The role of Bapx1 (Nkx3.2) in the development and evolution of the axial skeleton

The bagpipe-related homeobox-containing genes are members of the NK family. bagpipe (bap) was first identified in Drosophila and there are three diffe rent bagpipe-related genes in vertebrates. Only two of these are found in m ammals, the Nkx3.1 and the Bapx1 (Nkx3.2) gene. The targeted mutation in th e mouse Bapx1 gene shows a vertebral phenotype in which the ventromedial el ements are lacking; these are the centra and the intervertebral discs. In a ddition, a region of gastric mesenchyme is abnormal. This mesenchyme surrou nds the posterior region of the presumptive stomach and duodenum, and in th e mutant fails to support normal development of the spleen. In Drosophila, bagpipe has a role in gut mesoderm and the mutant embryos have no midgut mu sculature. Thus bap related genes in mouse and Drosophila have roles in pat terning gut mesoderm; however, neither of the mammalian genes has a discern ible role in the gut musculature. In contrast, both mammalian genes have ro les in developmental processes that have appeared recently in evolution. Th e Bapx1 gene found in fish, amphibians, birds and mammals appears to have d erived vertebrate specific functions sometime after the split between the j awless fish and gnathostomes.