HETEROCHRONIC DIFFERENCES OF HOXA-11 EXPRESSION IN XENOPUS FORE-LIMB AND HIND-LIMB DEVELOPMENT - EVIDENCE FOR LOWER-LIMB IDENTITY OF THE ANURAN ANKLE BONES

The wrist (carpus) and ankle (tarsus) of most tetrapods, as well as th e wrist of anurans, contains relatively small nodular skeletal element s. The anuran tarsus, however, comprises a pair of long bones, the pro ximal tarsals tibiale and fibulare, which resemble the lower leg bones , tibia and fibula (zeugopodium). In this paper we investigate whether the proximal tarsals of Xenopus are of zeugopodial character identity , i.e. whether they develop under the influence of the same genes that pattern the lower limb. We compare Hoxa-11 expression in the forelimb bud with that in the hind limb bud by wholemount in situ hybridizatio n. Hoxa-11 has been implicated in the development of the lower limb. I n Xenopus we note three differences between Hoxa-11 expression in fore - and hind limb buds: (1) Hoxa-11 expression is maintained until the h ind limb bud reaches a larger size (2 mm) than that of the forelimb bu d (1.5 mm); (2) Hoxa-11 expression is maintained over larger spatial d omains than in the forelimb; and (3) Hoxa-11 expression has a pronounc ed posterior polarity in the hind limb, but not in the forelimb. Hind limb expression of Hoxa-11 can be understood as a heterochronic prolon ging of the expression dynamic in the forelimb. Finally we found that the proximal tarsals start to develop within the expression domain of Hoxa-11, while in the forelimb the lower arm elements reach the distal expression limit of Hoxa-11. The gene expression data presented here support the notion of a zeugopodial identity of the proximal tarsal el ements in Xenopus.