The Questions of Developmental Biology

Hox Genes: Descent with Modification
As mentioned throughout this book, the expression of Hox genes provides the basis for
anterior-posterior axis specification throughout the animal kingdom. This means that the
enormous variation of morphological form in the animal kingdom is underlain by a common set
of instructions. Indeed, one of the most remarkable pieces of evidence for deep homologies
among all the animals of the world is provided by the Hox genes. As mentioned in Chapter 11,
not only are the Hox genes themselves homologous, but they are in the same order on their
respective chromsomes. The expression patterns are also remarkably similar between the Hox
genes of different phyla: the genes at the 3 end are expressed anteriorly, while those at the 5 end
are expressed more posteriorly (Figure 22.2). As if this evidence of homology were not enough,
Malicki and colleagues (1992) demonstrated that the human HOXB4 gene could mimic the
function of its Drosophila homologue, deformed, when introduced into Dfd-deficient Drosophila
embryos. Slack and his colleagues (1993) postulated that the Hox gene expression pattern defines
the development of all animals, and that the pattern of Hox gene expression is constant for all
phyla.*
If the underlying Hox gene expression is uniform, how did the differences among the
phyla emerge? It is thought that they arose from differences in how the Hox genes are regulated
and what target genes the Hox-encoded proteins regulate. Gellon and McGinnis (1998) have
catalogued four critical ways in which variation in Hox expression patterns might lead to
evolutionary change (Figure 22.3):
Changes in the Hox protein-responsive elements of downstream genes
Changes in Hox gene transcription patterns within a portion of the body
Changes in Hox gene transcription patterns between portions of the body
Changes in the number of Hox genes