Evidence for Chromosomal Macronuclear Substructures in Tetrahymena*†

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Abstract
Under the growth conditions employed, the G1 macronucleus of T. pyriformis strain HSM contains 7.4 .times. 10-12 g DNA, the G2 micronucleus 0.42 .times. 10-12 g. DNA content from the T. thermophila macronucleus did not significantly differ from that of HSM, but the micronucleus contained about twice as much DNA as the micronucleus of the HSM cells. The T. thermophila macronucleus contained on average enough DNA for .apprx. 35 haploid micronuclear copies. A new spreading technique allowed separation of macronuclear substructures from cells of late G2 to early G1. Photometric determination of DNA content of 345 individual structures suggested the existence of 5 different-sized macronuclear structures with a DNA content corresponding to 2, 4, 8 and 16 times the basic values. Comparison of the DNA content of these structures with mitotic micronuclear chromosomes and meiotic micronuclear chromosomes of T. thermophila cells suggests that the 5 basic values of macronuclear structures derive from structures of micronuclear chromosomes. The micronuclear chromosomes of T. pyriformis may be oligotenic. These results may further understanding of macronuclear organization.