m 



Colchicine 



SPECIES A 



A, A, 



II 



SPECIES B 



B, B, 



X 



DIPLOID 2n 



DIPL0ID2n 



A, B 



DIPLOID HYBRID 



Jl 



CHROMOSOME DOUBLING byCOLCHICINEl 



I 



T 



f\i Ml Ml A I 



nil 



AUTOTETRAPLOID 



A, A, B, B, 



4n 

 A, A, 



II 



AMPHIPLOID 

 4n 



T 



X 



B, B, B, B, 



AUTOTETRAPLOID 

 4n 



B, B, 



GAMETE 2n 



GAMETE 2n 



Fig. 11.1 — Use of colchicine to make autotetraploids. Doubling the chromosomes of in- 

 terspecific diploid hybrid. Amphiploids made by hybridizing two autotetraploid species. 



(After Wexelsen) 



Thi.s kind of evolution was loinuilated as the A 'X B hypothesis 

 by VVinge in 1917 before any examples were well known, although 

 the doubling of Primula keivensis was on record. •'•' According to the 

 A \ B hypothesis, a polyploid series with a basic number of 7 would 

 read 21, 28, and 42; or triploid, tetraploid, and hexaploid, respec- 

 tively. These can originate as follows: A triploid, sterile hybrid 

 arises from the hybridization ])ctwcen the diploid, 2u =i 14, and a 



