Chapter 10: Sea Urchins & Tunicates: Deuterostome Development

Sea Urchins & Tunicates: Deuterostome Development exploration into invertebrate deuterostomes focuses on the early development of sea urchins and tunicates, highlighting their critical roles in understanding fundamental biological mechanisms and their evolutionary connections to vertebrates. Sea urchin development begins with radial holoblastic cleavage, resulting in the formation of specific cell tiers including mesomeres, macromeres, and the highly influential micromeres. These micromeres exhibit autonomous specification by inheriting maternal factors such as Disheveled and B-catenin, which activate a complex gene regulatory network (GRN). This network utilizes a double-negative gate—specifically the Pmar1 and HesC circuit—to unlock genes required for skeletal formation while simultaneously providing signals that conditionally specify neighboring cells to become endoderm and secondary mesenchyme. Gastrulation in these organisms involves a sophisticated epithelial-to-mesenchymal transition (EMT), where cells lose their initial adhesions to enter the blastocoel and form the larval skeleton, followed by the invagination and convergent extension of the archenteron. Transitioning to tunicates, these organisms demonstrate bilateral holoblastic cleavage, where the first division establishes the primary axis of symmetry. Tunicate embryos are notable for their use of localized cytoplasmic determinants, such as the yellow crescent and the Macho-1 transcription factor, which autonomously direct muscle cell differentiation. While much of their development is mosaic, they also rely on inductive interactions; for instance, endodermal cells secrete fibroblast growth factors (FGFs) to induce the expression of the Brachyury gene, which is essential for notochord development. Ultimately, these model organisms illustrate how integrated logic circuits in the genome, such as feedforward loops and feedback mechanisms, coordinate the complex transitions from a single zygote into a functional larval body plan.