Page 161 - Introduction to Paleobiology and The Fossil Record

148 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

developmental genes that are widely shared pairs long (see p. 186) and encodes transcrip-
among organisms and that determine funda- tion factors, proteins that switch on cascades
mental aspects of form such as symmetry, of other genes, for example all the genes
anteroposterior orientation and limb differen- required to make an arm or a leg. In this sense
tiation. Since the 1980s a major new research homeobox genes are regulatory genes; they act

field has emerged, sometimes called “evo- early in development and regulate many other
devo” (short for evolution–development), genes that have more specialist functions.

that investigates these developmental genes. The Hox genes are a specific set of homeo-
This field is exciting for paleontologists box genes that are found in a special gene

because the developmental genes control cluster, the Hox cluster or complex that is
aspects of form on a macroevolutionary scale, physically located in one region within a chro-
and so major evolutionary transitions can be mosome. Hox genes function in patterning
interpreted successfully in terms of develop- the body axis by fi xing the anteroposterior
mental genes. orientation of the early embryo (which is
The most famous developmental genes are front and which is back?), they specify posi-
the homeobox genes, identifi ed first in the tions along the anteroposterior axis, marking

experimental geneticist’s greatest ally, the fruit where other regulatory genes determine the
fl y Drosophila, but since found in a wide range segmentation of the body, especially seen in
of eukaryotes from slime molds to humans, arthropods (see p. 362), and they also mark
and yeast to daffodils. Homeobox genes the position and sequence of differentiation
contain a conserved region that is 180 base of the limbs (Box 6.3).

Box 6.3 Hox genes and the vertebrate limb

One of the greatest transitions of form in vertebrate evolution was the remodeling of a fi sh into a
tetrapod, a process that occurred more than 400 Ma in the Devonian (see p. 442). The fossils show

how the internal skeleton of a swimming fin was transformed into a walking limb. A crucial part
of this repatterning from fin to limb seemed to be the pentadactyl limb, the classic arm or leg with

fi ve fingers or toes seen in humans and most other tetrapods. But then paleontologists began to fi nd

Late Devonian tetrapods with six, seven or eight digits. How could this be explained in a world
where there was supposed to be a gene for each digit, and five was the norm?

The tetrapod limb can be divided into three portions that appear in the embryo one after the
other, and that appeared in evolutionary history in the same sequence. First is the proximal portion
of the limb, the stylopod (the upper arm or thigh), then the middle portion of the limb, the zeugopod

(the forearm or calf), and finally the distal portion, the autopod (the hand and wrist or foot and
ankle).
This evolutionary sequence is replicated during development of the embryo (Shubin et al. 1997;
Coates et al. 2002; Tickle 2006; Zakany & Duboule 2007). At an early phase, the limb is represented
simply by a limb bud, a small lateral outgrowth from the body wall. Limb growth is controlled by
Hox genes. Early in fi sh evolution, five of the 13 Hox genes, numbered 9–13, were coopted to control

limb bud development. Manipulation of embryos during three phases of development has shown
how this works (Fig. 6.9a). In phase I, the stylopod in the limb bud sprouts, and this is associated
with expression of the genes HoxD-9 and HoxD-10. In phase II, the zeugopod sprouts at the end

of the limb bud, and the tissues are mapped into five zones from back to front by different nested
clusters of all the limb bud genes HoxD-9 to HoxD-13. Finally, in phase III, the distal tip of the
lengthening limb bud is divided into three anteroposterior zones, each associated with a different
combination of genes HoxD-10 to HoxD-13. Phases I and II have been observed in bony fi sh devel-
opment, but phase III appears to be unique to tetrapods.
In the development of vertebrate embryos, there is no fixed plan for every detail of the limb. A

developmental axis runs from the side of the body through the limb, and cartilages condense from