Ssm

A/P axis: Hox genes

Regionalized central nervous system otd, ems, Hox

segmentation hairy, engrailed, Notch

Regionalized gut ParaHox

circulatory pump tinman/NK2.5

Figure 4.12

Rebuilding "Urbilateria"

Figure 4.12

Rebuilding "Urbilateria"

The possible features of the common bilaterian ancestor are deduced from the conservation of genes and their developmental functions between arthropods (Drosophila) and vertebrates (mouse). (top) Patterning of the D/V axis may have been controled by ancestral genes of the short gastrulation (sog )/chordin and TGF-b families. The A/P axis was probably subdivided by nested, overlapping domains of Hox gene expression. (middle) Different tissue layers were regionally patterned along the A/P axis, including the gut (ParaHoxgenes) and the nervous system (otd, ems, Hox). Segmentation may have evolved under the regulation of ancestral hairy, engrailed, and Notch genes. (bottom) Primitive versions of a photoreceptor organ, a circulatory pump, and a body wall outgrowth might have been present in the bilaterian ancestor, under the regulatory control of the ancestral Pax6, tinman, and Dll genes, respectively.

Deuterostomes

Sponges Cnidaria

Multicellular Ancestor

Urbilateria

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