Genetic Systems II I 195 



Autoallooctoploid 



2-14 IV's 

 24-0 II's 



Autotetraploid 

 AAAA 

 1-7 IV's 

 12-0 IFs 



,1 



Species A 

 AA 

 7 II's - 



Autoallohexaploid 

 AAB.B^ Bj B^ 



1-7 IV's 



13-7 II's 



Allotetraploid 



AAB^B^ 



14 II's 



AB^ 



i4rs 



Triploid Fi 



AB.Bi 

 7 II's 7 I's 



Segmental 



allotetraploid 



B,B,B.,B., 



0-7 IV's 



14-0 II's 



1. 



Allohexaploid 



AABiBiCC 



21 II's 



1 



-•^ Triploid Fi 

 AB,C 

 21 I's 



Species Bj 

 BiB, 



7 irs -^ 



BiB., 

 7 II's 



Species B^ 



B2B2 

 *— 7 II's 



Species C 



CC 



7 II's 



Diploid species, n = 7 



Fig. 9.6 I Diagram showing various levels and types of polyploidy, their 

 genome constitution, and their mode of origin. Letters represent genomes; 

 roman numerals indicate pairing behavior, e.g., I = univalent, II = bi- 

 valent, etc. Thus 24-0 II's ^^ 24 to bivalents. (From Stebbins, 1950, 

 Variation and Evolution in Plants, Columbia University Press. ) 



is not invoked in explanation. As has been seen, centric fusion is 

 thought to be the cause. 



The effects of polyploidy as a genetic system are varied and at 

 times opposed. For example, autopolyploidy may severely restrict 

 or eliminate genetic recombination in one organism, only to act as 

 a bridge between an allopolyploid and one of its parents. A strict 

 amphidiploid is limited in recombination potential to that of its 

 parents, but in segmental alloploidy recombination of the parental 

 characteristics, including ecological preferences, may occur. In gen- 

 eral, one might say that polyploidy may act to increase the scope of 

 evolutionary units, enhancing the occasional interbreeding through 

 time. The situation resembles that postulated by Wright for most 



