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In the Echinoida species, Bra was expressed in the vestibule at the six-armed pluteus stage.Īrenas-Mena C, Cameron AR, Davidson EH (2000) Spatial expression of Hox cluster genes in the ontogeny of a sea urchin. (3) In the direct-developing sand dollar embryos, Bra was expressed with an oral-aboral asymmetry in the vegetal ring and with a left-right asymmetry in the oral ectoderm.

(2) Two of the three indirect-developing sand dollar species that were examined exhibited a third domain, in which Bra was expressed on the oral side of the archenteron. The characteristic Bra expression in the vegetal ring of the sand dollar embryos was thought to be involved in the mode of gastrulation, in which involution continues from the beginning of invagination until the end of gastrulation. (1) The vegetal ring expressing Bra in the sand dollars was much wider and was located at a higher position along the AV axis, compared with that in the Echinoida species. The following heterotopic changes in the expression of the Bra genes were found among the sand dollar species and between the sand dollars and the Echinoida species. The sand dollar species shared two expression domains of the Bra orthologs with the Echinoida species, in the vegetal ring (the first domain) and the oral ectoderm (the second domain). erythrogramma diverged from an ancestor with planktotrophic development only about 10 mya, indicating that morphogenetic mechanisms of early development can undergo substantial evolutionary changes, even after long periods of stasis.The expression patterns of Brachyury ( Bra) orthologs in the development of four species of sand dollars (order: Clypeasteroida), including a direct-developing species, and of a sea urchin species (order: Echinoida) were investigated during the period from blastula to the pluteus stage, with special attention paid to the relationship between the expression pattern and the mode of gastrulation. The conservation of gastrulation mechanisms among phylogenetically divergent echinoids with planktotrophic development suggests that the plesiomorphic condition has persisted at least 250 million years and perhaps much longer. erythrogramma thus include utilization of a different type of cellular movement as well as the imposition of dorsoventral asymmetry in cellular movements. Evolutionary modifications to archenteron extension in H. erythrogramma, however, prolonged movement of cells over the ventral lip of the blastopore accompanies extension of the archenteron. Cell marking experiments indicate that in H. Radially symmetrical cell rearrangements and changes in cell shape drive elongation of the archenteron in the ancestral mode of echinoid gastrulation. This study documents evolutionary modifications in mechanisms of gastrulation in Heliocidaris erythrogramma, an echinoid with lecithotrophic larvae.

Sea urchin blastopore