Quantity of prelocalized maternal factor is associated with the timing of initiation of an epidermis-specific gene expression of the ascidian embryo.

We produced half-egg-volume ascidian embryos by dividing the unfertilized egg of Halocynthia roretzi at the equatorial plane, and investigated the timing of the initiation of the expression of three tissue-specific genes, a muscle-specific actin gene HrMA4, a notochord-specific gene As-T and an epidermis-specific gene HrEpiC in the half-egg-volume embryos of the animal side and those of the vegetal side. The timing of the onset of HrMA4 and As-T expression in both the animal- and vegetal-half embryos and that of HrEpiC expression in the animal-half embryos were essentially the same as that of normal embryos. In contrast, the timing of HrEpiC expression in the vegetal-half embryos was delayed by one division cycle compared with the normal embryos. This delay was partially recovered by increasing the amount of unfertilized egg cytoplasm of the animal hemisphere, suggesting that the timing of HrEpiC expression is regulated by the amount of a maternal factor which is distributed abundantly in the animal hemisphere of the unfertilized egg.