Template properties of embryo chromatin from eggs of the milkweed bug, Oncopeltus fasciatus

Histone content in relation to amount of heterochromatin and developmental stage in three species of Drosophila

Relative amounts of various histone fractions in Drosophila chromatin were estimated densitometrically on electrophoretic gel separations. Several consistent and highly significant differences were obtained between larval and adult chromatin. The arginine-rich histones showed the most conspicuous changes: higher amounts of H4 in larvae, higher H3 in adults. The level of modification of these histones was clearly higher in larval than in adult chromatin. The modification of the two slower subfractions of H4 involved, in all probability, phosphorylation as well as acetylation. In all types of Drosophila chromatin studied 50% or more of the H2a molecules were phosphorylated--a remarkably high proportion. The species differences observed in relative amounts of histone were consistent in both stages of development. D. melanogaster differed from D. hydei and D. virilis in all histones except H2b, while the latter two species were generally similar. The interspecific variation in histone pattern was generally not correlated to differences in content of heterochromatin. The level of modification of H2 was, however, presumably an exception, as it was significantly lower for both larvae and adults in D. virilis than in the other two species. These differ from D. virilis in containing appreciably lower proportions of heterochromatic chromosome segments.

[Autoradiographic Studies on RNA and Protein Synthesis in the Early Embryogenesis ofMusca domestica]

1) ³H-Uridine incorporation into the RNA of eggs ofMusca domestica was measured by means of autoradiography. In addition, protein synthesis was studied by measuring incorporation of³H-amino acids. 2) The onset of embryonic RNA synthesis was found to occur after the 10. division of the cleavage nuclei. 3) Protein synthesis occurs in mature oocytes as well as in freshly laid eggs. The rate of synthesis in fertilized eggs begins to increase when the cleavage nuclei penetrate the periplasma. At the end of blastodermal development, the rate of protein synthesis reaches a value which remains constant until differentiation of the germ layers. Immediately after deposition of the eggs, the rate of incorporation of³H-amino acids is higher in unfertilized eggs than in fertilized ones. 4) At oviposition the radioactivity is uniformly distributed over the whole egg. During further development more radioactivity is accumulated in the periplasm than in the inner part of the egg. Already before penetration of the cleavage nuclei, a higher rate of protein synthesis is observed in the posterior pole-plasm than in any other part of the periplasm. 5) Immediately before the beginning of organogenesis, a region specific incorporation of³H-amino acids into nuclei can be observed. 6) The rÔle of the maternal RNA in protein synthesis in the developing egg as well as its rÔle in the determination of certain egg regions (especially of the posterior pole-plasm) is discussed. The beginning of RNA synthesis and the simultaneous increase in the rate of protein synthesis are interpreted as processes of embryonic differentiation. It is assumed that the maternal RNA stored in the pole-plasm has a determinative function.