and secondary. A primary polyploid (Figure 8-1) is one which arises 

 directly from a given organism by chromosome doubling and must there- 

 fore always be even-numbered, while a secondary polyploid is one which 

 arises from crossing — especially, but not necessarily, between organisms 

 of different ploidy levels; for example, 



(doubled) 



2x > 4x (primary) X 2x > 3x (secondary) 



Since cytological fertility is a function of the degree of association 

 in pairs, even-numbered allopolyploids are more likely to be fertile 

 than even-numbered autopolyploids. Odd-numbered polyploids, however 

 (triploids, pentaploids, etc.), tend to be sterile whether they be allo- 

 polyploids or autopolyploids simply because they have a high possibility 

 of giving rise to gametes of unbalanced chromosome constitution. 



Polyploidy in Plants 



Polyploidy appears to occur naturally in all groups of plants with the 

 possible exception of the fungi, but as pointed out by Stebbins (1950) 

 the distribution is so irregular that there seems to be little phylogenetic 

 significance. Natural autopolyploids are rather rare, though many poly- 

 ploid species and races seem to be near-autoploids as judged by fre- 

 quency of multiple associations. Among such are several tetraploid 

 species of Tradescantia and the triploid banana and plantains (Figure 

 8-2). Presumably such near autoploids have arisen by doubling the 

 chromosome number in a hybrid containing two similar but not strictly 

 homologous sets of chromosomes. Neither artificially induced nor natural 

 autoploids do well above the tetraploid level. While no clear-cut reason 

 for this has so far been established, there is some reason to suspect that 

 the changed surface-volume relationship in the characteristically larger 

 cells of the polyploid may lead to dysfunction. Nonetheless, increase in 

 chromosome number is not always reflected by measurable change in 

 cell size, to say nothing of organ or whole plant size. 



Most naturally occurring polyploid plant groups are alloploid or 

 complexes of alio- and autopolyploids and may reach rather high degrees 

 of ploidy. For example, the cotton plant of North American commerce 

 is apparently an allotetraploid of the AABB type derived from a hybrid 

 between Gossypium arboreuin and G. rarimondii. The wheat, Ttiticum 

 vulgare, is an allohexaploid of the AABBCC type derived from two 

 species of wheat and a wheat grass by a successive series of hybridization 

 and chromosome doubling. Polyploid species such as Phleiim protense, 

 Helianthos tuberosum, and Solarium nigrum are hexaploids which, on 



CYTOGENETICS / 177 



