The Nucleolus of wheat stem rust uredospores.

A cytological study of sporogenesis, mature uredospores and hydrated uredospores of Puccinia graminis tritici was made to determine if a definitive nucleolus was present. Electron microscopy has shown that the nuclei in immature uredospores and associated fungal tissue of the uredosorus possess prominent nucleoli. Nucleoli whose average diameter was 1.7 microns were observed in 58 percent of the nuclear sections from immature uredospores. Presence of a nucleolus in ultrathin sections of mature uredospores is established. Nucleoli whose average diameter was 0.5 microns were observed in 6 percent of the nuclear sections from mature uredospores. Nucleoli were not observed in hydrated uredospores which had resumed active metabolism. The reduction in the size of the nucleoli in mature uredospores and absence of nucleoli in hydrated uredospores may indicate that ribosomal RNA synthesis is repressed as uredospores mature.Land and Food Systems, Faculty ofGraduat