Evolution of the notochord

The presence of a notochord unites the chordates and is the distinguishing feature that inspired their name. The notochord is a stiff, axial rod of cells that represents the functional precursor of the vertebral column in basal chordates. It acts as an organizer for the early development of the CNS and adjacent axial mesoderm. Efforts to determine the origin of the notochord have often focused on the urochordates because of their relatively simple body plan and their basal position within the chordates. The larval ascidian tail consists of the noto-chord, muscle cells, and a dorsal nervous system. At metamorphosis, ascidian larvae lose their entire tail (including the notochord) and become sessile, filter-feeding adults.

The evolution of the notochord involved the co-option of an ancestral regulatory gene, Brachyury (T), a member of the T-box class of transcription factors (Fig. 6.7). In vertebrates, T is expressed in developing notochord cells and other mesodermal derivatives and is required for notochord differentiation. Similarly, the urochordate (ascidian and larvacean) T genes are expressed in cells that form the notochord and are sufficient to confer notochord fate. In larvaceans as well as hemichordates and invertebrates that lack a notochord, divergence of Brachyury target genes

Figure 6.7

Recruitment of the Brachyury gene during evolution of the notochord

The Brachyury (T) gene predates the origin of the chordate notochord. During the early evolution of the chordates, expression of the T gene was recruited to pattern the notochord. Targets of the T gene have evolved as the chordate lineages have diverged.

- Vertebrates

Figure 6.7

Recruitment of the Brachyury gene during evolution of the notochord

The Brachyury (T) gene predates the origin of the chordate notochord. During the early evolution of the chordates, expression of the T gene was recruited to pattern the notochord. Targets of the T gene have evolved as the chordate lineages have diverged.

Brachyury recruited for notochord development

- Cephalochordates

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