I 



INDUCED CHROMOSOMAL ALTERATIONS 1291 



16) which appeared in an Xz plant and has been investigated through 

 A^'e. All derivative lines have proved to be genetically asynaptic, 

 although great variation is shown in the amount of pairing in different 

 plants, at different stages of maturity of the same plant and under 

 different environmental conditions. Although no monosomic types 

 have appeared, a large number of polysemies have been found. Ten 

 of the distinct trisomic types are undoubtedly "primaries" and among 

 other trisomies which have occurred the remaining two will doubtless 

 be identified. In addition, a number of double trisomies and four 

 tetrasomics have been recognized. An illustration of the other type of 

 secondary effect producing aneuploidy is provided by the case of the 

 "deformed" X-ray derivative of N. Tabacum which is referred to in what 

 follows. 



As already pointed out, chromosome fusions, whether preceded or 

 followed by fragmentation, are characteristic products of irradiation in 

 plants. Thus, Lewitsky and Araratian (26) describe and illustrate an 

 induced association of two homologous and of two nonhomologous 

 chromosomes in Crepis capillaris. An instance of attachment involving 

 two homologous chromosomes is provided by the A'^. Tabacum X-ray 

 derivative "deformed" (cf. Goodspeed and Avery, 19). Reference 

 has already been made to the "sticky" character often visibly assumed 

 by chromosomes following irradiation, and it is postulated that such 

 initial alteration is responsible for the more or less permanent chromo- 

 somal attachment involved in the origin of "deformed." A "deformed" 

 plant exhibits tissue abnormalities and mosaicism as a result of elimina- 

 tion in certain cell lines of attached chromosomes. Its progeny consists 

 of "deformed" plants together with monosomies, trisomies, tetrasomics, 

 and simple and complex products of chromosomal reorganization, certain 

 of the latter being related to breakages of the original chromosomal 

 attachment. Similar attachments involving the major portions of two 

 chromosomes and resulting in the production of chromosomes possessing 

 two insertion regions have been shown by McClintock (30) to be products 

 of translocation induced in maize by X-radiation. Such abnormally 

 constituted chromosomes are assumed to be direct products of trans- 

 location and, also, to occur as a result of crossing over between the 

 translocated chromosome and one of its normal homologs. 



As already noted, fragmentation is certainly the most conspicuous 

 and probably the most typical chromosomal alteration produced in 

 plants by high-frequency radiation. With the rather meager evidence 

 available it is not, however, clear whether chromosomal breakage 

 generally follows, precedes, or is necessarily accompanied by fusion or 

 attachment of chromosomes or chromosome segments. Serebrovsky (39) 

 has suggested that breakage following attachment is the significant 

 process, whereas Stadler (44) contends that initial breakage of the 



