294 Colchicine 



Suppose that lines are isolated by selection after interspecific 

 segregation among progenies of aniphiploids. One cannot expect these 

 lines to compete in nature as successful independent aniphiploids in 

 the same rank as a distinct and differentiated species. From an agri- 

 cultural standpoint these lines need not be new species, and they may 

 or may not be valuable as new i:)olyploids. If the transfer of genetic 

 traits is made from one parental species to another, and the species 

 of commercial importance is improved, the result is not a new poly- 

 jiloid.-" For example, mosaic resistance was transferred from N. 

 ghiti)iosa to the A^ (ahacitrn genome. "^ The characteristics of com- 

 mercial tobacco plants were not changed, but the disease resistant 

 factor was added. Chromosome numbers were finally stabilized by 

 selection after backcrossing at the same number as .V. tabacum 48, and 

 after specific selection only a few traits were transferred from N. ghi- 

 tinosa. All but the resistance to disease were eliminated. As an am- 

 phiploid then, the new A', tabacum with only the disease-resistance 

 characteristic added can hardly be considered as an independent t\pe. 



Stability of a new amphiploid is proportional to the gene exchange 

 between the two parental genomes. Lack of interchange favors rela- 

 tive constancy; conversely, interchange promotes instability. Ex])cri- 

 mentally produced aniphiploids of all gradations from those with 

 much interchange to others with very little, offer excellent oppor- 

 tunity to explore certain basic propositions controlled and observed 

 after selection, ^f''^' ^ either in nature or under guidance. 



Doubling of the chromosomes among sterile diploid hybrids may 

 be done either through gametic j^rocesscs, i.e., production of un- 

 reduced gametes, or by somatic doubling. The accidental doubling 

 in nature has occurred largely by the gametic processes. On the other 

 hand, colchicine is most effectively apj)lied to somatic tissues. The 

 differences between these methods of doubling the chromosomes are 

 imj)ortant and should be compared when such comparisons can be 

 made. 



12.2: Amphiploidy in the Gramineae 



Economically, the grasses comprise the most important family 

 among all plants. Polyploidy is common in many groujis including 

 agricidtural species. Generally, their origin has been through hybridi- 

 zation and doubling of the chromosomes. Autoploidy is limited as 

 a method of speciation-"'-^ in grasses compared with amphiploidy.^'*-^ 

 Polyploidy among grasses presents problems-"'"' i*^- ^i- -">• ''^ that in- 

 volve both theoretical and practical aspects."- -•'• ■*'• ■'■*■ ^^'' '^*'' '^"' ^"^ 

 The origin of hexaploid wheat^'"^ has many theoretical phases,'^*'' ^"'^ 

 and no one can escape the practical importance attached to this one 

 species, Triticurn aestivutn L.^^^ 



