Talpid2 limb bud mesoderm does not express GHox-8 and has an altered expression pattern of GHox-7

We have studied the expression patterns of the chick homeobox-containing genes, GHox-7 and GHox-8, in the talpid2 (ta2) chick mutant whose limbs have abnormal pattern. These studies provide new insight into how homeobox gene expression and limb patterning may be related. This is the first study demonstrating a natural change in GHox-7 and GHox-8 along the anteroposterior axis. While GHox-7 is expressed asymmetrically in normal limb buds, it is expressed at a uniform level across the anteroposterior axis of ta2 limb buds. GHox-8 is expressed in anterior mesoderm of normal limb buds, but is undetectable in ta2 limb bud mesoderm. These data are consistent with the subtle anteroposterior polarity in ta2 limbs, and allow us to propose that ta2 limb buds lack anterior positional information, but have a narrow range of posterior positional values. We suggest that in normal limb buds GHox-8 may establish the anterior limb bud boundary. Furthermore, we point out that coexpression of GHox-7 and GHox-8 in normal anterior limb bud mesoderm can be correlated with the reduced apical ridge maintenance activity of this tissue, while the lack of coexpression in ta2 limb buds is correlated with the strong ridge maintenance activity in the mutant's anterior limb bud mesoderm. Last, ta2 limbs contain no dying cells in their anterior and posterior border mesoderm; nevertheless, they express GHox-7 in these regions. These data challenge the proposal that this gene determines cell death.