INDUCED CHROMOSOMAL ALTERATIONS 



1293 



homozygous condition. Thus, in N. Tabacum, true-breeding derivatives, 

 distinct morphologically and giving evidence of chromosomal losses, 

 have appeared. Chromosomal reorganizations such as translocation, 

 duplication, deficiency and deletion undoubtedly were concerned in the 

 origin of the A^. Tabacum derivatives just referred to and also have occurred 

 as secondary products in later generations to have a part in the definition 

 of these types. For example, a type distinct in morphological characters 

 (Fig. 46) has been derived from a plant normal in external morphology 

 which occurred in the Xi progeny referred to above (page 1886), in which 

 the majority of the plants were striking variants. When crossed with 

 control, this type shows, at first meiotic metaphase, a ring configuration 

 involving four chromosomes (Fig. 5a). 

 Such a configuration indicates that 

 portions of two chromosomes have 

 been interchanged. While the inter- 

 change may not have been directly 

 concerned in the differentiation of the 

 new morphological characters which ^ ? ^ . , • i -j r • 



^ ° Fig. 5. — -Cytological evidence of in- 



distinguish the particular type in- duced chromosomal interchange and 



VOlved, its occurrence indicates that reorganization in N Tabacum var. 



' purpurea, (a) Ring of four chromosomes 



such chromosomal reorganizations at first meiotic metaphase in i^i of type 



have accompanied this differentiation. ^^"^Z^ >" ^'^■. ^^^ flossed with control. 



^ (ft) First meiotic metaphase in type shown 



Again, in another distinct derivative in Fig. 4c: 21 bivalents, 1 fragment 

 type (Fig. 4c) of the same Zi plant, p^^^ (^)' ^^^^ ^ trivaient {y). 

 there are at first metaphase 21 bivalents, 1 trivaient, and 1 frag- 

 ment pair (Fig. 56). The presence of the fragment pair and the 

 fact that the monosomic chromosome conjugates with a bivalent 

 show that the chromosomes concerned are products of induced 

 reorganization. In the investigations on chromosomal alteration 

 following irradiation, in Nicotiana and in certain other genera, the 

 reorganizations involving sections of chromosomes cannot be so 

 thoroughly analyzed cytogenetically as they can be in Zea Mays. 

 Anderson discusses such reorganization in that species in Paper XLII. 



REFERENCES 



1. Belling, J. Crossing over and gene rearrangement in flowering plants. Gene- 

 tics 18: 388-413. 1933. 



2. Bergner, a. D., S. Satina, and A. F. Blakeslee. Radium experiments with 

 Datura. III. (Abst.) Anat. Rec. 41 : 99. 1928. 



3. Bergner, A. D., S. Satina, and A. F. Blakeslee. Prime types in Datura. 

 Proc. Nation. Acad. Sci. 19: 103-115. 1933. 



4. Blakeslee, A. F. The chromosomal constitution of Nubbin, a compound 

 (2n + 1) type in Datura. Proc. Nation. Acad. Sci. 13: 79-85. 1927. 



5. Bl.^keslee, a. F. Nubbin, a compound chromosomal type in Datura. Ann. 

 New York Acad. Sci. 30: 1-29. 1927. 



6. Blakeslee, A. F., S. Satina, A. D. Bergner, J. S. Potter, and A. G. Avery. 

 Fi's from radium treated pollen. (Abst.) Anat. Rec. 47: 383. 1930. 



