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Evolutionarily conserved developmental pathways
The bithorax cluster in Drosophila regulates the segmental identity of the anterior segments of the fly. Moving from anterior to posterior, the Bithorax cluster genes controlling head segmental identities are labial, proboscipedia, Deformed and Sex combs reduced. They regulate (respectively) the procephalon, labial and maxillary palps, maxillary segment and labial and first thoracic segment. These Drosophila genes are homologous to vertebrate paralogs Hox1, Hox2, Hox4 and Hox5. The Drosophila genes regulate the segmental identity neuromeres posterior to the superesophagael ganglion, comprising the main structure of the brain.
Behind the third brain neuromere, the cord of the nervous system surrounds the esophagus on its way to the ventral thorax and abdomen. The portion of the brain ventral and posterior to the superesophagael ganglion is the subesophagael ganglion. The identity of neuromeres comprising the subesophagael ganglion is determined by the above mentioned four genes of the bithorax complex.
The subesophageal gangleon is homologous to the vertebrate hindbrain. For example, a conserved region located in the second intron of proboscipedia is essential for proper formation of the adult mouthparts. This region directs pb expression of embryonic labial, maxillary and mandibular head segments, subesophageal ganglia, and supraesophageal ganglia, and in larvae, the labial discs, sub- and supraesophageal ganglia. Deformed regulates innervation of the proboscis and maxillary palps, innervated by the SEG.
Homeobox genes Hoxa-1 and Hoxa-2 are homologs of the labial and proboscipedia Drosophila genes. There are three independent enhancers that direct distinct portions of the Hoxa-1 and Hoxa-2 expression domains during early murine embryogenesis. Two enhancers mediate hindbrain-specific expression. Analysis of the murine Hoxa-2 rhombdomere 2-specific enhancer in Drosophila embryos reveals a distinct expression domain within the fly head segments, which parallels the expression domain of proboscipedia. These results suggest an evolutionary conservation between Hox family members, including a conservation of certain DNA regulatory elements and possible regulatory cascades.
The expression of Hox genes was analyzed in theneural tube of chick and quail embryos after rhombdomere (r) heterotopic transplantations within therhombencephalic area. Whena rhombdomere is transplanted from a caudal to a more rostral position, it expresses the same set of Hox genesas in situ. By contrast, in many cases if rhombdomeres are transplanted from rostral to caudal their Hox geneexpression pattern is modified. They express genes normally activated at the new location of the explant, asevidenced by unilateral grafting. Transplantation of 5 consecutive rhombdomeres (i.e. r2 to r6), to the r8 level leads to theinduction of Hoxb-4 (Drosophila homolog: Deformed) in the two posteriormost rhombdomeres but not in r2,3,4. Transplantations to more caudalregions (posterior to somite 3) result in some cases in the induction of Hoxb-4 in the whole transplant.Neither the mesoderm lateral to the graft nor the notochord is responsible for the induction. Thus, theinductive signal emanates from the neural tube itself, suggesting that planar signaling and predominance ofposterior properties are involved in the patterning of the neural primordium.
date revised: 15 NOV 96
Developmental Pathways conserved in Evolution
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