Evolution from a basic five-digit structure occurred through the specification of fewer digits during patterning of the limb in early development, or the aborted development of some of the digits, either by reduced cell proliferation or increased cell death, which can resculpt the tissues during later stages of development. Until now, the developmental mechanisms of digit loss have only been studied in earless skinks, but little is known about how this occurs in other species. To investigate further, Kimberly Cooper of Harvard Medical School and her colleagues used various staining techniques to analyse the expression of key limb patterning genes during embryonic development across different species.The team examined the odd-toed jerboa, a hopping desert rodent that has lost digits from only its hindlimbs, and found that this digit loss occurs through cell death within the tissue around the remaining toes. These experiments were repeated on camel and horse embryos to similar results. In pigs, however, digit loss seems to be orchestrated by another mechanism, involving altered expression of a critical limb patterning gene called Patched-1. The results point to plasticity during vertebrate limb evolution. Loss of the first and fifth digits in the odd-toed jerboa for instance seems to have occurred by extensive cell death during later stages of limb development, such that the developing limb bud is sculpted after patterning. In horses and cows, however, digit loss occurred by an earlier mechanism that prevents some digits from forming during limb patterning. “A clear take-home message from it is that the mechanisms employed by different animals, even ones that may be relatively closely related, to produce equivalent structures, can be quite distinct,” says Malcolm Logan of King’s College London.
doi:10.1038/nmiddleeast.2014.174
Cooper, K. L.
et al. Patterning and post-patterning modes of evolutionary digit loss in mammals. Naturedoi:10.1038/nature13496 (2014).
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