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 H3 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.