chromosome aberrations in Tradescantia 715 



ever, that a certain degree of genetically controlled difference in sensi- 

 tivity among species or within a given species may exist. 



The stage in the life cycle of Tradescantia which has proved most 

 favorable for observations of radiation effects on chromosomes is the 

 first postmeiotic mitosis in the developing microspores. Following 

 meiosis, the haploid microspores (having six chromosomes) go through a 

 regular mitotic cycle, and the chromosomes can be observed at metaphase 

 or anaphase, utilizing simple acetocarmine or other smear techniques. 

 The various mitotic stages are usually fairly well synchronized in the six 

 anthers of one bud, and a smear preparation of an appropriately selected 

 single bud will usually contain from 100-400 cells in a stage of mitosis 

 suitable for chromosome observations. Since the different buds on a 

 single inflorescence contain microspores in successive stages of develop- 

 ment, it is possible with one radiation exposure to treat cells in different 

 stages of mitosis. Observations of nuclei at metaphase or anaphase can 

 then be made at intervals after irradiation and the types and frequencies 

 of various chromosome aberrations determined. Since the duration of 

 the various stages in microspores has been determined (Sax and Edmonds, 

 1933) , it is possible to make certain correlations of aberration types and 

 frequencies with particular stages in the mitotic cycle, especially with the 

 resting stage and prophase. 



When inflorescences are exposed to penetrating radiation and the cells 

 in metaphase and anaphase examined immediately afterward, the chromo- 

 somes can be seen to exhibit a type of stickiness during which certain 

 chromosome parts, especially ends, tend to adhere. This adherence may 

 lead occasionally to bridging and fragmentation at anaphase. Such a 

 condition has been attributed to an effect of the radiation on the nucleic 

 acid forming the matrical material of the chromosome (Darlington, 1942). 

 This type of transient aberrant behavior is soon replaced, as seen in cells 

 examined a few hours after exposure, by the typical permanent aberration 

 types resulting from chromosome breakage and the reunion of broken 

 ends with which the subsequent discussion will be concerned. 



TYPES OF ABERRATIONS 



Observations at increasing intervals of time following irradiation have 

 shown the existence of two major categories of aberration types differing 

 in whether they are derived from chromosomes which are effectively 

 single or double at the time of irradiation. 



Chromatid Aberrations. The first category consists of types produced 

 in chromosomes which are effectively double at the time of irradiation. 

 Chromatid aberrations appear first, within a few hours after irradiation, 

 reach a peak 12-20 hours later, and are the only types present up to about 

 26 hours (under spring and summer growing conditions; see Sax, 1941). 

 Following a relatively brief transition period, in which both chromatid 



