NARITA, Y; KURATANI, S; RIKEN CDB (Center for Developmental Biology), Kobe; RIKEN CDB (Center for Developmental Biology), Kobe: Evolution of the vertebral colum and Hox code in mammals � a perspective from the developmental and phylogenetic constraints

Mammalian vertebral column consists of morphologically well differentiated groups of vertebrae such as, cervical, thoracic, lumbar, sacral and caudal. Such a morphological specification is now understood as regulated developmentally by the nested expression patterns of the Hox genes, along the anterior-posterior axis of the embryonic body. In mammals, numbers of vertebral groups are apparently variable except for that of the cervical, which is almost fixed to 7. Evolutionary changes in the numbers of vertebral groups are assumed to be correlated with the regulation of the Hox genes that could have also changed through phylogenetic evolution. To understand the evolutionary diversity of the mammalian vertebral formula in terms of developmental gene regulation, it becomes first necessary to identify the presence and loss of developmental constraints that define the vertebral formula along the mammalian phylogenetic tree. Based on the catalogue of Hanterian Collection by Richard Owen (1853; Descriptive Catalogue of the Osteological Series contained in the Museum of the Royal College of Surgeons of England), we plotted the vertebral formula of each animal on the phylogenetic tree obtained by recent molecular data, and found that not only the number of cervical, but also the sum of thoracic and lumbar vertebrae was highly conserved in each taxon and the change of the latter could be assumed to have occurred in the ancestor of each taxon. Thus, the distribution pattern of the vertebral formula, namely, specific developmental constraints, was consistent with the body plans of mammalian groups that define the taxa. As an example of evolutionary changes of Hox gene regulation that defines body plans, we farther report the comapartive analyses between chick and mouse HoxC8 enhancer.