Conserved genetic mechanisms for embryonic brain patterning.

A wealth of comparative embryological studies on the expression and function of homeotic genes and cephalic gap genes indicates that both gene groups are important for establishing and specifying the anteroposterior body axis during embryogenesis in bilaterian animals. Recently, studies of this kind have been extended to embryonic brain development in two genetic model systems, Drosophila and mouse. These studies demonstrate striking similarities in the pattern of expression and mode of action of these developmental control genes during embryonic patterning of the brain in both species. Thus, in both insect and mammalian species, members of the homeotic gene complex are involved in patterning the posterior brain anlage, where they control regionalized neuronal identity, and members of the cephalic gap genes, notably the otd/Otx gene family, are involved in patterning the anterior brain anlage where they control regionalized neurogenesis and neuronal identity. Furthermore, striking cross-phylum rescue experiments show that insect and mammalian members of the orthodenticle gene family can functionally replace each other in embryonic brain and CNS patterning. Comparable cross-phylum rescue experiments have now also been carried out for the empty spiracles cephalic gap gene family. Taken together, these experiments suggest that the genetic mechanisms involved in embryonic brain development are conserved and indicative of a common evolutionary origin of the insect and vertebrate brain. For a more extensive and quantitative investigation of the molecular conservation of developmental mechanisms for brain patterning, functional genomic experiments are now underway in Drosophila. These experiments exploit the advent of sequenced genome information and the technology for large scale transcript imaging, with the goal of identifying the entire set of downstream genes which is under the control of these regulatory genes in embryonic brain development.