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WIREs Dev Biol
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The evolution and development of mammalian flight

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Abstract Mammals have evolved a stunning diversity of limb morphologies (e.g., wings, flippers, hands, and paws) that allowed access to a wide range of habitats. Over 50 million years ago, bats (Order Chiroptera) evolved a wing (composed of a thin membrane encasing long digits) and thereby achieved powered flight. Unfortunately, the fossil record currently lacks any transitional fossils between a rodent‐like ancestor and a winged bat. To reconstruct how this important evolutionary transition occurred, researchers have begun to employ an evolutionary developmental approach. This approach has revealed some of the embryological and molecular changes that have contributed to the evolution of the bat wing. For example, bat and mouse forelimb morphologies are similar during earliest limb development. Despite this, some key signaling centers for limb development are already divergent in bat and mouse at these early stages. Bat and mouse limb development continues to diverge, such that at later stages many differences are apparent. For example, at these later stages bats redeploy expression of toolkit genes (i.e., Fgf, Shh, Bmp, Grem) in a novel expression domain to inhibit apoptosis of the interdigital tissues. When results are taken together, a broad picture of the developmental changes that drove the transition from a hand to a wing over 50 million years ago is beginning to take shape. Moreover, studies seem to suggest that small changes in gene regulation during organogenesis can generate large evolutionary changes in phenotype. WIREs Dev Biol 2012 doi: 10.1002/wdev.50 For further resources related to this article, please visit the WIREs website.

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Forelimb skeleton of Seba's short‐tailed bat (Carollia perspicillata) showing the lengthened digits of the autopod III–V. Scale bar is 1 cm in length.

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Developing forelimb of a fetal bat displays interdigital signals that allow for the maintenance of interdigital tissues8 and restarting of the FgfShh feedback loop that allows for lengthening of the limb.12

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Developing limbs of bats and mice exhibit different sized signaling centers. Bats have a larger zone of polarizing activity (ZPA) (b), compared to mice (a).12 Bats (d) also have a dorsoventrally wider apical ectodermal ridge (AER) compared to mice (c).39

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Wild bats hanging from the ceiling of a culvert in the island country of Trinidad. Source: Photo by Merla Hubler.

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