A polydactyl cat is a cat with a congenital physical anomaly called feline polydactyly (also known as polydactylism or hyperdactyly), which causes the cat to be born with more than the usual number of toes on one or more of its paws. A point mutation in the noncoding region of the ZRS regulatory sequence causes abnormal SHH expression during limb development, causing variable polydactyly through following developmental processes rather than the mutation alone, explaining why the phenotypic expression of polydactyly varies. Cats with this genetically inherited trait are most commonly found along the East Coast of North America (in the United States and Canada) and in South West England and Wales. The breed, American polydactyl cats, are bred with normal-toed cats by breeders, since the trait is inherited dominantly and can be passed on to offspring reliably.These cats spread through maritime trade, notably from Boston, since sailors valued polydactyl cats and saw them as lucky. They became known as "Hemingway cats" after Ernest Hemingway's love for his own collection of polydactyl cats.
Genetics
Embryologists distinguish two principal forms of feline polydactyly: preaxial and postaxial. Preaxial polydactyly involves supernumerary digits on the medial (inner) side of the paw, corresponding to the thumb side in humans, whereas postaxial polydactyly affects the lateral (outer) side, corresponding to the little finger. Postaxial polydactyly is comparatively rare in cats, and the additional digits in such cases are often incompletely formed. By contrast, preaxial extra digits are typically well developed.
Polydactyly is a congenital malformation that is most commonly inherited in an autosomal dominant pattern. In several cases, the condition is associated with mutations affecting regulatory elements that control expression of the sonic hedgehog (SHH) gene during limb development. SHH encodes a signalling protein that plays a central role in embryonic pattern formation, including the specification and growth of limbs and digits. Genetic work studying the DNA basis of the condition indicates that many different mutations in the same ZRS area can all lead to polydactyly. In the case of preaxial polydactyly of the Maine Coon cat (Hemingway mutant; Hw) a mutation of the cis-regulatory element ZRS (ZPA regulator sequence) is associated. ZRS is a noncoding element, 800 kilobase pairs (kb) remote to the target gene SHH. An ectopic expression of SHH is seen on the anterior side of the limb. Normally SHH is expressed in an organizer region, called the zone of polarising activity (ZPA) on the posterior limb side. From there it diffuses anteriorly, laterally to the growth direction of the limb. In the mutant mirroring smaller ectopic expression in a new organizer region is seen on the posterior side of the limb. This ectopic expression causes cell proliferation delivering the raw material for one or more new digits. An identical sequence at this position serves the same function in human and mice and cause similar symptoms when mutated. Different mutations have different specific effects: for example, while the Hw mutant tends to mostly induce extra fingers in the fore limbs, many other mutations affect the posterior limbs too. Polydactyly is a spontaneous complex phenotypic variation, developed in one generation. In the concrete preaxial form of the Hw mutant the variation is induced by a single point mutation in a noncoding cis-regulatory element for SHH. In an extensive phenotypic variation like this, one or more complete digits at each single limb are developed including nerves, blood vessels, muscles and ligaments. The physiology of the digits can be perfect. This complex phenotypic result cannot be explained by the mutation alone. The mutation can only induce the variation. In the consequence of the mutation, thousands of events, each different from the wild type, occur on different organisation layers, such as expression changes of other genes, cell-cell signal exchange, cell differentiation, cell and tissue growth. The summarized small random changes on all layers build the raw material and the process steps for the generation of the plastic variation.
The mentioned form of polydactyly of the Hw mutant shows a biased variation. In a recent empirical study first the number of extra toes of 375 mutant Maine Coon cats were variable (polyphenism) and second, the number of extra toes followed a discontinuous statistical distribution. They were not equally distributed as one might expect of an identical single point mutation. The example demonstrates that the variation is not explained completely by the mutation alone.
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