Polydactyly

Polydactyly (from Greek πολύς polys, much ' and δάκτυλος daktylos, finger', ie literally " Vielfingerigkeit " ) refers to a hereditary, congenital, anatomical peculiarity in the number of hand and / or Fußgliedmaßen. The special can happen in humans, but also in cats, dogs and other animals. People with polydactyly have more than the usual number of fingers or toes. Especially frequently found one or both sides a respective sixth finger or toe ( hexadactyly ). Polydactyly is inherited as an autosomal dominant trait.

With more than 90 different syndromes occurs polydactyly on as often festzustellendes symptom, for example, in the Patau syndrome ( trisomy 13), Edwards syndrome ( trisomy 18), the C syndrome, Ellis -van Creveld syndrome, Laurence -Moon - Bardet -Biedl syndrome, Meckel - Gruber syndrome and the forms of Kurzripp - polydactyly syndrome ( Type I, II, III, IV).

Epidemiology

The isolated polydactyly - the most common variant is the hexadactyly - has in Europe, Asia and North America, an incidence of 1:3000, 1:300 in Africa. In about 40 percent of cases, the change occurs on both sides. Polydactyly is generally indicated with an incidence of 1:500 live births. Axial polydactyly of the hand is a common deformity among black children in Africa. An autosomal dominant mode of inheritance is assumed here. Axial polydactyly is more common in blacks than in whites and more frequently in males. In comparison, postaxial Polydaktlie typically forms in white children from syndromes and is associated with an autosomal recessive mode of inheritance. A 2008 study combined data from Jefferson County Alabama, USA and Uppsala, Sweden. This study is the frequency of all aggregated Polydaktylieformen to 2.3 per 1000 male white, 0.6 per 1000 female white, 13.5 per 1000 male black and 11.1 per 1000 live births female black.

Polydakytlie in other animals

Cats have five toes on the front foot and normal four toes on the hind foot, 18 toes in total. The polydadaktylen cat breeds include primarily the Maine Coon, which forms a special form präaxialer polydactyly, the Hemingway mutants, so called because Ernest Hemingway at his home in Key West, Florida, a population of such cats owned. For this mutant describe Lange et al. 2013 up to 26 toes, in a statistical frequency distribution with a maximum at 20 toes ( Fig. 5). Another breed of cat with polydactyly is the Pixiebob. In contrast to the Maine Coon, the feature of extra toes in the Pixiebob in breed standards is permitted. A number of mutations of the gene LMBR1 can cause polydactyly in dogs, humans and mice. Dogs have the wild type like other canids four toes on the hind feet. A fifth, the wolf 's claw, occurs in some breeds. Polydactyly is known at the Norwegian Lundehund and the Pyrenees Dog ( Chien de Montagne des Pyrénées). The Norwegian Lundehund is the only four-legged vertebrate ( tetrapod ), which is defined as standard with at least six toes on each foot and therefore typically equipped with this feature equipment, but also with more toes occurs. Polydactyly is also a common feature in some chicken breeds, including the silkie and in pigs ( Fig. 8). A comprehensive early study of polydactyly in an inbred guinea pig strain comes from evolutionary theorist Sewall Wright ( 1934) .. Extensive Polydaktyliestudien undertook the British evolutionary biologist William Bateson in 1894 with exact anatomical analysis of human, macaque monkeys, cats, horses ( atavism ), calves and other types. The studies should help support Bateson's critique of Darwin's evolutionary theory that gradual evolutionary steps do not dominate, but evolutionary change predominates in larger, discontinuous steps. Today we know that both occur.

Polydactyly is assumed even in early tetrapods such as ( Ichthyostega ) and Acanthostega, although this can not be excluded the possibility that the wild-type Ichthyostega possessed more than 5 toes per limb.

Polydaktylieformen

The most common distinguishing form is preaxial polydactyly ( thumb side, Figure 3, 4, 6, 7, 8), postaxial polydactyly (hand outside Fig. 1, 2, 6) and rarely central polydactyly on the ring - middle - or rather on the index finger. Central Polydaktlyie is often associated with syndactyly. The combination is to polysyndactyly. Other deformities of fingers and toes are dysmelia, brachydactyly and oligodactyly.

Extra fingers and toes can be surgically removed by amputation procedures. Most surgery for purely aesthetic reasons is performed. Operations can also serve to improve the grip and running ability of those affected.

Genetic causes

Polydactyly is associated with multiple mutations. These are gene mutations themselves but mutations in a cis- element, which is responsible for the control of the expression of a particular gene in the control. Frequently affected is the Sonic Hedgehog ( SHH) signaling pathway, the rare Indian hedgehog signaling pathway ( Double Mutant foot ), or BMP - Signalsweg. Furthermore, mutations in the Hoxa - Hoxd or clusters are described which lead to polydactyly. Interaction of Hoxd13 with Gli3 induced Synpolydactyly, a combination of additional grown together with your fingers or toes. Other important pathways in this context are the fibroblast growth factors (FGF), the Wnt signaling pathway and the Notch pathway.

In the specific case präaxialer polydactyly ( Hemingway mutant ) (Fig. 4 and 5 ), a mutation of the cis- regulatory element ZRS ( ZPA regulator sequence), a non- coding region of approximately 800,000 base pairs away from the target gene Shh. There is an ectopic expression of Shh on the anterior side of the limb bud ( Figure 4). Normally, Shh is in a particular organizer region, the zone of polarizing activity ( ZPA ) expresses and diffuses from there in the anterior direction transverse to the growth direction of the posterior limb buds on the outside. The mutation is, however, parallel to a mirror image, albeit smaller ectopic formation of a new organizer region on the anterior palm. This cell growth (proliferation ) stimulated and thus provided the raw material for one or more new preaxial fingers or toes there.

Alternatively, for the study of genetic causes of polydactyly pattern formation models are hand used to simulate malformations in the computer and to explain in this way the development process more clearly (see: limb development ).

Directional Polyphänismus a spontaneous complex variation

Polydactyly is a complex spontaneous phenotypic variation ' that occurs in a single generation. The feature is a Polyphänismus because a single point mutation identical to a number of varying forms leads. It is induced in Hemingway mutants by a point mutation in a non-coding regulatory element of the gene of Sonic hedgehog (Shh ). In an extensive phenotypic variation such as this with the development of one or even several complete new fingers and / or toes, including all blood vessels, nerves, muscles, tendons, as well as their complete functionality of the genetic mutation can not adequately explain the extensive phenotypic result. She says only how the variation is initiated. As a result, the mutation leads to tens of thousands of small events at different organizational levels, including expression changes of other genes, ectopic expression of Shh, cell signaling exchange, cell differentiation, cell and tissue growth. The summed random changes on all these levels then the material for the process steps of the formation of plastic variation, in this case for the development of one or more new finger.

In the said Polydakylieform of Hemingway mutants is based on a recent study by the polydatkylen toes figures of 375 Hemingway mutants directional variation in the sense before that, firstly, the number of extra digits is variable or plastic and, secondly, the number of extra toes a discontinuous follows the statistical distribution and is not evenly distributed, as would be expected for identical point mutations. The directionality is not a result of natural selection, as the Phänotypne be viewed at birth, and the selection here has no point of attack. The variation is a Polyphänismus because all phenotypic forms present one and the same point mutation.

The Maine Coon cat (wild-type: 18 toes) polydactyly occurs in some cases with 18 toes by extension of the first toe on a three articulated thumb; much more common, however, 20 toes and waning find more often then 22, 24 or 26 toes, rarely also odd combinations toes on the feet ( Fig. 6). Another statistical directionality is in the difference of the numbers toes on front and hind feet, and a left-right asymmetry in the number of toes can be observed.

Consequences for the theory of evolution

This work opens up about polydactyly, first, a broader view of how innovation can arise in evolution. Since polydactyl fingers or toes do not have a homologous feature, that is, as at the site of a new finger in the wild type, neither cells nor tissue exist, a polydaktyler finger or toe is seen as phenotypic innovation. Type and scope, as in Hemingway mutants new toes arise can be seen as an example of how in other cases, evolutionary new elements arise that are not homologous not only to the previous generation but also for the same organism. The example demonstrates secondly, that phenotypic variation by gene mutation alone can not be satisfactorily explained. Such a form of directedness (English: bias) is alien to the synthetic theory of evolution. At most, there can natural selection cause a directionality ( directional selection). Directional variation in polydactyly is a typical EvoDevo example. Here you can get natural selection by the self -organizing capacity of embryonic development a "turnkey solution " presents. Natural selection is not involved in the generation of extensive plastic variation. You can intervene only after the variation and select the plasticity of the various forms of what their theoretical importance in perspective. The discovery helps to better understand mechanisms of development for the evolution and to explore new the weight of the factors involved in the evolution (mutation, natural selection, embryonic development ) (see: EvoDevo ).

Scientific history of polydactyly

Main article: History of Science of polydactyly

Early scientific exploration of polydactyly go back to the Frenchman Pierre -Louis Moreau de Maupertuis (1751 ), Charles Darwin ( 1868) and the British William Bateson (1894 ). In the course of the history of science different priorities in the Polydaktylieforschung be clear of inheritance rules on the evolutionary significance, exact anatomical analyzes to study the molecular causes and recently reinforced limb development. An analysis of the variation of the toe numbers and the genotype-phenotype relationship back again stronger in the focus of research.

Fine Arts and Literature

In the visual arts, there are numerous representations of human beings or gods with six fingers or toes, including frescoes in Portugal, statues in Australia, Polynesia, South America, Mexico, reliefs in Egypt and medieval panel paintings in Europe. The earliest textual mention of a hexadactyly found in the Old Testament, 2 Samuel, 21.20: " And again it came to battle at Gath. There was a man of great stature, who had six fingers on his hands and six toes on his feet, 24 in number. " In the Gothic pilgrimage church in the Austrian Khajuraho am Jauerling is a miraculous image of St. Mary worshiped with six fingers. Raphael is in the painting "The Marriage of the Virgin " 1504 St.. Joseph with six toes dar. Even the St.. Sixtus Raphael's painting on the Sistine Madonna (1512-1513) see some viewers six fingers (the little "fingers" but below is part of the palm ). The Austrian author Martin Amanshauser presents in his 2004 novel Chicken Christl the narrator Mika as hexadactyl dar. Also in the Darkover series ( 1958-2013 ) by Marion Zimmer Bradley, the six - fingered indigenous race of Chieri portrayed. People with mixed human and Chieri - parents and their offspring (some of which the six - fingered hands of their forefathers inherit ) represent a significant proportion of the portrayed in the series main characters dar. In Hollywood movie Gattaca, which with genetic optimization in itself form a dystopia busy, there pianist with twelve fingers, which alone are able to play specially composed pieces for twelve fingers at all.