"Micromere" formation and expression of endomesoderm regulatory genes during embryogenesis of the primitive echinoid Prionocidaris baculosa.

Blastomere composition and expression profiles of wnt8 and hox11/13b orthologues were examined in the primitive indirect-developing echinoid Prionocidaris baculosa. We found that blastomere composition in the 16-cell-stage Prionocidaris embryos was different from that of the indirect-developing echinoids belonging to Euechinoidea, a derived group of the echinoids. The sizes of the blastomeres in the 16-cell-stage embryo varied, and no embryos formed a "micromere quartet," a group of four equal-sized micromeres. The smallest blastomere was usually located around the vegetal pole. We also found significant differences in early expression profiles of wnt8 orthologues of the Prionocidaris and euechinoids. Unlike euechinoids, the expression of wnt8 orthologue of Prionocidaris was not detected at the 16-cell stage; it began at the 32-cell stage in the broad area containing the vegetal pole. However, in later stages, the expression profiles of hox11/13b and wnt8 orthologues of Prionocidaris were similar to that of euechinoid orthologues. The present study suggests that there are considerable differences between Prionocidaris and euechinoids in early developmental mechanisms in the vicinity of the vegetal pole.