The amphioxus rab GDP-dissociation inhibitor (GDI) gene is neural-specific: Implications for the evolution of chordate rab GDI genes

The rab GDP-dissociation inhibitor (rab GDI) proteins are involved in the r egulation of vesicle-mediated cellular transport. We isolated the amphioxus rab GDI gene, analyzed its expression during amphioxus development, and pe rformed a phylogenetic analysis of the rab GDI family. In contrast to the t wo major rab GDI forms in mammals, the alpha and beta forms, there is only one rab GDI isoform in amphioxus. Our analysis indicates that the occurrenc e of the alpha and beta forms of mb GDI preceded the divergence of Lineages leading to birds and mammals, and that the amphioxus rab GDI may have evol ved directly from the common ancestor of both forms. While the mammalian ra b GDI beta genes are ubiquitously expressed, the rab GDI alpha genes are pr edominantly expressed in neural tissues. The expression analysis of the amp hioxus rab GDI gene shows predominantly neural expression similar to that o f the mammalian rab GDI alpha form, suggesting that the ancestral expressio n pattern of chordate rab GDI was neural. In addition, the chicken rab GDI beta-like gene also shows neural-specific expression, which indicates that the neural expression was retained in both early postduplication alpha and beta isoforms and that a novel function associated with ubiquitous expressi on may have evolved uniquely in mammals. These results reveal a Likely scen ario of functional divergence of the rab GDI genes after duplication of the ancestral gene. A similar pattern of evolution, in which one of the duplic ated genes retained a role similar to that of the ancestral one while other genes were recruited into novel roles, was also observed in the analysis o f chordate Otx and hedgehog genes. In the rab GDI, hedgehog, and Otx gene f amilies, the gene retaining the ancestral role shows a lower rate of sequen ce evolution than its counterpart, which was recruited for a novel function .