The Amphiploids 309 



By interspecific hybridizations and doubling of chromosomes, syn- 

 thetic tetraploids liave been made that resemble N. tabacum, yet lack 

 the same genotypical balance that exists in the natural species. Even 

 though the diploid species, A^ sylvestris, and certain diploitls of the 

 tonu'ntosa group may be combined to make a polyploid that re- 

 sembles A^ tabacuNi. the exact genetic duplication has not been ac- 

 complished.''*^ Usually the sterile hybrids doubled somatically are 

 female-sterile. Sterility is caused by failure at the embryo-sac stage. 

 When a long procedure of backcrossing was involved, a fairly fertile 

 synthetic A^ tabacum was obtained.*"' AVhen the synthetic was crossed 

 with a natural species, the segregation ni the second generations was 

 like the variability found between varietal crosses. 



A list of the amphiploids made with colchicine is necessarily 

 large. There are more objectives involved than have been out- 

 lined in this section. Nicotiana provides some good material for the 

 study of polyploidy both from a practical and a theoretical point 

 of view.'*'^' ■^i' -'^'' •^**' i*^^' ~^' -^' •'• ^' -' ^-' ^-' ^•^' *^- ^'■^- ^"'- 



12.5: Dysploidy Combined With Amphiploidy 



Within the Cruciferae a natural group called the Brassica com- 

 parium by Clausen, Keck, and Heisey, form a dys})loid series as fol- 

 lows: 71 = 8, n = 9, u =z\0. ?/ = 11, n = 12, n = 17, u = 18. If the 

 artificial amphiploids are added, the series rises to the hexa)3loid 

 level, i.e., dysploid, // = 27 and // = 28. At once some fundamental 

 problems can be predicted from what has been said before. 



Some notable historical events in cytogenetics occurred with this 

 groujj. The first cross between radish and cabbage was produced by 

 Sageret in 1826. One century later, Karpechenko demonstrated fertile 

 Raphanobrassica plants. -^ After Sageret's time, the cross was re- 

 peated by others. With colchicine, autotetraploid Raphanus was 

 crossed with autotetraploid Brassica thereby repeating the intergeneric 

 hybrid by another method.-"- •'^"- "-^ Previously the sterile diploid hy- 

 brid was made, and fertile plants were selected after unreduced 

 gametes united.'*^ 



Fruit structure in the Raphanobrassica polypkjids is j^rojjortion- 

 ally radish or cabbage, depending on the genomes present. Accord- 

 ingly, diploid, triploid, tetraploid, and pentaploid series can be ob- 

 tained with different doses of whole genomes.-^ 



Judging from the total lack of ])airing in the Fj hybrid at diploid 

 levels along with the independence maintained in the amphi|)loid. 

 gene exchange at dij^loid level is exceedingly limited. Hyi)ridi/ation 

 and the synthetic amphiploids have raised the level above tetraploidy 



