Evolutionary developmental biology: Use it or lose it.

Abstract

A comparison of the development of adapted limbs in mammals uncovers multiple avenues to digit loss and highlights that early patterning events are not untouchable by evolutionary tinkering. See Articles p.41 & p.46 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 camel, cell death sculpts the tissue in the emerging limb to leave the remaining toes. In other creatures, such as the pig, digit loss is orchestrated by earlier limb patterning events with no increase in cell death. Taken together these findings demonstrate remarkable plasticity in the mechanisms of limb evolution in hooved mammals and rodents, yet reveal a degree of evolutionary convergence.