The Amphiploids 293 



polyploids must meet exacting demands in order to succeed as new 

 crop species. 



The diploid, interspecific hybrid, if it is to become a successftd 

 pohjjloid, must have good vigor, excellent growth of vegetative 

 characters, and an all-around vegetative cycle that is in harmony with 

 its environment.-^ Combined with these characteristics, the two 

 parental genomes shovdd be incompatible in the diploid hybrid to 

 the extent that no interchange can occur between them. 1 here should 

 be no gene exchange betAveen the parental sets of chromosomes, 

 which means no intergenomal pairing. Briefly, the dijiloid hybrid 

 according to these requirements should be entirely sterile until a 

 doubling of the chromosomes occms. Working in almost direct 

 opposition to these conditions, describing the source of ami)hij:)loid 

 from dijjloid hybrids between sj^ccies, are biological laws that tend 

 to j)re\ent achiexing the best-suited sterile hybrid. To acqiure such 

 genome incompatibility between the parents, one immediately moves 

 the relationships of the two species farther aj^art. Usually the farther 

 apart they are, the more difficult the hybridization Avill be. Even 

 after the hybrid has been made, a more distant relationshijj often 

 results in plants that are weak, j)oor in vigor, and lacking in good 

 growth generally. A poorly growing diploid hybrid cannot be ex- 

 pected to change into a vigorous, successful amphiploid by merely 

 dotd^ling the number of chromosomes. 



If hybrids are made from species too closely related, gene ex- 

 changes between the parental sets of chromosomes occur. Then after 

 four or five generations, segregations tend to destroy the individuality 

 of the amphiploid from the parental type.-^ Of course, by gene ex- 

 change the transfer of a trait from one species to another at the poly- 

 ploidy level can occur. The moment gene exchanges take place, the 

 fiUme of the amjjhiploid as a distinct and isolated individual becomes 

 entlangered.^ Cytological mechanisms may automatically cause the 

 plants of later generations to drift to one or the other parental type. 



Experimentally produced amjihiploids have been studied for 

 enough generations to demonstrate that genetic exchanges can take 

 place between the two parental sets of chromosomes. From a jilant 

 breeder's point of view this woidd seem to offer opportunity. Other- 

 wise a strict independence between genomes, like those of Raphnno- 

 hrassica, permits a true breeding type distinct from either parent, f)tit 

 further hybridization with either jjarental species to improve the 

 amphiploid is ineffective. -'^ If the amphiploid is not like the Raphauo- 

 bra.ssica case and intergenomal pairing does occur, gene exchange 

 leads to segregation in F^ and later generations. Many segregates may 

 be weak, sterile, and jjoor. Occasionally, new and xigoious com- 

 binations may arise. Certainly a scries of new lines can be developed 

 when there is exchange between genomes."'' 



