Time is of the essence for ParaHox homeobox gene clustering
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COMMENTARY
Open Access
Time is of the essence for ParaHox homeobox gene clustering Myles Garstang and David EK Ferrier*
Abstract ParaHox genes, and their evolutionary sisters the Hox genes, are integral to patterning the anterior-posterior axis of most animals. Like the Hox genes, ParaHox genes can be clustered and exhibit the phenomenon of colinearity - gene order within the cluster matching gene activation. Two new instances of ParaHox clustering provide the first examples of intact clusters outside chordates, with gene expression lending weight to the argument that temporal colinearity is the key to understanding clustering. See research articles: http://www.biomedcentral.com/1741-7007/11/68 and http://www.biomedcentral.com/1471-2148/13/129
Homeobox cluster integrity and colinearity The ParaHox genes consist of the Gsx, Xlox and Cdx families, involved in the anterior-posterior development of the nervous systems and guts of animals. The discovery of the ParaHox gene cluster in the basal lineage of chordates, the Cephalochordata or amphioxus, revolutionized our understanding about the origins and evolution of the paradigmatic Hox gene cluster, famed for its role in patterning the anterior-posterior axis in animal embryogenesis [1]. Instead of the Hox cluster evolving in isolation as a single homeobox gene cluster that arose via successive tandem duplications of an ancestral UrHox gene, an ancestral ProtoHox cluster seems more likely, this ProtoHox cluster then duplicating or splitting to give rise to the Hox and the ParaHox clusters. This ProtoHox hypothesis is based upon the three ParaHox genes (Gsx, Xlox and Cdx) not only being another example of a homeobox gene cluster, but the genes also being intermingled with the Hox genes in molecular phylogenetic trees, and the ParaHox cluster also * Correspondence: [email protected] The Scottish Oceans Institute, University of St Andrews, East Sands, St Andrews, Fife, KY16 8LB, UK
exhibiting the phenomenon of colinearity. We now have ParaHox clusters from an echinoderm, the sea star Patiria miniata, as well as the hemichordate Ptychodera flava, to compare to those of chordates to further resolve the parameters of homeobox clustering, colinearity and the ancestral functions of the ParaHox genes [2,3]. Colinearity can take various guises. In its original formulation colinearity was recognized as the order of the genes along the cluster matching the order of their expression domains along the anterior-posterior axis during embryogenesis: spatial colinearity. Further forms of colinearity have been recognized, such as temporal colinearity, in which the order of the genes along the cluster now corresponds to the order in which the expression of each gene is initiated. As taxon sampling has increased and Hox and ParaHox genes have been isolated from a wider range of species than just the traditional model organisms used in developmental biology, like Drosophila melanogaster and the mouse, it has become clear that there is a significant degree of evolutionary flexibility
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