Ultrastructure of the macronuclear anlage in Stylonychia mytilus

Abstract

The development of the macronuclear anlage of Stylonychia mytilus was studied by means of electron and light microscopy of some timed stages following conjugation. In the 16-h-old anlage, the chromosomes occur as condensed short rods consisting of coiled threads about 220 Å wide. In the 32-h-old anlage, the chromosomes appear as elongated elements in which more opaque dense regions (about 0.5 μm in diameter) alternate with less dense expanded areas (1.5 μm or more in diameter). The fine structure of such extended regions reveals both fibrils and granules within. Concomitant with the chromosomes at such a puffed stage numerous peripheral nucleolus-like bodies (0.3–1.2 μm in diameter) are present in the macronuclear anlage. In the 40-h-old anlage well developed banded polytene chromosomes are observed. Some of them exhibit deeply stained thickenings comparable to DNA puffs. At that stage, the peripheral spongy nucleolus-like bodies are absent. The nuclear envelope is smooth in early macronuclear anlagen but appears folded in the older ones, showing abundant evaginations that occasionally lead to extrusion of nuclear material into the cytoplasm. These findings are discussed in relation to the synthetic activity of the exconjugants.