EVOLUTION OF THE FIBROPELLIN GENE FAMILY AND PATTERNS OF FIBROPELLIN GENE-EXPRESSION IN SEA-URCHIN PHYLOGENY

This study documents evolutionary modifications in the expression patt erns of the sea urchin EGF I and EGF III genes, which encode a family of extracellular matrix proteins, the fibropellins. We show that the s ea urchin apical lamina, a macromolecular extracellular matrix that su rrounds the sea urchin embryo and is made up of the fibropellins, has been conserved through at least 250 million years of echinoid evolutio n. The contribution of different fibropellin family members to this st ructure has, however, changed over the course of sea urchin phylogeny, and between two congeneric species that exhibit different development al modes. Mapping the evolutionary history of the EGF genes on a clado gram of relationships among sea urchins reveals that EGF I is present in all echinoids examined, while EGF III appears to have arisen by dup lication and divergence from EGF I during the radiation of a suborder of the camarodont sea urchins some 35-45 million years ago. Alteration s in the temporal expression patterns of these genes as well as the lo ss of one of the two EGF I transcripts and encoded protein are coincid ent with the evolution of a direct-developing larval form in Heliocida ris erythrogramma. H. erythrogramma and its congener Heliocidaris tube rculata, which develops via a typical echinopluteus larva, shared a co mmon ancestor about 10 million years ago. The differences in fibropell in representation within the apical lamina of the various taxa indicat e that a homologous embryonic structure can undergo substantial change s in composition during its evolutionary history.