The Amphiploids 299 



Cliioinosonial pairing in the tli|>l()itl hybrid, or the lack oi pairing is 

 not necessarily an index of homology. The intergeneric aniphiploid 

 Aegilops iimbelhdata X Hayuoldia villosa has a reduced lertility.^"" 

 The particular strain made a difference in pairing; environmental 

 and genetic factors, also, influence pairing of chromosomes. 1 wo dis- 

 tantly related species may introduce physiological upsets that cause 



TABLE 12.1 

 Divergent and Convergent Evolution of Hexaploids 

 (Adapted from McFadden and Sears) 



Primary Form 

 Diploid 



Divergent Form 

 Diploid 



Convergent Form 

 Polyploid 



Agropyron genome B . 



Unknown 



Trilicum genome A 



/ 



, AB 



Trilicum 



tetraploid 



Trilicum 



hexaploid 



ABD 



'Aegilops genome D — Aegilops 



diploid 

 D 



meiotic irregularities." The rule cannot be established that uni- 

 \alen(y in the F, is j)rcdictable evidence for obtaining good fertile 

 amphijjloids. 



Evolution in wheat that finalh led to hexaploids may be charted 

 as a divergence in the early period following convergent evolution 

 giving rise to the tetraploid and hexaploid sj)ccies. Some tmknown 

 diploid form evolved into three basic genera: (1) Agropyron, (2) 

 Triticuni, and (3) Aegilops. The first two hybridized and gave rise 

 to a series of tetraploid species. A second step in evolution involved 

 the combinations between tetraploid Triticum and Aegilops. A chart 

 is used to help \isualize these evolutionary patterns (Table 12.1) . 



Since such valuable species have arisen throtigh combinations of 

 genomes, this approach was suggested as a "radical" method of wheat 

 breeding. Desirable characters would be transferred to T. aeslnnim L. 

 by using specific series of synthesized amphiploids. Four were sug- 

 gested. The first series involves the D genome from Aegilops squar- 

 rosa added to various tetraploids because the hybrids are more fertile 

 than crosses between tetraploids and hcxa]:)loids within Triticum. A 

 second series involves combinations between tetrapltjid wheat and 



