David Epel

We report the sequence and analysis of the 814-megabase genome of the sea urchin Strongylocentrotus purpuratus, a model for developmental and systems biology. The sequencing strategy combined whole-genome shotgun and bacterial artificial chromosome (BAC) sequences. This use of BAC clones, aided by a pooling strategy, overcame difficulties associated with high heterozygosity of the genome. The genome encodes about 23,300 genes, including many previously thought to be vertebrate innovations or known only outside the deuterostomes. This echinoderm genome provides an evolutionary outgroup for the chordates and yields insights into the evolution of deuterostomes.

Micromolar amounts of the divalent ionophore A23187 can activate echinoderm eggs. The activations by ionophore A23187 were examined in terms of membrane elevation, the program of membrane conductance changes, the respiratory burst, and the increases in protein and DNA synthesis which normally accompany activation by sperm. In all these respects activation by the ionophore was fairly normal although subsequent cleavage and embryonic development was limited. Ionophore A23187 activations of the cortex of Lytechinus pictus and Strongylocentrotus purpuratus eggs were compared in various ionic media and were found to be completely independent of the ionic composition of the external solution. Respiration and protein synthesis of L. pictus eggs in singly substituted ionic media also indicated that these activations were independent of external sodium, calcium, or magnesium. These results suggest that the ionophore acts by releasing intracellular Ca(++). Consistent with this interpretation is the finding that eggs preloaded with (45)Ca show a 20-fold increase in (45)Ca-efflux when activated by ionophore A23187 or fertilization. Measurements of the "free" and "bound" calcium and magnesium in homogenates of the unfertilized eggs show that most of the Mg(++) is already available in the soluble form, whereas Ca(++) is sequestered but available for release. We propose that both normal fertilization and ionophore activation affect the metabolism of the egg by releasing Ca(++) sequestered in intracellular stores.

Contrary to the view that embryos and larvae are the most fragile stages of life, development is stable under real-world conditions. Early cleavage embryos are prepared for environmental vagaries by having high levels of cellular defenses already present in the egg before fertilization. Later in development, adaptive responses to the environment either buffer stress or produce alternative developmental phenotypes. These buffers, defenses, and alternative pathways set physiological limits for development under expected conditions; teratology occurs when embryos encounter unexpected environmental changes and when stress exceeds these limits. Of concern is that rapid anthropogenic changes to the environment are beyond the range of these protective mechanisms.