sparrow: cytogenetic effects of ionizing radiations 77 



9. Death of nuclei or cells 



Death of a nucleus can lead to phenotypic change in binucleate 

 cells or heterokaryons. It could also lead to the production of haploids 

 if either a male or female nucleus had been sufficiently damaged prior 

 to fertilization but survived long enough for pseudo-fertilization to 

 occur and for development to begin. Such an effect, known as andro- 

 genesis, has been reported in Habrobracon (192) and, as already men- 

 tioned, some cases of haploidy resulting from exposure to ionizing 

 radiation have been reported in plants (158, 160). More extensive 

 cell death has been cited by Sagawa and Mehlquist (135) as the cause 

 of the reversion of a periclinal chimera of Dianthus back to a non- 

 chimera. Phenotypically such a reversion resembles a mutant but 

 obviously it should not be so classified. Very high doses will also cause 

 sufficient damage to kill whole meristems, organs, or even whole 

 plants. 



10. Sterility or partial sterility of various types may be produced 



Some of these are the result of genie or chromosomal change, but 

 other types are not. (See Section V.) 



11. Miscellaneous nuclear or chromosomal changes 



There are a number of changes known to occur in irradiated 

 cells whose significance with respect to genetic integrity is vague or 

 unknown. These are listed below along with possible modes of effects 

 in some cases. 



a. Chromosome paling. — The production of pale regions (also 

 called gaps) has been studied particularly in certain animal cells by- 

 means of microbeam irradiation (205). An increase in the number of 

 understained regions in plant chromosomes following exposure to 

 ionizing radiation has also been reported (31). 



b. Chromosome stickiness and clumping. — These effects occur 

 at late prophase, metaphase, and anaphase after sufficiently high doses 

 shortly after irradiation and can lead to secondary consequences such 

 as fragmentation and possibly polyploidy (183). 



c. Abnormal spiralization of chromonemata. — Abnormal spiral- 

 ization has been reported in meiotic chromosomes of Trillium (169) 

 and could lead to breakage due to entanglement of unspiralled 

 chromosome arms or possibly to restitution at anaphase due to the 



