For evolution to occur, there must be modifications in developmental process. However, the relationship between development and evolution is only beginning to receive experimental investigation. The studies proposed here experimentally address the nature of changes in gene expression that underly dramatic shifts in timing and cell fate determination that accompany replacement of typical larval development by direct development in a sea urchin. The use of this system allows us to test mechanisms of change in development at the level of cell lineage determination and behavior at the level of gene activity. We are able to directly examine the mechanistic underpinnings of one of the key concepts in the evolution of development, that is heterochrony, or changes in relative timing of developmental events. The experimental system is a molecular and developmental comparison of a direct developing sea urchin versus its dosest typical developing species. We have already demonstrated that homologous cell lineages can be identified, and we have observed heterochronies at the cellular and molecular levels in these species. We also have begun tracing cell lineages and cloning cell lineage-specific expressed genes. The major questions to be studied include the following: Are cell fates modified by autonomous determination or by modified patterns of induction? What are the mechanisms for the observed heterochronies in the evolution of direct development? How are gene expression programs changed in modified cell lineages? How distant are typical and direct developing species, and what are their precise phylogenetic relationships? The study of the evolution of developmental processes adds a new and potentially powerful means of understanding the underlying processes of developing systems, because closely related organisms that differ in specific developmental processes provide us with natural variants in these processes. The details of process modification by which these variants have come to differ provides a way of probing basic developmental processes and their controls.
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