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Evolutionarily conserved developmental pathways
The segmental subdivision of the brain is one of the more dramatic examples of how genes work to define specific strucures. Each segment of the brain, called a neuromere, is determined by a set of genes whose transcription is directed to that segment. The Drosophila brain consists of three neuromeres. These neuromeres are continuous with the rest of the central nervous system, including the ventral cord.
Two homeodomain genes, orthodenticle and empty spiracles, are expressed in progenitor cells that give rise to the adult brain. The two genes are considered to be gap genes, as mutation of either deletes specific brain structures. otd is essential for defining the antennal segment, which determines both the eye and the antenna, as well as the first (procerebral) brain neuromere. Mutation of empty spiracles eliminates the second (deutocerebral) and third (tritocerebral)neuromeres.
The roles of homeodomain proteins otd and ems in segmental subdivision of the brain are evolutionarily conserved. Structurally, vertebrate Emx1 and Emx2 are related to Empty spiracles and vertebrate Otx1and Otx2 are related to Orthodenticle. The four vertebrate genes have a role in establishing cell fates variousembryonic brain regions. Each is expressed in a specific manner in the developing rostral brain. Otx2 is expressed in dorsal and most ventralregions of telencephalon, diencephalon and mesencephalon. The Otx1 expression domain is similarto that of Otx2, but contained within it. The Emx2 expression domain is comprised of dorsaltelencephalon and small diencephalic regions, both dorsally and ventrally. Finally, Emx1 expressionis exclusively confined to the dorsal telencephalon
Other conserved proteins are involved in the subdivision of the brain. These include Nkx2, Pax6, Tbr1, Sim1, and HNF3 beta vertebrate homologs of Drosophila VND/NK2, Eyeless, Optomotor blind, Single minded and Forkhead respectively. These transcription factors possess a homeodomain, paired domain and homeodomain, T (brachyury) domain, basic helix-loop-helix and forkhead domain respectively. All but the last of the Drosophila genes have roles in brain neurogenesis, while the vertebrate genes have regional roles similar to those of Otx and Emx. Other Drosophila genes and their vertebrate homologs are expressed in the brain. The Drosophila genes include apterous, decapentaplegic, distal-less, engrailed, hedgehog, tailless, and wingless. The final word on conservation of regulation and function of each gene awaits better knowledge of the regulatory pathways for the induction of each gene, and knowledge of their regulatory targets.
date revised: 15 NOV 96
Developmental Pathways conserved in Evolution
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