The effects of growth in vitro on the chromosome complement of Daucus carota (L.) suspension cultures

Abstract

The chromosome number distributions and modal karyotypes of several suspension culture lines of Daucus carota L. have been analysed at various times after initiation. All lines had stable modal chromosome numbers and karyotypes, with small but significant variation about the modes. Some lines showed a predominance of diploid cells with a karyotype similar to the plant. Polyploid multiples of the modal chromosome number were present in all lines at low frequency. Variation of the 2,4-D concentration in the culture medium produced little alteration of the chromosome number distributions, but omission of 2,4-D produced a significant drop in the frequency of multipolar mitoses in those culture lines in which this treatment induced differentiation. There was no evidence of any direct effect of 2,4-D on general mitotic dynamics. Alteration of the frequency with which cultures were transferred to fresh medium showed that stationary phase was critical in the maintenance of the low frequency of tetraploids present in a predominantly diploid culture line. The results are explicable in terms of a competitive selection for cells with the dominant modal chromosome number in the presence of various mechanisms continuously producing polyploid, aneuploid and structurally altered karyotypes.