[Changing number of histone structural genes in Drosophila]

It has been shown in the previous paper that if one Drosophila chromosome lacks histone genes, the intact homologous chromosome has an increased number of histone genes. The present study shows that the extent of compensatory multiplication of histone genes depends on the nature of the deficient chromosome. An extra Y chromosome in the genome of flies without the deficiency does not affect their histone gene content. In heterozygotes with a deficiency of histone genes the number of these genes grows gradually and reaches 90% of the norm in the eight generation (magnification). After the deficient chromosome is eliminated the increased number of histone genes is not stably inherited and reverts to the normal level in the course of 5--7 generations. Deficiency-heterozygous males of the first generation contain extrachromosomal histone DNA and have a changed ratio of the two types of histone gene blocks. The multiplication of histone genes is compared with the compensation and magnification of rDNA.