242 



Most of our cultivated polyploids 

 are of ancient origin. Take the culti- 

 vated wheats. It had been known for 

 years that the wheats of the world 

 were more than just a large number of 

 varieties of wheat. Tliey belonged in 

 three or more great groups; crosses be- 

 tween some of the groups were hard to 

 make and such hybrids were more or 

 less sterile. The cytologists showed that 

 there was a very real background for 

 these groupings. Einkorn wheat, an 

 ancient cereal of Neolithic times, now 

 practicallv disappeared as a crop plant, 

 was a diploid; all the other cultivated 

 wheats were polyploid. Some of them 

 were tetraploid, including those pro- 

 tein-rich varieties which make a sticky 

 flour and are known as "the macaroni 

 wheats." The most important wheats 

 of all, the bread wheats, were hexa- 

 ploid, that is, they had six full sets of 

 chromosomes. If each of the capital 

 letters A and B and D designates a 

 set of seven chromosomes, then the 

 einkoms were of formula AA, the em- 

 mer and the macaroni wheats were 

 AABB, and the bread wheats were 

 AABBDD. Presumably the einkorn 

 wheats had evolved out of the wild 

 wheats of the Near East, since some 

 of these were also simple diploids of 

 the constitution AA, but where did the 

 BB's and the DD's come from? This 

 fascinating puzzle is not yet solved 

 down to proving the last detail, but we 

 are already reasonably certain of the 

 general outline of the story and even 

 have exact experimental proof for 

 some of it. It is clearly apparent that 

 the cultivated wheats did not spring 

 from the wild wheats alone; the BB's 

 and the DD's must represent other dis- 

 tinct genera of plants. BB may well be 

 a quack grass, Agropyron triticeum, 

 which is wild in the eastern Mediter- 

 ranean region. A polyploid of it and 

 einkorn back in Neolithic times may 

 well have produced the first of the 



CYTOLOGY 



tetraploid AABB wheats from which 

 cmmer, the Persian wheats, and other 

 modem teraploids were eventually 

 bred. The hexaploid (AABBDD) 

 bread wheats were most probably pro- 

 duced from accidental hybridization 

 between the AABB tetraploids, and a 

 bristle-headed little weed of the Near 

 East, Aegilops squarrosa, which sup- 

 plied the necessary DD for the finished 

 hexaploid. 



One of the most brilliant pieces of 

 current biological research has been 

 the proof by two American scientists 

 that Aegilops played a leading role in 

 the evolution of our modem bread 

 wheats. From evidence too technical 

 to discuss profitably here, a Texas 

 wheat breeder named McFadden 

 came to the conclusion that Aegilops 

 squarrosa was the probable source of 

 the DD chromosomes. He accordingly 

 took emmer, one of our most primitive 

 tetraploid (AABB) wheats, and crossed 

 it with the diploid (DD) Aegilops 

 squarrosa. He eventually produced, as 

 one might have predicted, a sterile hy- 

 brid of the formula ABD. It had three ' 

 sets of chromosomes, AB from emmer 

 and D from Aegilops. Like most plants 

 with three sets of chromosomes it was 

 sterile, though otherwise it looked like 

 a primitive bread wheat. 



At this point a Missouri cytologist. 

 Dr. Ernest Sears, joined in the work. 

 He took McFadden's sterile hybrids 

 and treated them with colchicine, a 

 drug which if carefully regulated can 

 prevent cells from dividing when their 

 nuclei divide. With patience and good 

 luck one may get a sector of a plant 

 and eventually a whole plant which has 

 developed from these affected cells 

 and in which consequently the chro- 

 mosome number has been doubled. 

 By this technique Sears produced a 

 hexaploid AABBDD from McFadden's 

 sterile ABD triploids. As might have 

 been predicted it was fertile and true- 



