Hox cluster genes and collinearities throughout the tree of animal life.

The discovery of Hox gene clusters, first in Drosophila (a protostome) and then as homologues in vertebrates (deuterostomes), was a major step in our understanding of both developmental and evolutionary biology. Hox genes in both species perform the same overall function: that is, organization of the body along its head-tail axis. The conclusion is that the protostome-deuterostome ancestor, founder of 99% of all described animal species, must already have had this same basic Hox cluster, and that it probably used it in the same way to establish its body plan. A striking feature of Hox genes is the spatial collinearity rule: that order of the genes along the chromosome corresponds with the order of their expression domains along the embryo. For vertebrates, though not Drosophila, there is also the temporal collinearity rule: that order of genes along the chromosome corresponds with timing of Hox expressions in the embryo. Although Hox genes are clearly recognized in pre-bilaterians (Cnidaria), it is only in bilaterians that the characteristic clustered Hox arrangement and function is commonly found. Spatial collinearity in expression is conserved widely throughout Bilateria but temporal collinearity is so far limited to vertebrates, cephalochordates, and some arthropods and annelids. In addition to conserved use of Hox genes to pattern the head-tail axis, some animal groups, particularly lophotrochozoans, have extensively co-opted Hox genes, outside collinearity rules, to regulate development of novel structures. Satisfactory understanding of Hox cluster function requires better understanding of the bilaterian last common ancestor (Urbilateria). Xenacoelomorpha may provide useful living models of the ancestral bilaterian condition.