Chromatin elimination in the hypotrichous ciliate Stylonychia mytilus

Chromatin elimination in the hypotrichous ciliate Stylonychia mytilus was studied by means of electron microscopy using a microspreading procedure. In the polytene chromosomes of the macronuclear anlagen three organization patterns are observed: Bands of various size composed of 300 A chromatin fibers, large blocks of 300 A nucleofilaments which probably represent the "heterochromatic" regions of the chromosome and axial 120 A filaments. Those DNA sequences which become eliminated belong to the 300 A fiber type. The eliminated chromatin occurs in the form of rings of variable size corresponding to a DNA content between 18 and 160 Kb while the axial 120 A filaments appear to be preserved.