Development in the floating world: Defenses of eggs and embryos against damage from UV radiation

Eggs and embryos of many aquatic organisms develop in the water column and can experience ultraviolet radiation with potentially deleterious effects. This is especially vexing for floating embryos that develop in the surface or neuston layer. Radiation damage can be a particular problem for these em bryos since the cell division cycle during the cleavage period is quite sho rt and often these cycles do not have mitotic checkpoints to insure faithfu l transmission of DNA to the daughter cells. This could result in cell divi sion with unrepaired DNA in the blastomeres, which could impact embryogenes is and the transmission of the genome through the germ line. Described stra tegies to restrict radiation damage include mechanisms to limit oxidative d amage and the use of sunscreens such as the mycosporines to curb radiation to sensitive targets. We describe a particularly ingenious use of sunscreen s in the tunicate embryo, the use of extra-embryonic cells to shield the em bryo from potentially harmful UV-A and UV-B radiation. We also raise questi ons regarding the nature of UV damage to embryos (is it DNA or also protein ) and the characteristics of DNA repair in such embryos. It is likely that unique mechanisms are present in floating embryos that develop in this air- water interface to assure that cell and genomic integrity are maintained in this challenging environment.