Diplopodia

People have identified five recessive lethal mutations in chickens that cause bilateral diplopodia.^[5][13] The genes are numbered in the order of their discoveries, with the symbols dp-1, dp-2, dp-3, dp-4, and dp-5.

Chickens normally have an anisodactyl toe arrangement, with one short toe pointing backwards and three long toes pointing forwards. In diplopodia, the feet usually lack the normal hind toe and instead develop two to four extra toes at various positions between the hock joint and the front toes.^{[2][4][13][14][15]} The toes are almost always connected to extra tarso-metatarsal bones,^[2][4][5] though sometimes the toes do not contain any bone.^[4] Embryos with diplopodia have been found with numbers of digits on one wing ranging from one to seven,^[4][15][16] while normal embryos have three digits per wing. Diplopod limb buds first differ visibly from normal limb buds by their abnormally thick and long apical ectodermal ridges.^{[15][17][18][19]} Diplopodia usually delays the embryonic growth of cartilage, bone, and tendon by two days, particularly in the limbs.^[4] Other characteristics frequently seen in diplopodia mutants are shortened wings and legs,^[5][16] short upper beak,^[2][5][16] and smaller embryo size.^[5][14] Many embryos reach the final embryonic stage but then are unable to hatch,^[4][5][16] though, diplopod embryos can occasionally hatch and mature.^[2]

Single diplopodia mutations produce wide ranges of phenotypes sharing these general characteristics and can even affect individual chickens' left and right legs differently.^[14] The ranges of phenotypes produced by the different mutations also overlap broadly, so diplopodia mutations sometimes need to be distinguished by their phenotype distributions.^[14] Dr. Ursula Abbott has extensively studied the phenotypes and descriptions of the first four diplopodia mutations and has ranked them from least severe to most severe: dp-3, dp-1, dp-4, dp-2.^[16]

Diplopodia-1

This autosomal mutation gives homozygotes duplicated wing tips and feet in front of the usual structures, as in the experimentally induced diplopodia described above.^[2][13] However, unlike the experimentally manipulated embryos, these embryos show no change in shh expression.^[13] The embryos express hoxd-11, hoxd-12, hoxd-13, Bmp-2, and Fgf-4 along the whole edge of each limb bud, even though these genes are normally only expressed at the posterior edge.^[13]

Diplopodia-2

This is an autosomal trait that gives a very extreme diplopod phenotype, with up to eight toes on a foot.^[14] In some embryos, the two outer front toes on each foot are joined together.^[14] The trait is now extinct and was never fully characterized.^[16]

Diplopodia-3

This is an autosomal trait that may be the least severe of the five.

Diplopodia-4

This sex-linked mutation is similar to the previous mutations, but causes the embryo to grow thicker wing bones.^[19]

Diplopodia-5

This is an autosomal mutation that gives embryos webbing between the inner two front toes on each foot, in addition to the usual characteristics of diplopodia.^[5] Almost all of the embryos survive to the end of incubation, but they are unable to clear the fluid from their lungs or absorb the blood from their extraembryonic vessels.^[5] These problems always prevent them from hatching.^[5] Almost all diplopodia-5 embryos have only one or two extra toes on each foot, so this mutation causes the least extreme foot malformation.^[5]

• Limb development
• Congenital abnormality
• Dysmelia
• Sonic hedgehog
• Polydactyly