Degeneration and adaptive evolution of digits in ratite birds.

The amniote digits have undergone recurrent modifications, with the diversified molecular mechanisms more studied among mammals than reptiles. Here we focus on the emu wings and ostrich feet, both of which experienced species-specific digit changes driven respectively by secondary flight loss and adaptation to running. By comparing their digit transcriptomes to those of chicken and alligator, we identified different gene networks in skeleton/muscle development responsible for the degenerated digits in archosaur ancestors and emu, but those in epidermal development for the load-bearing digit of ostrich. These results provide new clues for developmental programs of different cell types between different digits, on which natural selection can convergently operate.