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Hox collinearity is a spectacular phenomenon that has excited life scientists since its discovery in 1978. Two mechanisms have been proposed to explain the spatially sequential pattern of Hox gene expression in animal embryonic development: interactions among Hox genes, or the progressive opening of chromatin in the Hox clusters, from 3' to 5'. A review of the evidence across different species and developmental stages points to the universal involvement of trans-acting factors and cell-cell interactions. The evidence focuses attention on interactions between Hox genes and on the vertebrate somitogenesis clock. These novel conclusions open new perspectives for the field.
Fig. 4. Temporal collinearity In the Xenopus gastrula. The figure shows Hox ex- pression patterns at sequential stages during gastrulation in Xenopus. The embryos are seen from underneath, where a ring (the blastopore) shows the position where mesodermtissue invaginates during gastrulation. This ring gets smaller as gastrulation proceeds and the upper tissues in the embryo spread out and cover the lower part of the embryo (epiboly). The expression of several different Hox genes, seen as blue colour by in situ hybridisation, is in each case initially in the gastrulamesoderm in the zone above (outside) the ring. Hox expression is thus seen as a blue ring, and since it is initially only in part of the mesoderm, the ring is initially broken. The ring of Hox expression gets smaller as the blatopore ring gets smaller and mesoderm invaginates into the embryo.The figure shows expression of a sequence of Hox genes with differ- ent paralogue numbers, from 1 to 9. It will be seen that the Hox gene with the lowest paralogue number starts expression first and later numbers start sequentially later. It will also be seen that the Hox genes in this time sequence include members of all of the 4 primary vertebrate paralogue groups (a,b,c,d).
