A new karyotype of <i>Allium cepa</i> 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.