The Experimental Polyploids 277 



Segmental allopolyploid is an amphiploid which shows character- 

 istics of autoploids with respect to pairing of chromosomes, resem- 

 blance to parents, and fertility; yet the amphij^loid exhibits enough 

 tlilference between the genomes contributed by the parents to fall 

 within the scope of amphiploids. Segmental types are important for 

 practical and theoretical reasons. Our discussion of the segmental 

 allopolyploid will be included in Chapter 12 (The Amphiploids) . 



Genome designates the set of chromosomes derived from a species; 

 the term may be used to express a relationship between species. Ex- 

 tensive use has been made of genomes since many intersi>ccific hy- 

 brids have been made and doubled with colchicine. Among species 

 of Gossypium the genome concept is related to geographical distribu- 

 tion of species. The genomes of Trituum refer to generic contribu- 

 tions. The original term was introduced by Winkler in 1920. 



Dysploidy refers to a series of polyploids in nature whose basic 

 numbers are not nuiltiples. A dysploidy is superimposed upon an 

 amphiploid series. A good example is found among the Cruciferae, 

 where basic numbers 5^ 6. 7, 9. 11 fall at levels of diploid, tetraploid, 

 and hexaploid status. 



Aneuploidy is a condition in which chromosomes are added or 

 lost from the diploid set of chromosomes. Aneuj:>loids may or may not 

 represent balanced genotypes. 1 he loss or addition may be found at 

 polyploid levels. For example, the nullisomic is essentially aneuploid. 



Cryptic structural hybridity*'*'' designates a chromosomal differentia- 

 tion in very small segments that does not readily find expression in 

 configuration at metaphase of meiosis. Pairing of chromosomes may 

 be bivalent and apj^arently normal, for the segments that are differ- 

 entiated are so small that no opportunity is afforded for abnormal 

 configurations during synapsis. For these reasons a structural hy- 

 bridity of this nature may be indistinguishable from the genetic 

 hybridity. 



11.3: Cataclysmic Origin of Species 



The origin of a new species by gene mutation or chromosomal 

 repatterning (inversions or translocations) is a slow process and re- 

 quires a long time. Surprisingly, there exists in nature, alongside 

 these slower processes, a very rapid method that can catajndt a new 

 species into existence within a generation or two.''' This sudden 

 origin is called "cataclysmic evolution." -^ By this process a new plant 

 is separated at once from its immediate jjarents and is destined to 

 occiipv new environments different from either, or both, of its pro- 

 genitors, (Fig. 11.1) .'"' 



