Physical forces may cause Hox gene collinearity in the primary and secondary axes of the developing vertebrates

The features of spatial and temporal Hox gene collinearity along the anteroposterior and secondary axes of vertebrate development have been extensively studied. However, the understanding of these features remains problematic. Some genetic engineering experiments were performed and the consequent modifications of the Hoxd gene expressions in the vertebrate limb and trunk were presented. A two-phases model was proposed to describe the above results but still many data cannot be explained. In the present work a different mechanism is put forward in order to deal with the above experiments. This alternative mechanism (coined biophysical model), is based on the hypothesis that physical forces decondense and 'loop out' the chromatin fiber causing the observed Hox gene collinearity phenomena at the early stages of axonal development. The two models are compared in detail. The biophysical model adequately explains the data even in cases where the results are characterized as unexpected. Furthermore, the biophysical model predicts that the Hox gene expressions are entangled in space and time and this coupling is compatible with the data of the early developmental stages. Additional experiments are proposed for a direct test of this model.