THE RELATIONSHIP BETWEEN CHROMOSOME 



FRAGMENTATION AND REJOINING IN 



TRILLIUM ERECTUM FOLLOWING THERMAL 



NEUTRON AND X-IRRADIATION 



Eleanor E. Deschner* 

 Brookhaven National Laboratory', Biology Dept., Upton, Long Island, New York 



An investigation into one of the factors affecting the degree of radio-sensi- 

 tivity of the nuclear cycle in Trillium erectum was reported by Sparrow and 

 Maldawer^ in 1950. Evidence was presented by them indicating that 

 greater rejoining occurred at a relatively radio-resistant stage, interphase, 

 accounting for a part of the difference in the total visible chromosome 

 fragmentation while a small amount of rejoining occurred after X-irradia- 

 tion at first meiotic metaphase, a highly sensitive stage. However, the 

 increased number of reunions at the apparently resistant interphase stage 

 was not sufficient to explain the decrease in total visible fragmentation. 



A further study along these lines has been carried out in an effort to 

 determine if this inverse relationship between breakage and I'ejoining would 

 hold true at other stages of meiosis and if, in general, the frequency of 

 reunions be closely associated with the degree of sensitivity. In addition, 

 a similar study was undertaken using thermal neutron irradiation in an 

 effort to learn how the two radiations differ, if at all, in their effect on the 

 processes of breakage and reunion^. 



Propiono-carmine smears of anthers irradiated at known stages of micro- 

 sporogenesis were examined at first anaphase and microspore anaphase 

 in order to pick up both immediately visible and delayed breakage. Frag- 

 ments were scored as an index of the breakage sensitivity and dicentric and 

 ring chromosomes were used as an index of the degree of reunion. 



Data collected at first anaphase as a result of 50r X-irradiation at zygotene, 

 pachytene, and diplotene indicates that while no statistically significant 

 difference in breakage sensitivity existed, the amount of recombination of 

 broken ends was significantly higher at diplotene. Cells irradiated at diplo- 

 tene displayed a higher breakage sensitivity at microspore anaphase than at 

 first anaphase although the same degree of rejoining was found to occur at 

 both stages. The combined data at first anaphase and microspore anaphase 

 revealed that a significantly greater degree of fragmentation and reunion 

 occurred after diplotene irradiation than after pachytene or zygotene 

 irradiation. 



In comparing the relative sensitivity of diplotene and first metaphase, an 

 analysis of variance showed that there was no significant difference in 



* Present address : Radiobiological Research Dept., Mount \'ernon Hospital. Northvvood, 

 Middlesex, England. 



275 



