Critical-Point Dried Macronuclear and Micronuclear Chromatin Fibers of Hypotrich Ciliates

Macronuclear and micronuclear chromatin fibers from several hypotrich ciliates were examined by electron microscopy after spreading on a Langmuir trough and subjecting to critical-point drying. Three size classes of "unstretched" macronuclear fibers were seen: (a) 10-15 nm, (b) 23-28 nm, and (c) 60-65 nm. The 60-65 nm fiber predominates in gently lysed macronuclei and is probably representative of the most highly supercoiled chromatin. Micronuclei exhibit a 25-nm fiber exclusively. Examination of numerous macronuclear spreads has revealed a tangle of fibers with no obvious organization above the fiber level. For example, they are not structured into small spherical bodies as are the fibers of Paramecium and Tetrahymena. On the basis of morphological evidence, the interdivisional hypotrich macronucleus appears to contain a moderate number of long chromatin units (large linkage groups) rather than numerous gene-sized units such as are found in purified hypotrich macronuclear DNA.