Changes Involving Unbroken Chromosomes 



155 



because of synapsed polynemes, offers a 

 unique opportunity to correlate genetical and 

 cytological events. ( It should be noted that, 

 as a rule, giant polyncmic chromosomes are 

 produced in cells which will never divide 

 again. ) 



At any given stage of a cell cycle, most 

 of the chromosomal material reacts similarly 

 to certain staining procedures and, therefore, 

 is called euchromatic (truly or correctly 

 colored). Other portions of the chromo- 

 somes stain either darker or lighter and are 

 said to be heterochromatic. Although het- 

 erochromatin may be located at various 

 places along a chromosome arm, it is nor- 

 mally found adjacent to the centromere and, 

 to a lesser extent, near the ends. Hetero- 

 chromatin also has the characteristic of being 

 less specific in synapsis than is euchromatin, 

 different heterochromatic regions located in 

 the same chromosome, its homolog, or in 

 nonhomologous chromosomes often being 

 found synapsed. In the giant salivary gland 

 nuclei of Drosophila larvae, the heterochro- 

 matic regions nearest the centromeres of all 

 chromosomes synapse to form one mass, 

 called the chromocenter. This is the center 

 from which the double cables radiate in 

 Figure 1 1-4 and at the left of Figure 1 1-6. 

 Also, the heterochromatic regions nearest 

 the ends are sometimes found synapsed with 

 other heterochromatic regions, especially the 

 chromocenter. In squashing the nuclei to 

 separate and flatten the salivary chromo- 

 somes, two synapsed heterochromatic re- 

 gions may be pulled apart, but show evi- 

 dence of synapsis because they are still 

 connected by strands of apparently sticky 

 material. The right end of the fourth 

 chromosome polynemes in Figure 1 1-6 

 shows such glutinous matter, probably 

 indicating synapsis with the chromocenter. 

 Heterochromatin is chromatic and is not to 

 be identified with the regions between bands; 

 interband regions do not seem to contain the 



Fculgcn-stainable material - and apparently 

 are achromatic, as is the spindle. 



3. Allopolyploidy 



Ploidy can increase another way besides 

 allopolyploidy. Two species can each con- 

 tribute two or more genomes to form a third 

 species which is called an allopolyploid. 

 Cultivated wheat is an allopolyploid. As 

 expected, allopolyploids often show a com- 

 bination of characteristics of their different 

 parent species. This type of polyploidy is 

 discussed in more detail in Chapter 18. 



Changes in genome number represent the 

 class of normal and mutational events in- 

 volving the largest unit of genetic material. 

 Although many plants are polyploid and one 

 plant has 512 chromosomes, polyploidy will 

 produce a chromosome number that is un- 

 wieldy in nuclear division if it occurs many 

 times in succession. It should also be noted 

 that certain other classes of mutation, like 

 those involving a single locus, have greater 

 difficulty expressing themselves in autopoly- 

 ploids than they have in haploids or diploids, 

 in which no other, or just one other, homol- 

 ogous locus is able to mask the mutant effect. 



Aneusomy 



The next category of mutations to be dis- 

 cussed involves the addition or subtraction 

 of part of a chromosome set. Such muta- 

 tions upset the normal chromosomal and 

 gene balance and produce aneuploid ("not 

 right-fold") chromosomal (genetic) consti- 

 tutions by having the incorrect number of 

 particular chromosomes (aneusomy). By 

 what mechanisms can single whole (un- 

 broken) chromosomes be added to or sub- 

 tracted from a genome? 



1 . In Drosophila 



Recall that nondisjunction in the germ 

 line of Drosophila can produce offspring, 



- See D. M. Steffensen (1963). 



