A new karyotype of Allium cepa L., containing three nucleolar chromosomes (A preliminary note)

Abstract

Root-tips of Allium cepa L. were pretreated with 0.5 per cent aqueous solution of chloralhydrate, fixed with La Cour 2BE, stained by Feulgen's technique and squashed in 45% acetic acid. The chromosome number is found to be 2n=16, of which 2(S₁S₁ type, Fig, 1a) or 3 chromosomes (S₁S₁S₂ type, Fig. 2a) being nucleolar. The former type is one reported repeatedly by the previous workers. The latter is found newly by the present study. These two types showed maximum number of nucleoli 2 and 3 (Figs. 1b, 2b) respectively according to the number of nucleolar chromosomes contained. Both size of satellite and length of connecting filament showed a considerable variation between indivduals, between cells of one and the same individual (Fig. 3b) or even between homologues within a cell (Figs. 1a, 3a). Among 50 bulbs studied, 10 revealed to be S₁S₁S₂ type, 36 to be S₁S₁ type and the remaining 4 were not identified as to this point. The new type nucleolar chromosomes, S, is suggested to originate from some structural rearrangement or from some alteration in the balance of nucleolar-forming capacities between the chromosomes in the nucleus.