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Why can't most mammals fly but birds can?

A new study in collaboration with Avihu Klar's group
Diverged modes of locomotion have evolved to support the movement of animals in different milieu: on the ground, under or above water and in the air. Most terrestrial animals walk via stepping – left/right alternation. This stepping pattern is mediated by neuronal network embedded with the spinal cord. However, aerial animals, like birds and bats, fly by flapping their wings together. A new study by the groups of Avihu Klar and Claudio Mello, Oregon Health & Science University, Portland, unraveled the genetic events that occurred in evolution that enable the joint flapping of birds’ wings. The researchers found that the wiring of circuits that instruct stepping in mice and wing flapping in birds are different. The difference is a consequence of mutations in an axon guidance molecule – ephrin-B3: The enhancer that controls the pattern of expression of the gene is missing in birds; the coding sequence is altered and thus the activity of the protein is diminished; and in some birds (chicken and quail) the gene is completely deleted. These natural mutations are implicated in a wiring that supports synchronous wing flapping in birds.
mutations in an axon guidance molecule – ephrin-B3
Credits: Opening picture - Hamutal Klar; Microscope image - Baruch Haimson
Birds in flight