Nucleolar fusion in wheat

Two pairs of major nucleoli form in cells of the wheat variety Chinese Spring and these frequently fuse during the cell cycle. The volume of the nucleolus formed at the organizer on chromosome iB is approximately twice the volume of the nucleolus formed at the organizer on chromosome 6B. The volumes of fused nucleoli are distributed around the additive means expected if the nucleoli of the two different sizes fused in all possible combinations. Thus nucleolar volume but not the surface area is additive on fusion. Four nucleoli can fuse to produce nuclei with nine different nucleolar patterns. All nine were found but the frequencies of the different fusion classes were different in different genotypes. The frequencies in euploid Chinese Spring and tetrasomie 3B showed small deviations from those expected if fusion between different nucleoli occurred with equal frequency, but in the aneuploids DT 5BL and DT sAL the fusion of heterologous nucleoli from chromosome 1B and 6B organizers occurred with a much higher frequency. It is suggested that the pattern of nucleolar fusion is determined in part by the position of the organizers in the nucleus, and their positions are altered in these ditelosomic stocks.