Figure 9: In the developing embryo, an animal's body plan
emerges under the guidance of developmental control genes, including a
cluster called the Hox genes. Hox genes are usually
activated and expressed in the animal body in the same order as their
position in the cluster. Molecular and developmental biologists now
believe that modifications in body plans arise from modifications-for
example, duplications of genes and subsequent sequence changes-of
developmental genes, such as those found in Hox clusters. Mapping Hox
genes on the evolutionary tree suggests that the common ancestor to
fruit flies (Drosophila) and mice had one gene (gray, red and
violet striped) that presumably controlled the development of the
animal's midsection. In Drosophila this gene has become three
separate genes (gray, red and violet). These genes underwent
further expansion, duplication and sequence modifications in evolution, so
that in the mouse there are a greater number of related genes to control
and direct the formation of an abdominal region that is much more complex
than those of more primitive animals. Similar expansions and modifications
can be seen in other Hox genes as one goes from more primitive to
more advanced animals and body plans. (The Drosophila zen gene is
hatched to indicate that it does not have the same developmental role as
genes of the Hox cluster.) (Adapted from Carroll 1995.) |