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Review
. 2013 Jun 26:11:72.
doi: 10.1186/1741-7007-11-72.

Time is of the essence for ParaHox homeobox gene clustering

Myles Garstang  1 , David E K Ferrier
Affiliations
Review

Time is of the essence for ParaHox homeobox gene clustering

Myles Garstang et al. BMC Biol. .

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.

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Figures

Figure 1
Figure 1
Animal phylogeny with the correlation between ParaHox cluster integrity and temporal colinearity indicated. The ParaHox cluster originated before the divergence of the Porifera. The protostome-deuterostome ancestor (PDA) had ParaHox expression in the gut and CNS; with Gsx (blue) anterior, Xlox (green) central and Cdx (pink) posterior (note, this is purely schematic and not intended to illustrate specific morphology or precise expression domain boundaries). Genomic organization of ParaHox genes for each species is shown, with gene linkage represented by a continuous line connecting individual genes. Double diagonals represent genes located on the same chromosome but separated by large distances, and the inclusion of a red ‘X’ indicates loss of one or more ParaHox genes. The Nematostella cluster has only 2 ParaHox genes, though it is unresolved whether one of these is a Cdx or Xlox homolog and a third gene has been lost relative to other cnidarians (hence the question mark). The order in which ParaHox genes are activated and expressed has been indicated numerically (Patiria Gsx activation in parentheses due to presumed later larval expression). The presence of an intact cluster or temporal colinearity is indicated by a check or cross. A horizontal line indicates that temporal colinearity cannot be resolved due to the absence of one or more ParaHox genes.
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