Attenuated sensing of SHH by Ptch1 underlies evolution of bovine limbs

The large spectrum of limb morphologies reflects the wide evolutionary diversification of the basic pentadactyl pattern in tetrapods. In even-toed ungulates (artiodactyls, including cattle), limbs are adapted for running as a consequence of progressive reduction of their distal skeleton to symmetrical and elongated middle digits with hoofed phalanges. Here we analyse bovine embryos to establish that polarized gene expression is progressively lost during limb development in comparison to the mouse. Notably, the transcriptional upregulation of the Ptch1 gene, which encodes a Sonic hedgehog (SHH) receptor, is disrupted specifically in the bovine limb bud mesenchyme. This is due to evolutionary alteration of a Ptch1 cis -regulatory module, which no longer responds to graded SHH signalling during bovine handplate development. Our study provides a molecular explanation for the loss of digit asymmetry in bovine limb buds and suggests that modifications affecting the Ptch1 cis -regulatory landscape have contributed to evolutionary diversification of artiodactyl limbs. The basic five-digit limb of tetrapods has been altered many times during evolution, usually by the progressive loss of digits — this study tracks the molecular underpinnings of this change, showing that in comparison to mouse, the polarized gene expression in the bovine limb bud is progressively lost due to evolutionary alteration of the cis -regulatory sequences that control Ptch1 expression in response to SHH signalling in the digit-forming handplate. Mechanisms of evolutionary limb loss The basic five-digit limb of tetrapods has been altered many times and in many ways during evolution, usually by the progressive loss of digits. Two papers published in this issue of Nature examine the developmental changes underlying digit reduction in mammals. Javier Lopez-Rios et al . look at cattle, where digits three and four are modified to form hooves; digits two and five are vestigial, and the first digit is lost. The first limb bud is shown to be progressively lost as it develops. The Ptch1 gene, which encodes a receptor for the limb-development morphogen Sonic hedgehog (SHH), is upregulated due to evolutionary alteration of a Ptch1 cis -regulatory module that no longer responds to graded SHH signalling during bovine handplate development. Kimberly Cooper et al . show, using a wide range of mammals, that mechanisms of digit loss vary in different lineages. In creatures as varied as the jerboa and the came