A CYTOPHOTOMETRIC INVESTIGATION OF THE RELATIONSHIP OF DNA AND RNA SYNTHESIS TO ASCUS DEVELOPMENT IN SORDARIA FIMICOLA

Abstract

Results from quantitative cytochemical analyses of Feulgen-DNA and azure B-RNA content of different stages of ascus development in the pyrenomycete Sordaria fimicola Ces. and DeNot. may be summarized as follows: (a) DNA replication in nuclei of the prefusion ascus (penultimate cell) precedes karyogamy, hence the prefusion nuclei are 2C (replicated haploid), and the fusion nucleus 4C (replicated diploid) in DNA content. These data show that premeiotic S phase and genetic recombination cannot be coincident since DNA replication is complete prior to nuclear fusion in the crozier penultimate cell. (b) DNA replication in the basal and terminal cell nuclei of the crazier does not occur until DNA replication is complete in the subterminal (penultimate) cell. Therefore, DNA replication occurs first in the cell which forms the primary ascus. (c) During ascus enlargement, which occurs during pachytene and diplotene, the total ascus RNA content increases from 18.9 to 195.4 arbitrary units (a.u.), while at the same time nucleolar RNA increases from 0.5 to 1.7 a.u. The concurrent increase in nucleolar and ascus RNA suggests that a rather substantial proportion of the increase in total ascus RNA may be due to the synthesis of ribosomal RNA. (d) In asci which contain binucleate ascospores the total RNA content has increased to 281.5 units. This increase may represent a storage of RNA to be utilized for protein synthesis during the subsequent germination of the ascospore. (e) Ascospores in mature asci of eight day old cultures were always multinucleate. The multinucleate condition is presumably established in preparation for ascospore germination and the initial growth of the germ tube.