Cytogenetic Studies on Haplopappus gracilis Cells Cultured on Agar and in Liquid Media

Abstract

Cytogenetic behaviour of Haplopappus gracilis (Nutt.) Gray cells cultured on agar or in liquid B5 medium containing 4.5×10^-6 M 2, 4-D, was studied. The nuclei in explants (hypocotyl cells from three-day old seedlings) had 2C, 4C and 8C DNA values and showed a continuous distribution. The explants fixed 72 hrs after they were placed on the culture medium, showed only diploid and tetraploid mitoses. On the agar medium, the frequency of polyploid cells increased with time, while in the suspension cultures, a drastic increase in the frequency of diploids was observed. Longer transfer intervals in the suspension cultures resulted in an increase in the frequency of tetraploid cells. It is suggested that endoreduplication is the major factor involved in the increase of the polyploid frequency.
The frequency of chromosome I was greater than expected in the majority of the aneuploid cells, and in the suspension cultures many diploid cells had a deleted chromosome II. It is suggested that the genes present in chromosome II are comparatively less essential than those present in chromosome I for the in vitro survival of H. gracilis cells in the culture systems used.