Early gene expression along the animal-vegetal axis in sea urchin embryoids and grafted embryos.

The HE gene is the earliest strictly zygotic gene activated during sea urchin embryogenesis. It is transiently expressed in a radially symmetrical domain covering the animal-most two-thirds of the blastula. The border of this domain, which is orthogonal to the primordial animal-vegetal axis, is shifted towards the animal pole in Li+-treated embryos. Exogenous micromeres implanted at the animal pole of whole embryos, animal or vegetal halves do not modify the extent and localization of the HE expression domain. In grafted embryos or animal halves, the Li+ effect is not affected by the presence of ectopic micromeres at the animal pole. A Li+-induced shift of the border, similar to that seen in whole embryos, occurs in embryoids developing from animal halves isolated from 8-cell stage embryos or dissected from unfertilised eggs. Therefore, the spatial restriction of the HE gene is not controlled by the inductive cascade emanating from the micromeres and the patterning along the AV-axis revealed by Li+ does not require interactions between cells from the animal and vegetal halves. This suggests that maternal primary patterning in the sea urchin embryo is not limited to a small vegetal center but extends along the entire AV axis.