184 I The Process of Evolution 



total number of chromosomes in any nucleus is its chromosome com- 

 plement. It is easy to see that the complement may include one or 

 more genomes or parts of genomes. In discussing the regular alterna- 

 tion of gametic and zygotic chromosome numbers, it is convenient 

 to use a diflferent symbol n. The haploid and diploid numbers of 

 all organisms are n and 2n. Organisms that have experienced an 

 increase in chromosome number are called polyploid. Here n in- 

 cludes more than one x or portions of x. In polyploid organisms n 

 and 2n usually are called haploid and diploid, even though in a tetra- 

 ploid, for example, n = 2x and 2n = 4x. The commonest type of 

 change is eupolyploidy, which is irreversible increase by whole 

 genomes. The oldest members of a eupolyploid series are those with 

 the lowest numbers. Thus if a plant is found to have 2n = 22 

 (x=ll) chromosomes, its closely related tetraploid derivative 

 would have 2n = 44 (x = 11). The number of chromosomes in one 

 genome may also change, leading to aneupolyploidy. Strictly speak- 

 ing, an aneupolyploid series is a series of numbers, such as 11, 12, 13, 

 15, not a series of organisms; aneuploidy is reversible and may arise 

 in several ways. 



Aneupolyploidy 



There is cytogenetic evidence that centromeres do not arise anew in 

 populations in nature. Nor can genetically active chromosomes be 

 added or subtracted from the genome. Nevertheless, conditions 

 favoring the loss or gain of chromosomes can be brought about by 

 unequal reciprocal translocations. In many organisms the region of 

 a chromosome adjacent to the centromere is genetically inert ( hetero- 

 chromatic). Darlington has suggested that, if the active euchromatic 

 arms are translocated to other chromosomes, the remaining hetero- 

 chromatic centric fragment usually will not pair properly and may be 

 lost. Thus a chromosome is removed from the genome, but the same 

 amount of genetic material remains (Fig. 9.3). The resulting off- 

 spring is isolated cytogenetically from its parent; if it forms a new 

 population, further difiFerentiation may be expected to occur. On the 

 other hand, a second translocation might apportion some active 

 material back to the centric fragment, dividing the genetic material 

 of another chromosome. Again pairing will be upset and a variety 

 of gametes will be produced. A zygote that has an extra chromosome 

 added to its genome may be formed. The original amount of genetic 

 material must be present or the combination will not be viable. 



Aneuploid change in chromosome number has been studied exten- 

 sively in plants. Generally a decrease in basic chromosome number 



