Chapter 7. Axis formation and the rapid evolutionary transformation of larval form.

Marine invertebrate embryos and larvae are diverse and can evolve rapidly, providing a link between early developmental and evolutionary mechanisms. We here discuss the role of evolutionary changes in axis formation, which is a crucial part of the patterning of marine embryos and larvae. We focus on sea urchin embryos, where axial features are well defined and subject to active current investigation. The genetic control of processes of formation of the three axial systems, animal-vegetal, dorsal-ventral, and left-right, is becoming established for species that undergo development via the feeding pluteus larva. These species represent the primitive condition among living sea urchins. We compare their developmental processes to the highly modified development of a species that has evolved a nonfeeding larva. This derived form has accelerated some elements of axis formation, and eliminated or modified others. Three features of embryonic/larval evolution stand out (1) evolution of developmental features occurs rapidly over geological time; (2) upstream gene regulatory systems of axis formation are conserved, whereas downstream features evolve rapidly; and (3) heterochronies play an important role.