- Hemichordates ancestor
Protostomes homologs of the T gene are expressed in posterior gut tissues. This expression may reflect the ancestral function of T before the evolution of the notochord. This gene began to play a new role in notochord development after the chordate and hemichordate/echinoderm lineages became separated, as the hemichordate Tgene is not expressed in the stomochord (a possible precursor of the notochord).
Sonic hedgehog is also expressed in the notochord of cephalochordates and vertebrates and is required for maintenance of the notochord during development. By contrast, neither of the two ascidian Hedgehog genes are expressed in the notochord. In the ascidian Ciona intestinalis, Hedgehog2 is expressed in the ventral portion of the nerve cord, similar to Shh expression in the floorplate of the neural tube in amphioxus and vertebrates. The expression of Shh in the mouse notochord and floorplate is controlled by distinct upstream regulators. Together, these data suggest that Shh was recruited for its role in the notochord after the divergence of the urochordate and cephalochordate/vertebrate lineages. Novel structures are likely to arise and become further elaborated through series of gene co-option events. Thus, changes in gene expression in the notochords of higher chordates may reflect new features of the notochord in the evolution of the chordate lineage.
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