RAFF, R. A.; Indiana Univ.: �From pattern to process: learning mechanisms of evolution from comparative developmental biology�

We use several strategies to study radical evolutionary transformation in developmental modes of these closely related species Australian sea urchin species, Heliocidaris erythrogramma and H. tuberculata. Our approaches include the production of cross species hybrids between these two species to reveal genes that underlie the evolutionary transformation. First, these studies show that evolution of developmental mechanisms in embryos is consistent with punctuated equilibrium. We have found that losses in processes that produce individual features of the ancestral feeding larval mode of development of H. tuberculata have taken place. However, novel features also have arisen. Gains in maternal information governing initial steps of development have occurred in the evolution of the direct-developing form H. erythrogramma. Cross species hybrids with distantly related independently evolved direct-developing sea urchins suggest that major developmental changes have been convergent in which mechanisms have been exploited in the evolution of similar outcomes. Second, we have identified and cloned sets of candidate regulatory genes, which we are experimentally expressing by microinjection of mRNAs in embryos to produce phenotypes that show the roles of candidate genes in the evolution. Third, we have carried out microsurgical creation of chimaeras between embryonic tissues of hybrid and parental species to investigate how signals between tissues and their responses have evolved. The results of our studies show that major changes in early development can occur in a short span of evolutionary time, that a small number of regulatory genes may underlie much of the observed changes in life history and development, and that major evolutionary changes can be strongly convergent in underlying mechanisms.