Epigenetics of Reproduction in Animals - Building the Most Complex Structure on Earth - page 105

of animals, 298,000 plants, 36,000 protozoan species, and 611,000 fungi ( Mora
et al., 2011 ). In another estimate, the number of fungi species alone may be between
3.5 and 5.1 million. Such data led to the idea that the above figures may be an under-
estimation of the real number of extant living forms on Earth. When one takes into
consideration the repeated mass extinctions of species during the last few hundred
million years, that number increases dramatically.
In 1983, researchers in Walter Gehring's laboratory in Basel, Switzerland, discov-
ered the Hox genes. Hox (from Homeobox ) genes are a group of genes that have in
common a DNA sequence (homeobox) that resides in one chromosome as a clus-
ter. Of paramount importance to the development of multicellulars, they are TFs
that bind to specific gene enhancers and function as switches for other genes. The
expression patterns of these genes determine the establishment of the body axis and
embryonic regions in eumetazoans. Several Hox genes seem to have existed in the
first eumetazoans, the common ancestor of Cnidaria and Bilateria ( Merabet et al.,
2010 ). They are conserved among animal taxa with a clear trend of increasing num-
bers, ascending the evolutionary tree in eumetazoans.
The discovery of these “master” control genes and, more recently, the unexpected
finding that no relationship exists between the number of genes and the evolution-
ary complexity or progress (a sheep has more genes than a human) contributed to
the idea that these and other genes, such as those for cell membrane receptor pro-
teins, cell adhesion proteins, and so on, represent the genetic or developmental-
genetic toolkit. The differential patterns of activation of the toolkit genes, rather than
changes in the number of genes or in DNA, determine the evolution of forms in the
living world.
The concept of the developmental genetic toolkit contributed to further amplify
scientists' amazement about the immense diversity of living forms on Earth.
However, although the discovery of the Hox genes and other regulatory genes as
the key to understanding mechanisms of animal development inspired great enthu-
siasm, it raises another problem. The high conservancy of Hox genes and many other
developmentally important genes among animal taxa do not help much, if at all, to
explain the tremendous diversification in morphology and functions within the ani-
mal kingdom. Gerhart and Kirschner (1997 ) have readily admitted that “where we
most expect to find variation, we find conservation, a lack of change.”
The fact that conservation of the genetic toolkit led to accelerated diversification
in the multicellular world is a great paradox. The solution of this conundrum requires
biologists to search for a “user” of the genetic toolkit.
A User of the Genetic Toolkit
If the genetic toolkit and other developmentally important genes are conserved, as
biologists have found, the source of the enormous diversity of living forms cannot be
related to these genes. Biologists now believe that “the animal form is not so much a
matter of the genes an animal has, but how they are used during the process of devel-
opment, of going from a single egg to the complete mature animal, the building of