Tinkering: new embryos from old--rapidly and cheaply.

Marine embryos and larvae reflect distinct life histories and body plans from their adults, and are relatively simple in morphology and genetic regulation. Evolution of development to produce highly modified larval forms can be rapid among closely related species. We have studied the mechanisms by which the non-feeding direct-developing larva of the sea urchin Heliocidaris erythrogramma evolved from an indirect-developing feeding larva, the pluteus. H. erythrogramma diverged within 4 million years from its sister species, H. tuberculata, which has a typical pluteus larva. Radical evolution of H. erythrogramma early development allows it to reach metamorphosis in three to four days versus the several weeks required for the pluteus. We have used embryology, cross species hybrids, and manipulation of gene expression in embryos to dissect developmental changes and the genic controls that underlie these changes. Evolution of a new larval form resulted largely from several heterochronies in which conserved regulatory pathways are shifted in timing, producing new temporal relationships to other developmental events. Other changes in gene regulation also have contributed to rapid evolution of larval features, including the origin of unexpected and novel tissue identities that transcend changes within homologous features.