The evolution of embryonic gene expression in sea urchins

Many evolutionary modifications in development and life history derive from changes in embryonic gene expression. However, the genetic variation affecting gene expression in natural 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, providing an informative basis for analyzing the developmental and evolutionary mechanisms that alter gene expression. Comparative functional analyses have been carried out for several genes. These case studies indicate a complex relationship between sequence divergence and gene expression: in some cases, gene expression is conserved despite extensive divergence in cis-regulatory sequences, while in others the basis for a change in gene expression does not reside locally but rather in the expression or activity of transcription factors that regulate its expression. Diverse evolutionary mechanisms apparently operate on cis-regulatory regions, including negative, balancing, and stabilizing selection.