We have hypothesized that a changing distribution of lateral plate mesoderm over evolutionary time led to the emergence of the paired appendages. The lateral plate mesoderm of Agnathan lampreys is confined to the coelomic lining indicating this as the likely ancestral, pre-appendicular condition. Conversely, lineage analyses of lateral plate mesoderm in chick and mouse indicate that it contributes to the connective tissue of amniote limbs (see other pages on this site). The composition and boundaries of mesodermal lineages remains unmapped in amphibians. We are using isotopic transplants of mesoderm populations between wild type and GFP expressing axolotl embryos to generate chimeric salamanders. We will use this to determine the distribution of the mesoderm populations comprising the abaxial and primaxial domains in amphibians and the adult structures to which they contribute.
Comparative studies examining the changing developmental domains in various taxa allow us to generate evolutionary narratives explaining how morphological changes arise over time. As the first animals with paired appendages adapted to a terrestrial environment, amphibians represent a key point in the evolution of the tetrapod limb and the changing roles of the tissue domains from which they develop.
There is a growing body of work suggesting the lateral somitic frontier is a transition between areas of different patterning information. Previous experiments in chick have shown that transplanted somitic cells retain donor identity in the primaxial domain but those which migrate into the abaxial domain integrate normally into hosts. This implies that those cells crossing the frontier are repatterned to a host-specific developmental fate. Whether or not this phenomenon is taxon specific is unknown so far as experiments have been limited to birds and mammals. We are performing embryological challenge experiments in axolotls to determine if the primaxial and abaxial domains are developmental modules in amphibians. To test this, we are performing isotopic and heterotopic GFP labeled mesoderm populations to wildtype host salamanders and observing the contribution these transplanted tissues make to development. As these domains contribute to the entire post-cranial musculoskeletal system, determining the nature of their development has broad implications for our understanding of genetic patterning in development and how this patterning has changed during the course of vertebrate evolution.
