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Two types of autosomal mutations have been induced by EMS. One type (enhancer) has the same effect on the zeste phenotype as have additional white subloci, the other (suppressor) acts similar to white subloci deficiencies. Enhancers and suppressors are dominant mutations with recessive lethal effects. Their interactions with different zeste and white alleles are discussed.

The amount of histone H1 relative to core histones has been determined in three Drosophila species (D. melanogaster, D. texana and D. virilis) in chromatin from several tissues differing in chromatin structure and genetic activity. Low levels of H1 were found in relatively undifferentiated, early embryos as well as in a line of cultured cells. In late embryos the content of H1 was highest in D. virilis which possesses larger amounts of and a partially more compacted constitutive heterochromatin than the two other species. Polytene chromatin from larval salivary glands showed increased levels of H1 compared with diploid chromatin and the degree of phosphorylation of this histone was relatively low. The degree of phosphorylation of H2A was found to be drastically reduced in polytene as compared with diploid embryonic chromatin, which parallels the extensive underreplication of constitutive heterochromatin. Also, in diploid chromatin a qualitative correlation was observed between the relative amounts of heterochromatin and the levels of H2A phosphorylation. These findings suggest a connection between H2A phosphorylation and heavy compaction of interphase chromatin.