Journal of Cell Science, Vol 6, 615-627, Copyright © 1970 by Company of Biologists

Submitted on October 13, 1969

Differential Effects of K⁺ and Na⁺ on Specific Bands of Isolated Polytene Chromosomes of Chironomus Tentans

¹ Department of Biological Sciences, Northwestern University, Evanston, Illinois 60201, U.S.A.

Previous investigations on the effects of monovalent cations on the gene activities (formation of puffs) of polytene chromosomes utilized whole salivary glands or nuclei and suggested that the K⁺/Na⁺ ratio in the nuclear sap was of primary importance in influencing gene activities. To determine whether these effects of ions were direct or indirect, we have employed isolated polytene chromosomes incubated in salt solutions of varying ionic composition. Our observations reveal differential effects of K⁺, Na⁺, Mg²⁺, Ca²⁺ and combinations thereof on specific chromosome regions. The band at region I-18-C that is specifically affected by K⁺ is the same one that puffs when salivary gland nuclei are incubated in a K⁺-rich medium and this is believed to be an ecdysone-specific puff. Bands at region I-19-A are specifically affected by Na⁺ and are the same ones that form puffs when salivary gland nuclei are incubated in a Na⁺-rich medium. This region is believed to be juvenile hormone-specific. Effects on the chromosome are also described that are the result of incubation in solutions containing Mg²⁺ and Ca²⁺. Analogies are drawn between the observed effects of ions on isolated chromosomes and their effects on chromatin isolated from mammalian cells. For example, when chromatin is exposed to solutions of increasing ionic strength, hydration is followed by aggregation and histone dissociation. This may be analogous to what is observed in the isolated chromosome under similar conditions; namely, expansion, contraction and expansion.