Webbed feet were acquired independently at least fourteen times in modern birds (Fig. Webbed feet are observed in five clades of modern birds: (1) order Anseriformes including palmate-footed ducks and geese and semipalmate-footed magpie geese 1, 4, 5 (2) order Gruiformes including lobate-footed sungrebes and coots 1, 4, 6 (3) the clade composed of order Phoenicopteriformes including the palmate-footed flamingos and order Podicipediformes including the lobate-footed grebes 1, 4, 7 (4) order Charadriiformes including the palmate-footed gulls, semipalmate-footed shorebirds, and lobate-footed genus Phalaropes in the family Scolopacidae 1, 4, 8 (5) the large clade including the orders Eurypygiformes, Phaethontiformes, Gaviiformes, Sphenisciformes, Procellariiformes, Ciconiiformes, Suliformes, and Pelecaniformes with palmate (loons, penguins, and shearwaters) or totipalmate feet (Phaethontiformes and Suliformes) 1, 4, 9. Phylogenetic tree in ( B) was prepared by authors using Mesquite 3.01 38. Illustrations of avian feet in ( A) were prepared by authors after modification of those in Evans (2016) 49. ( B) Phylogenetic position of the waterbird taxa that possess webbed feet within the modern birds (Neornithes). ( A) Simplified illustrations showing webbed foot types in birds. Morphological diversity of webbed feet in birds. The webbing of the cormorant’s totipalmate foot and duck’s palmate foot may have risen from distinct developmental mechanisms. 31, but its expression disappeared except along the toes by St. In the totipalmate-footed great cormorant, Gremlin1 was expressed in all interdigital tissues at St. Differences in Gremlin1 expression pattern and proliferating cell distribution pattern in the toe tissues of the common coot and little grebe support the convergent evolution of lobate feet. Gremlin1, which encodes a bone morphogenetic protein (BMP) antagonist and inhibits interdigital cell death (ICD) in the foot plate of avian embryos, remained expressed in the interdigital tissues of webbed feet in the duck, common coot, little grebe, and great cormorant. Ancestral state reconstruction based on phylogeny assumed that the lobate feet possessed by the common coot and little grebe arose independently, perhaps through distinct developmental mechanisms. To understand the developmental mechanisms underlying this morphological diversity, we conducted a series of comparative analyses. ![]() The webbed feet of waterbirds are morphologically diverse and classified into four types: the palmate foot, semipalmate foot, totipalmate foot, and lobate foot.
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