METABOLIC INHIBITORS AND CHROMOSOME REJOINING

BEATTY, ALVIN V., and JEANNE W. BEATTY. (Emory U., Atlanta, Ga.) Metabolic inhibitors and chromosome rejoining. Amer. Jour. Bot. 46(5): 317-323. 1959.-X ray-induced chromosomal aberrations of the dicentric and ring types in the first microspore division in Tradescantia pcaludosa were used to study the effects of temperature and chemical substances acting as metabolic modifiers on rejoining under both aerobic and anaerobic conditions. A total dose of 400 r of x rays was administered at 50 r/min. at temperatures ranging from 0.3?C. to 45?C. In the aerobic temperature experiments, 5% oxygen was used in either helium or carbon monoxide and in the anaerobic ones helium was used. In 5% oxygen in helium, the highest number of aberrations was produced at the lowest temperature. When the amount of oxygen in solution was made constant at all temperatures by adjusting the positive pressure of gas in experiments carried out above zero degrees, there was a temperature effect at 0.3?C. and 10?C., but no effect from 200C. to 450C. Temperature was believed not to influence chromosome breakage but only rejoining, through its influence on oxidative metabolism, thereby removing the source of energy. In the anaerobic temperature experiments, a low aberration yield of 0.14 occurred at 0.30C., while a high of 0.66 was recorded at 400C. Cytochrome oxidase was found not to be involved in the energy supply for rejoining in anaerobic experiments, since exposures in alkaline pyrogallatewashed carbon monoxide gas gave the same results as helium, indicating that both gases only provide anoxic conditions. Under anoxic conditions, pretreatment in M/1000 sodium iodoacetate or M/10,000 p-chloromercuribenzoate solutions had no effect on non-irradiated plants thus treated, but increased the average number of chromosome aberrations per cell produced by x rays at least 2-fold over the irradiated controls. The effect of these 2 salts, as used in these experiments, is believed to have taken place through their inactivation of triosephosphate dehydrogenase, thus inhibiting glycolysis at an early stage before any energy was released. THE CALCULATIONS by Lea (1947), that 95% of the primary chromosome breaks produced by radiation rejoin so that the chromosomes are returned to their pre-irradiated state, suggest that cell constituents may have an amazing inherent capacity to recover from radiation damage. Why the cell does not completely recover and what can be done to facilitate recovery present an intriguing area of investigation. Most of the recent work with protective agents has shown that such agents, presumably by their influence on the radiochemical events in the cell, probably operate to reduce the primary damage. A few investigations, however, indicate that some agents might operate on the recovery process (Wolff and Luippold, 1956). Since a tabulation of the number of actual chromosome breaks produced at the time of irradiation cannot be made at the present time, Beatty et al. (1956) and Beatty and Beatty (1957) have attempted to learn something about the frequency of breaks in Tradescantia chromosomes by an indirect method. The procedure employed was to attempt a delay of the rejoining process, so that a maximum utilization of the original breaks would be possible. Using 5%o oxygen in helium and a total dose of 400 r of x rays at intensities of 1 r, 50 r and 400 r/min., aberration yields of 0.17, 0.48 and 0.58 'Received for publication July 29, 1957. Work performed under Contract No. AT-(40-1) -1345 with the United States Atomic Energy Commission. were obtained; but when the oxygen was used in carbon monoxide instead of in helium, the aberration yield in each case was around 0.88. At 400 r/min., irradiation in the oxygen-helium mixture, followed by post-treatment in carbon monoxide, also gave about the same aberration yield. These results (substantiated by other unpublished data) led the authors to conclude that a total dose of 400 r of x radiation in 5%o oxygen, within the limits of these experiments, must produce a constant number of primary breaks from which there must be a maximum number of possible chromosome interchanges. The results indicated that this maximum interchange yield was about 0.88 per cell. Also, Beatty et al. (1956) and Beatty and Beatty (1957), using a helium atmosphere to maintain an anoxic state, irradiated the material with a total dose of 400 r in each of the following experiments: (1) at 1 r/min; (2) pretreatment in helium for 400 min. followed by irradiation at 50 r/min.; (3) pretreatment for 400 min. in helium, followed by irradiation in combination with carbon monoxide; (4) pretreatment in helium for 400 min. at 30?C., followed by irradiation at a temperature of 1?C.; (5) a combination of all of the last 3; and (6) experiments carried out at 40?C. with no pretreatment but with immediate irradiation at 50r/min. The authors concluded that in an anaerobic state, 400 r of x radiation must produce a constant number of primary breaks, differing from that in 5% oxy[The Journal for April (46: 283-316) was issued April 23, 1959] AMERICAN JOURNAL OF BOTANY, Vol. 46, No. 5, May, 1959