ISOLATION AND CHARACTERIZATION OF 3 MESSENGER-RNAS ENRICHED IN EMBRYOS OF THE DIRECT-DEVELOPING SEA-URCHIN HELIOCIDARIS-ERYTHROGRAMMA - EVOLUTION OF LARVAL ECTODERM

The Australian sea urchin Heliocidaris erythrogramma utilizes a derive d direct developmental mode that evolved 8-12 million years ago. From a differential screen we have isolated a small set of cDNAs correspond ing to genes more greatly expressed in embryos of H. erythrogramma tha n in those of its indirect-developing nearest relative, H. tuberculata . The method was biased towards abundant transcripts and did not allow detection of modifications of usage of highly conserved gene family m embers. Three differentially expressed abundant transcripts were found that potentially encode secreted proteins. Two of these, the arylsulf atase HeARS and the putative lectin HeEL-1, were identifiable as homol ogues of known proteins. Another gene, HeET-1, may be exclusively expr essed in the H erythrogramma embryo. In situ hybridization experiments demonstrate that all three transcripts are localized to the ectoderm. Two of them, HeET-1 and HeEL-1, are transcribed in an identical domai n comprising the larval ectoderm. This region of gene expression has a cquired a novel columnar cytology during the evolution of the H. eryth rogramma embryo. The third sequence, HeARS, encodes an arylsulfatase h omologue. Its expression is uniform in the gastrula, but as the rudime nt develops it accumulates to the greatest extent in the invaginating vestibular ectoderm. Through comparisons with indirect-developing spec ies, we show that this concentration of arylsulfatase mRNA in the rudi ment is a novel feature of H. erythrogramma development. These data su ggest that H. erythrogramma has a unique arrangement of ectodermal gen e expression territories. We propose that these reflect larval adaptat ions that have occurred in the lineage leading to H. erythrogramma, an d enabled the evolution of direct development.