Maternal mRNA encoding the orphan steroid receptor SpCOUP-TF is localized in sea urchin eggs

The SpCOUP-TF gene is a highly conserved sea urchin homologue of the vertebrate COUP-TFs and the Drosophila seven up subfamily of transcription factors, which are members of the orphan steroid hormone receptors. Whole-mount in situ hybridization experiments, using three sea urchin species, detect the maternal SpCOUP-TF mRNA deposited unevenly in the oocytes, mature eggs and the blastomeres of the early embryo. The localization pattern indicates that, in all three sea urchin species, the maternal SpCOUP-TF mRNA is placed in the egg in a fixed position relative to the embryonic axes, i.e. lateral to the animal-vegetal and at 45 degree angle to the oral-aboral (ventral-dorsal) axis. The embryonic expression of the SpCOUP-TF gene is spatially restricted in the oral ectoderm of the early embryo and, at later stages, in the cells of the ciliated band, the neurogenic cell lineage of the sea urchin embryo. The SpCOUP-TF mRNA, the first sea urchin maternal mRNA encoding a transcription factor that is specifically localized with respect to both embryonic axes in the egg, could be involved in early specification events in the sea urchin embryo.