CHROMOSOME FRAGMENTATION AND REJOINING IN TRILLIUM ERECTUM 



fragmentation occurring at microspore anaphase between stages nor was 

 there a difference in the number of reunions taking place as a result of 

 diplotene and first metaphase irradiation. However, when the total 

 amount of reunion occurring as a result of diplotene irradiation was calcu- 

 lated, that is, the combined rejoining at first anaphase and microspore 

 anaphase, it was found that rejoining was significantly higher in cells irradi- 

 ated at diplotene than in cells irradiated at first metaphase, although both 

 stages were equally sensitive to breakage. The combined data of first 

 anaphase and microspore anaphase also indicates that rejoining calculated 

 as a function of fragmentation occurs 2-3 times more frequently after 

 diplotene and interphase than after first metaphase X-irradiation. 



Ratios formed between the number of fragments and rejoins observed for 

 each stage irradiated with thermal neutrons and X-rays indicate an over-all 

 similarity in response to both types of radiation. Diplotene chromosomes 

 are comparably sensitive and appear to have a greater ability to rejoin than 

 chromosomes at other stages while first metaphase irradiated material 

 consistently shows high sensitivity and low rejoining capacity. 



In conclusion, no consistent relationship has been found to exist between 

 the rejoining capacity and degrees of sensitivity exhibited by chromosomes 

 of Trillium erectum exposed to irradiation at various stages of microsporo- 

 genesis. Consideration of the data seems to indicate that the processes of 

 breakage and reunion may be controlled by two independent mechanisms 

 or alternately that they are two processes whose relationship in time is 

 obscured because while they are closely associated, both phenomena do not 

 proceed at the same rate. In addition, it would seem that not only are 

 there differences in radio-sensitivity of various stages to chromosome break- 

 age, as for example, first metaphase and interphase, but also, stages similar 

 in sensitivity to fragmentation, like first metaphase and diplotene, differ 

 with respect to the degree of rejoining which follows. 



REFERENCES 



1 Sparrow, A. H. and Maldawer, M. Proc. Nat. Acad. Sci. 1950, 36 636-643. 



2 Deschner, Eleanor E. and Sparrow, A. H. Genetics, in press. 



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