Morphology and development of the macronuclei of the ciliates Stylonychia mytilus and Euplotes aediculatus

Some stages of macronuclear anlagen development, known from earlier investigations (see Fig. 1), were studied in detail. The results are: a) The giant chromosomes of Stylonychia mytilus are not somatically paired, but are connected end-to-end to form one or a few composite chromosomes. When they later disintegrate, the bands become isolated granules. b) Spectrophotometric measurements show that during the DNA-poor stage which follows the disintegration of the chromosomes, the macronuclear anlagen of Euplotes have a DNA content of 21 c, while the syncaryotic (deriving from syncarya) and hemicaryotic (deriving from haploid hemicarya) anlagen of Stylonychia have the DNA content of diploid micronuclei (2c). Nevertheless the syncaryotic anlagen of Stylonychia and Euplotes initially develop two nucleoli at the end of this stage, the hemicaryotic anlagen of Stylonychia only one. From this it is concluded that the genes of one giant chromosome band stay together in one granule, c) Labeled DNA from the giant chromosomes which remains in the anlagen during the DNA-poor stage is distributed approximately equally to the daughter nuclei during the first few fissions of the exconjugants.-Autoradiographic experiments showed that the DNA of the macronuclei of Stylonychia that is duplicated at one time in a replication band is not duplicated simultaneously during the next DNA-duplication. The DNA duplications during the second polyploidization stage of the macronuclear anlagen development are exceptions, because the mixing of the macronuclear DNA which occurs before every fission does not occur during the second polyploidization stage.—The pseudomicronuclei which sometimes are formed from the macronuclei in emicronucleated strains of Stylonychia contain numerous elements which are much smaller than the chromosomes.—The macronucleus of Stylonychia is very insensitive to irradiation with X-rays.—The results lead to the following hypothesis: The macronuclei of the two hypotrich ciliates contain unconnected chromomeres or small aggregates which are distributed at random to the two daughter nuclei during the divisions.Research supported by the Deutsche Forschungsgemeinschaft.