Novel interactions between vertebrate Hox genes

Understanding why metazoan Hox/HOM-C genes are expressed in spatiotemporal sequences showing colinearity with their genomic sequence is a central chal lenge in developmental biology. Here, we studied the consequences of ectopi cally expressing Hox genes to investigate whether Hox-Hox interactions migh t help to order gene expression during very early vertebrate embryogenesis. Our study revealed conserved autoregulatory loops for the Hox4 and Hox7 pa ralogue groups, detected following ectopic expression Hoxb-4 or HOXD4, and Hoxa-7, respectively. We also detected specific induction of 5' posterior H ox genes; Hoxb-5 to Hoxb-9, following ectopic expression of Hoxb-4/HOXD4; H oxb-8 and Hoxb-9 following ectopic expression of Hoxa-7. Additionally, we o bserved specific repression of 3' anterior genes, following ectopic express ion of Hox4 and Hox7 paralogues. We found that induction of Hoxb-4 and Hoxb -5 by Hoxb-4 can be direct, whereas induction of Hoxb-7 is indirect, sugges ting the possibility of an activating cascade. Finally, we found that activ ation of Hoxb-4 itself and of posterior Hox genes by Hoxb-4 can be both non -cell-autonomous, as well as direct. We believe that our findings could be important for understanding how a highly ordered Hox expression sequence is set up in the early vertebrate embryo.