The developmental origin of zygodactyl feet and its possible loss in the evolution of Passeriformes

摘要: The zygodactyl orientation of toes (digits II and III pointing forwards, digits I IV backwards) evolved independently in different extant bird taxa. To understand the origin this trait modern birds, we investigated development foot budgerigar (Psittaciformes). We compared its muscular with that anisodactyl quail (Galliformes) show while musculus abductor digiti (ABDIV) becomes strongly developed at HH36 both species, extensor brevis (EBDIV) degenerates almost disappears only budgerigar. asymmetric action those muscles early causes retroversion digit (dIV). Paralysed embryos do not revert dIV are anisodactyl. Both molecular phylogenetic analysis palaeontological information suggest ancestor passerines could have been zygodactyl. followed zebra finch (Passeriformes) found primordia ABDIV EBDIV fail to develop. These data loss forces activity exerted on dIV, caused by absence ABDIV, resulted secondary anisodactyly Passeriformes.