Electron-Microscopic Observations on the Macronuclear Development of Stylonychia Mytilus and Tetrahymena Pyriformis (Ciliophora-Protozoa)

This report describes an ultrastructural investigation of macronuclear development following conjugation in Stylonychia mytilus (a spirotrichous ciliate) and Tetrahymena pyriformis (a holotrichous ciliate). In S. mytilus, polytene chromosomes are formed in the young macronucleus (macronuclear Anlage). They are subsequently broken between the bands by ‘membranous’ partitions; the assembly of the membranes appears to be concomitant with the formation of the polytene chromosomes. The membranes in the Anlage appear to originate from fibrous material seen in the early Anlage. This fibrous material in the earlier stages is seen concentrated at several points along the border of the inner nuclear membrane. In the later stages it is seen in the interior of the Anlage, outlining the developing polytene chromosomes. As the chromosomes reach the maximum degree of polyteny, the fibrous material condenses to acquire a membranous appearance and extends into the interband regions. The Anlage throughout this period shows a progressive increase in size. Subsequently, the membranes enclose individually each band plus portions of the 2 adjacent interband regions of the polytene chromosomes to form a large number of vesicles. After vesicle formation the Anlage shrinks, and the chromatin inside the vesicles shows degradative changes. Finally, the vesicles disappear, the membrane degradation products appear at the nuclear membrane, and the Anlage now contains nucleoli. The Anlage increases its DNA content by multiple rounds of replication to become a mature macronucleus. The ultra-structural changes observed in the Anlage support the idea of genetic diminution (i.e. extensive DNA synthesis, elimination of many DNA nucleotide sequences, and amplification of the remaining DNA nucleotide sequences in a second period of DNA synthesis) proposed earlier on the basis of cytochemical, biochemical, and limited electron-microscope studies. In T. pyriformis, the macronuclear development differs substantially from that of Stylonychia. Features such as the formation and degradation of polytene chromosomes are absent in the macronuclear development of Tetrahymena; the young macronucleus in this cell becomes a mature macronucleus by progressive increment in size and chromatin content with no apparent genetic diminution. These observations agree with cytochemical studies on the macronuclear development of Tetrahymena.