Radiation and electron microscope studies on Euglenphyceae

Abstract

Two species of Euglena, Euglena gracilis and Eug1ena mutabilis, were cultured with the object of finding the conditions which would give the highest mitotic index at a predictable time. This was done, and the cell-cycle time under controlled conditions was found. Following this, mitosis was investigated by ordinary cytological methods and with the electron microscope. Using the microdensitometer, it was found that DNA replication occurred during interphase and in other organisms. Evidence for the presence of separated chromatids was found at the end of interphase (G2). Counting sectioned chromatin masses in sections photographed under the electron microscope showed the presence of separated chromatids at metaphase. It was found that irradiation made metaphase and particularly anaphase stages easily spread, and from these stages it was seen that the chromatid is characteristically V-shaped with the point of the V directed to the poles. At metaphase however the point was predominantly on the equator. Detailed study of the ultrastructure of the mitotic figures showed that microtubules are connected by arms or bridges; they are also attached laterally to the endosome and probably to the chromatin; in addition they pass through the chromatin. The relationship between chromosomes and microtubules has been followed by means of aerial transverse sections and in 1ongitudinal sections. Models for mitosis in Euglena have been devised. The ultrastructure of the endosome has been investigated and observations suggesting the presence within it a nucleolar-organising chromosomal region, have been made. The effects of irradiation have been investigated using gamma radiation in doses from 4Kr to 350Kr. A dose-tolerance curve has been plotted. LD.50 is 35Kr. The effects of radiation on the ultrastructure of the nucleus are described.