The Amphiploids 295 



12.2-1: Origin of hexaploid xvheat. Bread wheat, Triticiim aesti- 

 vum L. {T. vulgare Vil.) is mankind's most important single species 

 in culti\ation. Millions of people depend on the annual grain produc- 

 tion ol this plant. As an achie\emcnl in agriculture, the accession ol 

 this one species alone is man's important contribution as a plant 

 breeder. 



Historically, in terms of the long period of agriculture, the 42- 

 chromosome wheats are relatively new. Certainly the tetraploid 

 Avheats antedate hexa))loids, while diploid species preceded the tetra- 

 ploids. No hexaploids are known out of cultivation, whereas diploids 

 and tetraploids are represented by wild and cultivated species. Full 

 knowledge of the origin of bread wheat probably will never be ob- 

 tained, but some phases can be closely inspected by observing the 

 experimentally produced poly]:)loids. Colchicine has been a useful 

 tool in tracking down certain stejjs in the origin of the hexaploid 

 species, notably Triticiim spelta and related species. 122 



First, consideration should be given to Tritiniin monococciim L., 

 a 14-chromosome sj^ecies, to gain some idea of the oldest species of 

 w^heat in agricidture today. Another diploid, Agropyrou triticeum 

 Gaertn., is suspect in the hybridization with Triticiim which created 

 the tetraploid, or 28-chromosome, species.'*'- ^*"* These two parental 

 types may be called the A and B genomes, representing Trilicum and 

 Agropyron , respectively.^^ 



A large group of cultivated tetraploids, having either free-threshing 

 or invested grains, remain in cultivation as valuable economic species. 

 The emmer and durum types play an important role in agriculture.-*" 

 One of the most interesting tetraploids is the free-threshing Triticiim 

 persicum.-'^ 



Let us return to our hypothesis that Triticiim monococcum is the 

 genome A, and that the diploid genome B came from Agropyron 

 triticeum. ^^^ The true contribution made by Agropyron may now be 

 so remote that one cannot hoj:)e to retrace these steps. Let us assume 

 these diploids combined to make the tetraploid wheats. The evolu- 

 tion fiom tetraploid to hcxaj:)loid may be repeated more easily than 

 that from diploid to tetraploid. Bv crossing tetraploid Triticum 

 dicoccoides, 28-chromosomes, with diploid Aegilops squarrosa, a sterile 

 triploid hybrid was obtained.""- ^'^ This plant had 21 chromosomes, 

 was sterile, and resembled hexajiloid Triticum spelta, or spelt wheat. 

 Upon doubling the chromosomes, a 42-chromosome wheat was de- 

 veloped. This synthesized hexaploid hybridized with the natural 

 hexa])loid T. spelta. The selfed ]3rogenies from this hybrid did not 

 thro^v segregates as one might expect from a wide cross. In fact, no 

 segregation occurred. Pairing at meiosis among the F, hybrid did not 

 indicate widely differentiated cluomosomes of synthetic T. spelta 



