Diplopodia is a congenital anomaly in tetrapods that involves duplication of elements of the foot on the hind limb. It comes from the Greek roots diplo = "double" and pod = "foot". Diplopodia is often found in conjunction with other structural abnormalities and can be lethal. It is more extreme than polydactyly, the presence of extra digits. Description
The affected leg usually has one foot that is in an approximately normal position. The extra foot is composed of at least some metatarsal or tarsal bones and extra digits, though it is usually not complete. The feet can be joined together, so that the limb appears to have one large foot, or the extra foot can be joined to the limb separately, usually above the level of the other foot. Diplopodia affects one or both of the hind limbs. Causes
Recessive alleles of some genes involved in embryonic limb patterning produce bilateral diplopodia, and diplopodia can be experimentally induced in early embryos. Many instances of diplopodia in humans have no apparent cause.People have been able to produce diplopod limbs by increasing sonic hedgehog (shh) signaling in the limb buds of embryos. The zone of polarizing activity (ZPA) in the proximal posterior mesoderm of a tetrapod limb bud is responsible for maintaining the anterior-posterior axis of the growing limb. The ZPA secretes shh protein, which induces formation of the distal segment of the limb, or autopod, with its posterior side facing the ZPA.
Diplopod limb buds first differ visibly from normal limb buds by their abnormally thick and long apical ectodermal ridges. Diplopodia usually delays the embryonic growth of cartilage, bone, and tendon by two days, particularly in the limbs. Other characteristics frequently seen in diplopodia mutants are shortened wings and legs, short upper beak, and smaller embryo size. Many embryos reach the final embryonic stage but then are unable to hatch, though, diplopod embryos can occasionally hatch and mature.Single diplopodia mutations produce wide ranges of phenotypes sharing these general characteristics and can even affect individual chickens left and right legs differently. The ranges of phenotypes produced by the different mutations also overlap broadly, so diplopodia mutations sometimes need to be distinguished by their phenotype distributions. 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.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. However, unlike the experimentally manipulated embryos, these embryos show no change in shh expression. 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.Diplopodia-2
This is an autosomal trait that gives a very extreme diplopod phenotype, with up to eight toes on a foot. In some embryos, the two outer front toes on each foot are joined together.