Exites in Cambrian arthropods and homology of arthropod limb branches

The last common ancestor of all living arthropods had biramous postantennal appendages, with an endopodite and exopodite branching off the limb base. Morphological evidence for homology of these rami between crustaceans and chelicerates has, however, been challenged by data from clonal composition and from knockout of leg patterning genes. Cambrian arthropod fossils have been cited as providing support for competing hypotheses about biramy but have shed little light on additional lateral outgrowths, known as exites. Here we draw on microtomographic imaging of the Cambrian great-appendage arthropod Leanchoilia to reveal a previously undetected exite at the base of most appendages, composed of overlapping lamellae. A morphologically similar, and we infer homologous, exite is documented in the same position in members of the trilobite-allied Artiopoda. This early Cambrian exite morphology supplements an emerging picture from gene expression that exites may have a deeper origin in arthropod phylogeny than has been appreciated.

How to align arthropod leg segments

How to align leg segments between the four groups of arthropods (insects, crustaceans, myriapods, and chelicerates) has tantalized researchers for over a century. By comparing the loss-of-function phenotypes of leg patterning genes in diverged arthropod taxa, including a crustacean, insects, and arachnids, arthropod legs can be aligned in a one-to-one fashion. By comparing the expression of pannier and aurucan, the proximal leg segments can be aligned. A model is proposed wherein insects and myriapods incorporated the proximal leg region into the body wall, which moved an ancestral exite (for example, a gill) on the proximal leg into the body wall, where it invaginated independently in each lineage to form tracheae. For chelicerates with seven leg segments, it appears that one proximal leg segment was incorporated into the body wall. According to this model, the chelicerate exopod and the crustacean exopod emerge from different leg segments, and are therefore proposed to have arisen independently. A framework for how to align arthropod appendages now opens up a powerful system for studying the origins of novel structures, the plasticity of developmental fields across vast phylogenetic distances, and the convergent evolution of shared ancestral developmental fields.

BMP signaling is essential for sustaining proximo-distal progression in regenerating axolotl limbs

ABSTRACTAmputation of a salamander limb triggers a regeneration process that is perfect. A limited number of genes have been studied in this context and even fewer have been analyzed functionally. In this work, we use the BMP signaling inhibitor LDN193189 on Ambystoma mexicanum to explore the role of BMPs in regeneration. We find that BMP signaling is required for proper expression of various patterning genes and that its inhibition causes major defects in the regenerated limbs. Fgf8 is downregulated when BMP signaling is blocked, but ectopic injection of either human or axolotl protein did not rescue the defects. By administering LDN193189 treatments at different time points during regeneration, we show clearly that limb regeneration progresses in a proximal to distal fashion. This demonstrates that BMPs play a major role in patterning of regenerated limbs and that regeneration is a progressive process like development.