ULTRAVIOLET-SENSITIVE OOPLASMIC FACTORS ARE RESPONSIBLE FOR THE DEVELOPMENT OF AN ENDODERM-SPECIFIC ALKALINE-PHOSPHATASE IN THE ASCIDIAN EMBRYO

The ascidian egg contains muscle and endoderm determinants that play c ritical roles in the specification of muscle and endoderm cells, respe ctively. Endoderm cells of the ascidian embryo express alkaline phosph atase (AP) as a tissue-specific enzyme. We obtained egg fragments from the unfertilized eggs of Ciona savignyi by means of centrifugal force . The largest fragment (red fragments) contained the egg nucleus while other small fragments (black, clear and brown fragments) were anuclea te. When inseminated, only red fragments developed into partial embryo s, which showed only epidermis cell differentiation and, very rarely, AP activity. When red fragments were fused with other fragments, only black fragments promoted AP expression, suggesting that endoderm deter minants were concentrated in the black fragments. A lower dose (1500 J /m(2)) of ultraviolet (UV) light did not eliminate the AP-promoting ab ility of black fragments, while this dose significantly repressed the ability to promote the expression of the muscle-marker. A higher dose (4500 J/m(2)) of UV light markedly reduced the AP-promoting activity o f black fragments. These results suggest that factors for endodermal A P development are inactivated by UV irradiation, but are more resistan t than muscle determinants.