Other genetic changes implicated in webbed feet development in avians include reduction of TGFβ-induced chondrogenesis and reduction of msx-1 and msx-2 gene expression. The magnitude of attenuation in this pathway is correlated with the amount of interdigital tissue preserved. These results indicate that in avian lineages, the disruption of BMP signaling in interdigital tissue caused webbed feet to arise. In experiments with chickens, mutations to a BMP receptor disrupted the apoptosis of interdigital tissue and caused webbed feet similar to ducks to develop. BMP signaling molecules (BMPs) are expressed in the tissue regions between digits during development. One pathway implicated in interdigital necrosis is the bone morphogenetic protein (BMP) signaling pathway. These conditions also demonstrate a variety of genetic targets for mutation resulting in webbed feet, which may explain how this homologous structure could have arisen many times over the course of evolutionary history. While these conditions are disorders in humans, the variability in genetic cause of webbed digits informs our understanding of how this morphological change arose in species where webbed feet were selectively advantageous. In addition, the same genetic mutations can underlie different phenotypic expressions of syndactyly. In humans, syndactyly can arise from as many as nine unique subtypes with their own clinical, morphological, and genetic fingerprints. Reductions in the BMP-induced apoptosis likely allowed this trait to arise. Different vertebrate species with webbed feet have different mutations that disrupt this process, indicating that the structure arose independently in these lineages.īats have also developed interdigital webbing for flight. Apoptosis, or programmed cell death, in development is mediated by a variety of pathways, and normally causes the creation of digits by death of tissue separating the digits. Webbed feet are the result of mutations in genes that normally cause interdigital tissue between the toes to apoptose. This is in contrast to a more hydrofoil-like flipper of many permanently aquatic animals. The roughly triangular design of webbed feet, with a broad distal end, is specialized to increase propulsive efficiency by affecting a larger mass of water over generating increased lift. Paddles generate less lift than hydrofoils, and paddling is associated with drag-based control surfaces. Aquatic control surfaces of non-piscine vertebrates may be paddles or hydrofoils. Webbed feet are a compromise between aquatic and terrestrial locomotion. This shape allows for the production of large forces during swimming through both drag-based and lift-based propulsion. This delta wing shape is a solution that has convergently evolved in many taxa, and is also used in aircraft to allow for high lift forces at high attack angles. However, one of the most common is the delta (Δ) or triangular shape seen in most waterfowl and frogs. Webbed feet take on a variety of different shapes in birds, the webbing can even be discontinuous, as seen in lobate-footed birds like grebes. In birds, the legs utilize countercurrent heat exchange so that blood reaching the feet is already cooled by blood returning to the heart to minimize this effect. One of the consequences of this modification in some species, specifically birds, is that the feet are a major location for heat loss. This modification significantly increases the surface area of the feet. The webbing can consist of membrane, skin, or other connective tissue and varies widely in different taxa. Several distinct conditions can give rise to webbed feet, including interdigital webbing and syndactyly. Here, the foot has a delta (triangular) shape that allows for the formation of leading edge vortices and likely increases swimming efficiency.Ī webbed foot has connecting tissue between the toes of the foot. The webbed foot of Rana temporaria, the common frog.
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