WRAY, G. A.; WRAY, ; WRAY, ; WRAY, ; WRAY, ; WRAY, ; WRAY, ; WRAY, ; Duke University: Evolutionary mechanisms that operate on embryonic gene expression in purple sea urchin Strongylocentrotus purpuratus

Many evolutionary modifications in development derive from changes in embryonic gene expression. However, the genetic variation affecting gene expression that is segregating in wild populations is not well understood, nor are the evolutionary mechanisms that operate on that variation. The early embryonic gene network of the purple sea urchin (Strongylocentrotus purpuratus) has been studied in considerable detail. We are using this information as a basis for understanding the evolutionary mechanisms that alter gene interactions regulating transcription over short (population) to long-term (10s of millions of years) time scales. We have gathered detailed comparative data on the sequence and function of cis-regulatory regions from several early embryonic genes in several species of Strongylocentrotus. The results indicate that the intensity and mode of selection can differ among functional modules within a single cis-regulatory region, among paralogous loci, and among unrelated loci. Both negative and balancing selection are apparently common. Consistent with widespread balancing selection, we have found that nucleotides in empirically documented transcription factor binding sites turn over faster than nucleotides with no apparent function. In addition, we have found alleles containing repeat element insertions that are segregating in natural populations; in some cases, these allele confer higher fidelity of expression than the ancestral and more common alleles. Together, these data suggest that connections within the embryonic gene network of S. purpuratus are evolving rapidly.