The jaws of anuran tadpoles consist of several evolutionary novelties, which are unique within the vertebrates. The most noticeable of these novelties are two cartilaginous elements called rostralia and the derived organisation of cranial muscles, which form an entirely new feeding apparatus. The suprarostral cartilage is located in the upper and the infrarostral cartilages in the lower jaw. Between infrarostral and Meckel’s cartilage an intramandibular joint is present. The morphological diversity of anuran tadpoles and their different feeding modes could be associated with these cartilaginous innovations and the shifted insertions of various cranial muscles. The evolutionary origin of the rostralia and, more generally, of evolutionary novelties remains unclear most widely. Therefore, we investigated the molecular basis of the morphogenesis of the infrarostrals by functional knock-down of the bagpipe-related homeobox gene zampogna (zax) in Xenopus laevis . Its knockdown caused fatal deformation of the anterior part of the head and disappearance of the infrarostrals as well as shifted muscular insertions. Higher doses caused a total loss of head structures including mouth, eyes and cranial cartilages. Using quantitative PCR, we found a correlation between cartilage formation and the expression of zax . Our findings indicate that zax is part of a regulatory gene network controlling the development of the ventral part of the anuran head and the formation of the intramandibular joint. This supports the view that not the infrarostrals but rather the intramandibular joint, which forms in a zax -dependent manner, is the actual evolutionary novelty.

Bagpipe-related homeobox gene, morphogenesis, nkx 3 – 3, novelties, rostralia, Xenopus laevis