Section 5 — Mutagenesis 



5.94. Radiosensitivity of the Chromosomes. N. Ta- 



naka (Tokyo, Japan). 



Freshly rooted plantlets of Tradescantia palu- 

 dosa were divided into four groups: the first 

 group was grown in a culture solution, aerated 

 and supplemented with tritiated thymidine 

 (2uC/ml) for 8 to 12 hr (internal beta-ray irra- 

 diation); the second group was treated identically 

 as the first group but exposed simultaneously to 

 gamma-irradiation at a distance 40 cm from 

 Co 60 source (1.19 rad/hr) (both internal and ex- 

 ternal irradiation); the third group was exposed 

 only to the gamma-irradiation at the same in- 

 tensity as the second group (Co 60 external ir- 

 radiation); and the fourth group served as a 

 control. 



The root tips were fixed with acetic alcohol at 

 relevant time intervals. Radiosensitivity of the 

 chromosomes was measured in four ways by 

 observing frequency of the total aberrations, 

 fraction of sister chromatid reunion distal (SUD), 

 fraction of sister chromatid reunion proximal 

 (SUP), and fraction of erosion proximal (EP) in 

 the mitotic anaphases. 



The treatment was carried out at room tem- 

 perature 26 C and the mitotic cycle was estimated 

 to stand around 16 hr, being Gi=2.8 hr, S = 8.4 

 hr, G 2 = 2.8 hr, and M = 2.7 hr, where Gi, S, G 2 , 

 and M designated first growth period, DNA 

 synthetic period, second growth period, and 

 mitosis, respectively. 



Radiosensitive period as measured by fre- 

 quency of total reunion (SUD + SUP + EP) was 

 found at S and G2 in the first group and at S in 

 the second group. Frequency distributions of 

 each SUD, SUP, and EP in the second group 

 contrasted with each other, suggesting the com- 

 bined irradiation enhanced SUD in S and G2 

 while SUP in Gi and S, and for EP in G 2 . 



5.95. Chromosome Aberrations and the Mitotic Cycle 

 in Trillium. C. J. Grant (Oxford, Great 

 Britain). 



The mitotic parameters of root tips of Trillium 

 grandiflorum were measured by the flash label- 

 ing technique, and confirmed by the binucleate 

 cell method. At 25 C the period before DNA 

 synthesis Gi, the synthetic period S, and the 

 post-synthetic period G 2 , were of two, one and 

 two days duration respectively. 



It was found that the Xi, X 2 and X 3 divisions 

 were labelled in such a way as to confirm the 

 view that chromosomes duplicate before normal 

 cell division by a semi-conservative method. 



After X-irradiation of root tips, cells con- 



taining the so-called sub-chromatid, chromatid 

 and chromosome type aberrations are found. 

 Overlap of these types may occur within a single 

 cell, and in species with a short mitotic cycle this 

 makes it difficult to correlate the different aber- 

 ration types with the state of the cell at the time 

 of their induction. However the relatively long 

 cycle in Trillium results in little overlap and 

 chromosome type aberrations can be definitely 

 correlated with induction during Gi, Chromatid 

 types with G 2 , and sub-chromatid types with 

 prophase. Chemical agents induce chromatid 

 types during S. 



5.96. Cytological Effects of Tritiated Water on Root- 

 tip Meristems of Allium cepa L. G. Wittmer 

 (Rome, Italy). 



Series of onion bulbs were grown, after ger- 

 mination, in tritiated water having specific 

 activity of 2.5 mc/ml. Cytological examinations 

 performed at different times from exposure to 

 THO have shown: 



progressive mitoinhibition; after 42-48 hr, the 

 treatment is lethal; 



no physiological effect on the chromosomal 

 matrix: phenomena of stickiness were only 

 seldom observed; 



strong direct effect on mitotic chromosomes, 

 i.e. very high fragmentation in anaphase (as 

 far as 90 per cent of anaphases with frag- 

 ments); the fragmentation is highest after 

 24-32 hr, and at this time also the meta- 

 phases show, if even in a low grade, frag- 

 ments; the fragments are nearly always 

 acentric and free, rarely attached; 



these mitosis with aberrant anaphases (or 

 metaphases) may end their cycle and yeld 

 cells with 1-2 micronuclei near the nucleus; 



"restitution" of these breakages induced by 

 (3-rays occurs quite frequently, since root- 

 tips transferred in tap-water after the THO 

 treatment show progressive normalization 

 of the mitotoc activity, with only rare chro- 

 mosomal aberrations. 



The treatments were performed in the Labora- 

 tories of the Istituto di Chimica Farmaceutica 

 delTUniversita di Roma. 



5.97. Effect of X-rays on Chiasma Frequency in Coix 

 aquatica Roxb. K. C. Subbaiah (Bombay, 

 India). 



The present study relates to the effect of radia- 

 tion on chiasma frequency in Coix aquatica 



89 



