We examined the expression of a panel of Hox genes in tailbud stage embryos (unprocessed embryo shown Fig. S1).
For all Hox genes examined,we observe, as reported previously (Cohn and Tickle,1999), a strong homogenous expression in the posterior part of the trunk, but also a clear spatially collinear expression in the anterior part of their mesodermal expression domains (Figs. 2A–S and Table 1).
Most Hox genes investigated are also strongly expressed in neural tissue and spinal ganglia but since these expression domains are irrelevant to the changes in body plan investigated here, they are not described in detail.
However for clarity, these expression domains are, where possible, indicated in the figures.