The Questions of Developmental Biology

Another set of genes that controls the formation of the fly head (orthodenticle and empty
spiracles) has homologues in the mouse that show expression in the midbrain and forebrain.
While Hox genes appear to specify the anterior-posterior axis throughout the vertebrates,
we shall discuss mammals here, since the experimental evidence is particularly strong for this
class. The mammalian Hox/HOX genes are numbered from 1 to 13, starting from that end of each
complex that is expressed most anteriorly. Figure 11.36 shows the relationships between the
Drosophila and mouse homeotic gene sets. The equivalent genes in each mouse complex (such as
Hoxa-1, Hoxb-1, and Hoxd-1) are called a paralogous group. It is thought that the four
mammalian Hox complexes were formed from chromosome duplications. Because there is not a
one-to-one correspondence between the Drosophila Hom-C genes and the mouse Hox genes, it is
likely that independent gene duplications have occurred since these two animal branches diverged
(Hunt and Krumlauf 1992; see Chapter 22).
Expression of hox genes along the dorsal axis
Hox gene expression can be seen along the dorsal axis (in the neural tube, neural crest,
paraxial mesoderm, and surface ectoderm) from the anterior boundary of the hindbrain through
the tail. The different regions of the body from the midbrain through the tail are characterized by
different constellations of Hox gene expression, and the pattern of Hox gene expression is
thought to specify the different regions. In general, the genes of paralogous group 1 are expressed
from the tip of the tail to the most anterior border of the hindbrain. Paralogue 2 genes are
expressed throughout the spinal cord, but the anterior limit of expression stops two segments
more caudally than that of the paralogue 1 genes (see Figure 11.36; Wilkinson et al. 1989;
Keynes and Lumsden 1990). The higher-numbered Hox paralogues are expressed solely in the
posterior regions of the neural tube, where they also form a "nested" set.