However, we also detect collinear Hox expression boundaries within the dorsal homogenous rib-bearing part of the mesoderm and this differential expression does not correlate obviously with transitions in the morphology of the axial skeleton.
Genes such as HoxC6, HoxA7, HoxC8, HoxB9 and HoxC10, that normally act to modify the axial skeleton, display regionalized expression in “silent” domains (i.e. not reflected in anatomy), suggesting that they have lost their ancestral effects on vertebral regionalization.
The diffuse anterior expression boundaries of these genes in the snake also may reflect their loss of involvement in the regionalization of the axial skeleton due to a loss on the purifying selection for particular cis-regulatory elements that act to define exact somitic expression boundaries.