Figure 21–31 The vertebrate body plan as a dorsoventral inversion of the insect body plan. Note the correspondence with regard to the circulatory system as well as the gut and nervous system. In insects, the circulatory system is represented by a tubular heart and a main dorsal blood vessel, which pumps blood out into the tissue spaces through one set of apertures and receives blood back from the tissues through another set. Unlike vertebrates, insects have no system of capillary vessels to contain the blood as it percolates through the tissues. Nevertheless, heart development depends on homologous genes in vertebrates and insects, reinforcing the relationship between the two body plans. (after e.L. Ferguson, Curr. Opin. Genet. Dev. 6:424–431, 1996. With permission from elsevier.)
Bcd, Lab Pb a posterior character. Conversely, loss of posterior Hox genes allows the posterior tissue where they are normally expressed to adopt an anterior character (Figure 21–33). Because of a redundancy between genes in the four Hox gene clusters, the transformations observed in mouse Hox mutants are not always so straightforward as those in the fly, and they are often incomplete. Nonetheless, it seems clear that the fly and the mouse use essentially the same molecular machinery to impart individual characteristics to successive regions along at least a part of the A-P axis.
Just as there are genes that regulate pattern formation and segmental identity, there are genes whose products act as triggers for the development of a specific cell type or even a specific organ, initiating and coordinating the whole complex program of gene expression that is required. An example is the MyoD/myogenin
Figure 21–32 The Hox complexes of an insect and a mammal, compared and related to body regions. the genes of the Antennapedia and Bithorax complexes of Drosophila are shown in their chromosomal order in the top line. the corresponding genes of the four mammalian Hox complexes are shown below, also in chromosomal order. the gene expression domains in fly and mammal are indicated in a simplified form by color in the cartoons of animals above and below. there is a remarkable parallelism. however, the details of the patterns depend on developmental stage and vary somewhat from one mammalian Hox complex to another. also, in many cases, genes shown here as expressed in an anterior domain are also expressed more posteriorly, overlapping the domains of more posterior Hox genes.
the complexes are thought to have evolved as follows: first, in some common ancestor of worms, flies, and vertebrates, a single primordial homeotic selector gene underwent repeated duplication to form a series of such genes in tandem—the ancestral Hox complex. In the Drosophila sublineage, this single complex became split into separate Antennapedia and Bithorax complexes. Meanwhile, in the lineage leading to the mammals, the whole complex was repeatedly duplicated to give four Hox complexes. the parallelism is not perfect because apparently some individual genes have been duplicated and others lost. Still others have been co-opted for different purposes (genes in parentheses in the top line) over the time that has elapsed since the complexes diverged. (Based on a diagram courtesy of William McGinnis.)