Through detailed description of the ontogeny of wing muscles and their associated tissues in bat embryos, we have revealed that bat wing muscles are derived from multiple myogenic sources with different embryonic origins, and there is a spatiotemporal correlation between the outgrowth of wing membranes and the expansion of wing muscles into them. These muscles are considered to have essential roles in controlling the shape of the wing and allowing the bats to fly 14. In addition, they possess sheets of muscles composed of the coraco-cutaneus, the humeropatagialis and the plagiopatagialis within the lateral wing membrane (plagiopatagium), and the uropatagialis within the interfemoral wing membrane (uropatagium) ( Fig. Bats have an architecturally unique muscle complex called the occipito-pollicalis that extends along the anterior wing membrane (propatagium) and is necessary for powered flight ( Fig. In this paper we focused on the wing muscles-a synapomorphic character supporting the monophyly of bats 11, 12. In any case, molecular and cellular mechanisms underlying the evolution of the bat wing remain poorly understood. In that case, the evolution of the unique body plan of bats might be better explained by means of gradual processes, such as the divergence of many regulatory elements for several developmental control genes 10. However, the fossil record of bats is currently quite poor 9, so there is still a possibility that transitional fossils may be discovered in the future. This has led some authors to hypothesize that bats achieved powered flight in a relatively short period of geological time, possibly through small changes in gene regulation during organogenesis 7, 8. The earliest known fossil bat, Onychonycteris finneyi, from the early Eocene was clearly capable of powered flight, possessing a forelimb that was morphologically similar to that of extant bats 6. However, to date there are no mammalian fossils of intermediate morphology linking bats to gliding or flightless ancestors 5, implying the sudden appearance of this mammalian lineage with a unique body plan. It is almost universally accepted that bats evolved from small arboreal mammals 2, 4. Recent molecular phylogenetic analysis supports the monophyly of bats, suggesting a single origin of powered flight in mammals 3. They have adapted to various environments through tremendous variation on a basic single body plan and now constitute over 20% of extant mammalian species 1, 2. Bats (order Chiroptera) acquired wings to become the only mammalian lineage with the ability of powered flight.
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