per cell, a not too unreasonable choice. [...]
In [...], I show a graph if the number of cell types, based on
histological criteria, versus the total DNA content per cell, across a
wide range of phyla. The data, on a log-log plot, are quite close to
linear, indicating that the corellation between amount of DNA and number
of cell types is a power law. Specifically, the data suggest that the
number of cell types in an organism is crudely proportional to the square
root of the DNA content per cell. This means that, in evolution, adding
the next cell type has required ever greater amounts of DNA. Suppose
instead that we consider, for any organism, the number of cell types
versus the estimated number of RNA sequences transcribed in that
organism. Thus for yeast, the estimated number of cell types is 3 and the
number of transcribed genes is 4000. In Drosophila, as in annelids, the
number of cell types might be about 60 and the transcribed complexity
about 17000. In humans the number of cell types is 200 to 300 and the
transcribed complexity is about 100000. Thus on the basis of measured
complexity of RNA sequences, the number of cell types increases roughly
linearly with, or perhaps a bit faster than, the number of genes. It
certainly does not increase much more rapidly than linearly. That is, the
number of cell types does not increase as the number of genes squared or
as an exponential function of the number of genes. Since total DNA is an
overestimate of the number of genes and transcribed complexity is an
underestimate, we might safely guess that the true relation is somewhat
less than linear. This statement says that, in evolution, progressively
more genes are requred for each additional cell type. Although it fits
the general notion that more regulatory genes are needed to coordinate
gene expression, this image is quite naive. It is trivial to construct
model genetic networks in which the number of model cell types increases
_exponentially_ as the number of genes increases. Exponential growth is
faster than any power law.
In short, it is very much a nontrivial observation, across many phyla,
that the number of cell types increases at something like a linear or
square-root function of the total number of genes in an organism.
Obviously, we would like to know why that might be true. And we might
begin to wonder whether a property such as this splayed across many
phyla, represents selection or some deeper feature of genome regulatory
system. [...]