The Amphiplo'ids 307 



12.4: Nicotiana 



A theory of evolution was experimentally verified when N. digliita 

 was made in 1925. 1 he parental species, N. tahacum, a natural tetra- 

 ploid with 48 chromosomes {n = 12), and the diploid N. glutinosa 

 were hybridized to make the sterile triploid with 36 chromosomes. 

 A fertile hexaploid was isolated that had 72 chromosomes. This num- 

 ber was a new and high one for the genus. Previous to the develop- 

 ment of A', digliita, 48 chromosomes was the highest number.i^. 4o, 4i 

 Using colchicine, A', digluta was resynthesized. Since then numbers 

 higher than hexaploid have been built into polyploids of Nicotiana.^^ 

 These polyploids were made by bringing together the proper species 

 in hybridizations and doubling the chromosomes of the hybrids. A 

 combination of three natural tetraploids included 144 chromosomes 

 in one plant.s*' Another report of 176 chromosomes has been made.^o 

 The development of plants with high numbers is not the sole 

 objective. Of particular significance is the combining of widely diverse 

 genomes in order to establish higher polyploid-amphiploids that are 

 fertile, vigorous, and relatively stable in later generations of propaga- 

 tion. ^'^ The changes that take place in subsequent generations of these 

 polyploids show what mechanisms might operate genetically when 

 new species at new levels of chromosomal numbers become estab- 

 lished. Furthermore, the effects of selection upon these types are of 

 basic importance. i'^-^' ^ 



An important development that resulted from the synthesis o£ N. 

 digluta was the eventual transfer of mosaic resistance to the com- 

 mercial varieties of tobacco." ^ The necrotic factor from N. glutinosa 

 was transferred to the N. tahacum genome.^o. 38 An example of poly- 

 ploid breeding is illustrated by this program. After full review of 

 the work necessary to make the transfer, one becomes convinced that 

 these methods are not short cuts. 



Realizing all iliat \\as involved in the requirements for transfer 

 and the cyt'ological and genetic data at hand as late as 194.S, there 

 was no complete assurance that the factor for resistance in A\ glu- 

 tinosa could be incorporated in the genome of N. tahacum:-- Each 

 time the transfer was tried, disadvantageous traits were carried along 

 with the chromosome contributed by A', glutinosa. Therefore, the 

 problem was one of maintaining the good features of commercial 

 tobacco varieties and utilizing only the disease resistance of the 

 glutinosa type. Fortunately, some chromosomal change occurred 

 during generations of selection, and a true tobacco type with mosaic 

 resistance of the kind noted for A^ glutinosa ap):)eared in the cultures. 

 The plant had 48 chromosomes and possessed the resistance factor 

 incorporated in the tahacum genome. ^^ Perhaps one might call the 

 new varietv. N. tahacum var. 77)// after a type made by Kostoff.^''^ No 



