298 Colchicine 



convert this sterile hybrid to a fertile one, the necessary evidence le- 

 mained hidden until fertile hexaploids could be made.^^*^ 



The D genome represented in hexaploid wheat and the genomes 

 of modern diploid Aegilops squarrosa are probably very close in their 

 homologies. Also, this genome is not found in any species of wheat 

 tested that had fewer than 21 chromosomes. Tetraploid wheat lacks 

 this genome. Finally, taxonomic characters in Aegilops squarrosa 

 correspond to those traits that distinguish the hexaploid wheat from 

 tetraploids.^"" These are: the square-shouldered inflorescence, hollow 

 stem, and articulation of rachis, differentiating Triticum spelta from 

 the tetraploid Emmcr wheats.'" 



Taxonomic characters were used to trace the probable origin of 

 hexaploid wheat before cytogenetic evidences were at hand. The 

 fact that diploid Agropyron triticeum Gaertn. has features distinguish- 

 ing dijiloid T. monococciDU from tetraploid wheat arouses interest. ^"^" 

 Discovering more specifically how genome B was contributed and what 

 its relation to Agropyron is, becomes more involved. This genus also 

 has a polyploid series in its evolution. The base is ?; = 7 (Table 12.1) . 



Some intergencric hybrids involving Agropyron have been made. 5- 

 11-9 Wey^A^iloid T. aestivum {ri=:2\) -And Agropyron gknicinn (n =^ 

 21)^*^ were combined to make an amphiploid with 84 chromosomes. 

 Strong perennial tendencies arise with these high polyploids. In 

 another case, vigorous plants with 70 chromosomes were derived by 

 adding the hexaploid complements, 42 chromosomes, to the tetra- 

 ploid Agropyron intermedmm, 28 chromosomes. This particular 70- 

 chromosome fertile hybrid was the first amphiploid to be reported 

 from tests with colchicine.''^ 



The genus Triticum, represented by three chromosomal levels, 

 n r= 7, n =: 14, and ?/ = 21, provides much material following inter- 

 specific hybridization. A tetraploid, T. timopheevi, has the genome 

 G not common to other well-known species.-*' Another free-threshing 

 tetraploid species, T. persicutn, produces an interesting series when 

 crossed with Aegilops squarrosa/'^ Unquestionably, these amphi- 

 ploids have free-threshing hexaploid bread wheat features. 



Within short intervals after colchicine was discovered, more than 

 80 different amphiploids, involving tetraploid and hexaploid, as well 

 as diploid species of Triticum were produced in Russia. ii'^ Some 

 higher numbers proved to be interesting in their hybridization charac- 

 teristics in subsequent generations. Generally the sterility increased 

 when hybrids above the hexaploid level were created. The ordinary 

 wheat, usually self-pollinated, changed into a cross-fertilizing type as 

 higher-level amphiploids were reached. 



1 he complexity of sterility-fertility relationships appear in the 

 intergencric and interspecific hybrids among 'rriticinae.^^- i**' ^"" ■^"' ^^ 



