The Questions of Developmental Biology

But in another organism, the same pathway may activate the genes responsible for
making a photoreceptor. And in a third organism, the pathway activates the genes needed to
construct a vulva.
Pathways undergo descent with modification, too. This is readily seen in the Wnt
pathway that we have discussed through-out the book. Figure 22.13 shows how the Wnt pathway
is used in several different organisms. The pathways are homologous, but not identical. They are
thought to have originated in a common ancestral pathway that predated the deuterostomeprotostome
split.
Earlier in this chapter, we discussed genes such as Pax6 and tinman that appear to have
had their developmental functions before the protostome-deuterostome split. We have also
discussed homologous pathways that may or may not be used in similar structures.
However, there appear to be some pathways that are used to form the same structure in all
animals. When homologous pathways made of homologous parts are used for the same function
in both protostomes and deuterostomes, they are said to have deep homology (Shubin et al.
1997).
Instructions for forming the central nervous system
One example of deep homology has already been discussed in earlier chapters.
First, as seen in Chapter 10, the chordin/BMP4 pathway demonstrates that in both vertebrates and
invertebrates, chordin/Short-gastrulation (Sog) inhibits the lateralizing effects of
BMP4/Decapentaplegic (Dpp), thereby allowing the ectoderm protected by chordin/Sog to
become the neurogenic ectoderm. These reactions are so similar that Drosophila Dpp protein can
induce ventral fates in Xenopus and can substitute for the Sog protein (Holley et al. 1995).
In addition to this central inhibitory reaction of chordin/Sog inhibiting BMP4/Dpp, there are other
reactions that add to the deep homology of the instructions for forming the protostome and
deuterostome neural tube. For instance, the spread of Dpp in Drosophila is aided by Tolloid, a
metalloprotease that degrades Sog.