Meeting Abstract

123.5  Tuesday, Jan. 7 14:30  Experimentally induced homeotic shifts in anterior axial patterning mimic events in the evolution of the tetrapod skull MADDIN, H.C.*; PIEKARSKI, N.; HANKEN, J.; Harvard Univ.; Harvard Univ.; Harvard Univ. hillary.maddin@gmail.com

The occipital region of the skull incorporates the sclerotomal portions of several anterior somites. Among tetrapods, amniotes incorporate more somites (five) than amphibians (three, at most) and, therefore, have additional cranial nerves and foramina. This difference has led to the claim that ‘increased occipitalization’ characterizes the origin of amniote skull form. However, the fossil record suggests that the amniote condition evolved earlier, at the base of Tetrapoda, and that the amphibian condition is secondarily derived. The developmental basis of this transformation remains poorly understood. Here we show the number of occipital segments can be modified experimentally in the axolotl. In normal development the head-trunk boundary is located within somite 3 (S3). Treatment with retinoic acid (RA) produces additional vertebral segments anterior to the first cervical vertebra, which is suggestive of a homeotic shift in segment identity. This shift is confirmed by somite transplantation experiments in which S3 forms only vertebral segments when exposed to RA. The opposite is true when embryos are treated with RA inhibitors: additional segments are recruited into the occiput. These experimentally induced homeotic shifts mimic events that took place in the evolution of the tetrapod skull, such as the secondary transformation of occipital somites into trunk somites during the origin of amphibians. Perturbation of the RA signaling pathway may underlie the evolution of cranial diversity in tetrapods.