The effect of thiouracil on the viability of eggs and embryogeny in Pteridium aquilinum

Gametophytes of Pteridium aquilinum fed for six days with 50 and 75 p.p.m. thiouracil ceased to produce archegonia, but archegonia already initiated completed their development. Eggs produced in the presence of 50 p.p.m. thiouracil were viable, but embryogenesis was retarded. Gametophytes fed with 75 p.p.m. thiouracil mostly remained barren after insemination, the eggs being inviable, but others produced outgrowths from the archegoniate region which were either gametophytic, or sporophytic with a tendency to revert morphologically and functionally to gametophytic tissue.Thiouracil administered after fertilization either prevented embryogenesis, or retarded it, the sporophytes then being deformed and often showing a tendency to produce gametophytic tissue. The embryo became less sensitive to short periods of thiouracil as it developed, possibly because of the appearance of relatively quiescent regions, serving as reservoirs of unaffected cells.The results are held to support the view that the genes responsible for sporophytic growth become activated during oogenesis, and that the cytoplasm of the mature egg already contains the information leading to this kind of growth.

The synthesis and distribution of ribonucleic acid in developing archegonia of Pteridium aquilinum

Feeding gametophytes of Pteridium aquilinum with tritiated uridine, followed by autoradiography, revealed two waves of incorporation of the nucleoside into nuclei during oognesis. The first, affecting the nucleus of the primary cell of the archegonium, is interpreted as indicating the activation of the genes initiating oogenesis. The second, seen in the nucleus of the maturing egg, is believed to be concerned with the differentiation of the egg itself.The results of the autoradiography, corroborated by enzyme digestion and fluorescence microscopy, also demonstrated a high concentration of ribonucleic acid towards the periphery of the mature egg, some possibly located in nucleolus-like bodies found only in this region of the cytoplasm. It is suggested that part of this cytoplasmic ribonucleic acid may represent genetic information remaining untranslated until after fertilization.No evidence was found of any asymmetry in the distribution of the ribonucleic acid in mature eggs which might account for the polarity of developing zygotes.