MURAKAMI, Y; TAKIO, Y; PASQUALETTI, M; RIJLI, F; KURATANI, S; RIKEN, CDB, Kobe; RIKEN, CDB, Kobe; Insti. Genet. France; Insti. Genet. France; RIKEN, CDB, Kobe: Developmental plan of the lamprey CNS; with reference to the vertebrate evolution

To understand the vertebrate brain evolution, developmental pattern of lamprey neruomeres was studied . We have isolated cognates of several neuruomere marker genes in the lamprey. In the fore-midbrain, each of these genes was expressed in a specific domain in a pattern similar to that in gnathostomes, indicating the conserved morphogenetic program that would have been already established in the vertebrate common ancestor. In the hindbrain, however, Hox gene expression domains were often associated with posterior boundaries, unlike the condition in gnathostomes. Moreover, suggested paralogue groups of the lamprey Hox genes were not always expressed in the pharyngeal arch with gnathostome generic patterns. Thus the colinearity of Hoxgene expression may not be expected in the lamprey. Such the irregular patterns of hidbrain gene expressions may be associated with the neuronal development. We examined the position of reticulospinal and branchial motor neurons, which are largely associated with rhombomeres in ganthostomes. We found that positions of branchial neurons in the lamprey were exceptionally diverged among known vertebrates. Reticulospinal neurons were well specified in each domains, unlike the expected metamerical pattern. These results indicate that developmental organization of the lamprey hindbrain is either diversified as compared to gnathostomes, or the iterative morphological pattern that has been suggested from gnathostome embryos may represent a secondarily established one in this vertebrate lineage. We conclude that evolutionary pattern of the vertebrate body plan exhibits modularity, which could be subdivided into several anatomical domains, such as fore-midbrains and hindbrain-pharyngeal systems.