Expression of ptc and gli genes in talpid(3) suggests bifurcation in Shh pathway

talpid(3) is an embryonic-lethal chicken mutation in a molecularly un-chara cterised autosomal gene. The recessive, pleiotropic phenotype includes poly dactylous limbs with morphologically similar digits. Previous analysis esta blished that hox-D and bmp genes, that are normally expressed posteriorly i n the limb bud in response to a localised, posterior source of Sonic Hedgeh og (Shh) are expressed symmetrically across the entire anteroposterior axis in talpid(3) limb buds, In contrast, Shh expression itself is unaffected. Here we examine expression of patched (ptc), which encodes a component of t he Shh receptor, and is probably itself a direct target of Shh signalling, to establish whether talpid(3) acts in the Shh pathway. We find that ptc ex pression is significantly reduced in talpid(3) embryos. We also demonstrate that talpid(3) function is not required for Shh signal production but is r equired for normal response to Shh signals, implicating talpid(3) in transd uction of Shh signals in responding cells. Our analysis of expression of pu tative components of the Shh pathway, gli1, gli3 and coupTFII shows that ge nes regulated by Shh are either ectopically expressed or no longer responsi ve to Shh signals in talpid(3) limbs, suggesting possible bifurcation in th e Shh pathway. We also describe genetic mapping of gli1, ptc, shh and smoot hened in chickens and confirm by co-segregation analysis that none of these genes correspond to talpid(3).