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Why snakes don’t have legs

Gnangarra/Wikicommons Python: missing sequence of 17 base-pairs meant snakes grew no legsGnangarra/Wikicommons

TTCTGAGGTAACTTCCT. This sequence of 17 base-pairs – the chemical units that make up DNA – was missing along a stretch of snake genome, causing the reptiles to gradually lose their limbs (Cell, October 20, 2016). So concluded a study coordinated by researchers at the Lawrence Berkeley National Laboratory in Berkeley, California, with the participation of Brazilian molecular biologist Uirá Souto Melo. The sequence is from a regulatory region that governs the functioning of the sonic hedgehog gene (Shh), which produces a protein important to limb development. Called ZRS enhancer, this stretch of DNA is well conserved across the vertebrate genome but displays some deletions in snake DNA. In the case of modern, legless snakes, this 17 base-pair sequence is missing. Using the gene-editing tool CRISPR-Cas9 – a molecular biology technique that enables editing of targeted stretches of gene – researchers inserted and deleted the enhancer in transgenic mice. When they replaced the mouse enhancer with a version from legged vertebrates, like humans, horses, and chickens, the mice developed limbs. However, when they inserted the enhancer from one of the two snake species under study, the mice did not grow limbs. “We saw that the sequence is enough to control leg formation,” explains Melo, who completed part of his doctorate in 2016 at the Lawrence Berkeley National Laboratory and currently works at the Human Genome and Stem-Cell Research Center (HUG-CELL), of the University of São Paulo (USP).