Researchers have discovered an unexpected way for developmental changes to proceed that may explain the difference between webbed toes and distinct digits and give insight into convergent evolution. The National Institute of Genetics, Research Organization of Information and Systems (ROIS) scientists published their findings in the Proceedings of the National Academy of Sciences.
The researchers were studying a mutant mouse line called “Hammer toe” due to the webbed and sometimes fused appearance of the toes, as opposed to the distinct digits of normal mice. Initially, the team thought this mutation might be due to regulation of the Sonic hedgehog protein, but found instead that the Hammer toe mutation is due to a large insertion between the enhancer and the Sonic hedgehog gene. A non-coding DNA fragment from chromosome 14 found inserted into chromosome five, near the Sonic the hedgehog gene explains the emergence of Hammer toe synactylysim, as opposed to polydactylism, a mutation commonly observed from changes in Sonic hedgehog regulation.
"In the cell nucleus, a number of transcription factors interact with the genome to orchestrate gene expression," said Toshihiko Shiroishi, a professor in the Mammalian Genetic Laboratory at the National Institute of Genetics in Japan and a professor on the paper. "The addition of new information means that the gene receives a new access point for transcription factors. If the transcription factor is an activator, it would raise expression levels of that gene."
In the Hammer toe line, skeletal digits are already formed so newly added Sonic hedgehog activity allows the inter-digital cells to persist, changing the individual limbs into the webbed appearance.
These findings could have implications for understanding convergent evolution, such as the morphological similarity of webbed feet of frogs, water birds, capybara and even the webbed wings of bats.
Image: X-ray micro-CT images show that the Sonic hedgehog gene is ectopically expressed in the interdigital region, and causes interdigital webbing in Hammer toe. Image courtesy of the National Academy of Sciences of the United States of America.