Chromosomal analysis on Yoshida tumor cells sensitive and resistant to dibromodulcitol

Yoshida ascites tumor in Wistar rats was rendered resistant to dibromodulcitol. The modal number of chromosomes in the parent line was reduced from 39 to 36 in the resistant one. The parent tumor was a mixed cell population, containing large subtelocentric (LST) chromosome in 65 per cent of the cells, while in the remaining 35 per cent the LST chromosome was absent. In the resistant tumor line, however, the LST marker chromosome was present only in a few cells. The resistant line preserved this new karyotype throughout the 5 months of passaging even without dibromodulcitol, however, parallel to the loss of resistance this tumor line regained the original number of chromosomes, i. e. 39. Selection of a cell population with reduced number of chromosomes seems to be associated to the development of drug resistance.