152 



CHAPTER 11 



iiwri: 11-3. Normal {left) and triploid (right) D. melanogaster females. The body 

 of the 3N female is slightly larger than the 2N female and also has slightly larger cells. 

 (Drawn by E. M. Wallace.) 



in a variety of mammals, and even races of 

 some animals are polyploid. For example, 

 tetraploids of the following are known: the 

 water shrimp, Artemia; the sea urchin. 

 Echinus; the roundworm, Ascaris; and the 

 moth, Solenobia. Polyploid larvae of sala- 

 manders and of frogs also have been ob- 

 tained, although races are not formed. Poly- 

 ploidy is also found in Drosophila. Female 

 Drosophila have been found that are triploid 

 (3X + 3 sets of A) (Figure 11-3) and 

 tetraploid (4X + 4 sets of A). Somatic 

 parts of Drosophila individuals have been 

 found to be haploid (IX + 1A set). 



One way that ploidy can increase is by 

 the addition of genomes of the same kind 

 as those present — by allopolyploidy — as was 

 the case with Datura. Autopolyploidy can 

 arise several different ways: 



1 . Mitotic anaphase may be abnormal, so 

 that the doubled number of chromosomes 

 becomes incorporated into a single nucleus 

 which thereafter divides normally to produce 

 daughter polyploid nuclei and eventually — 

 by asexual reproduction — polyploid progeny. 



2. Sometimes two of the haploid nuclei 

 produced by meiosis fuse to form a diploid 

 gamete which, after fertilization with a hap- 

 loid gamete, forms a triploid zygote. (Com- 

 plementarily, fertilization of a gamete formed 

 without a nucleus may initiate development 

 of a haploid.) 



3. Haploid individuals may undergo meio- 

 sis and, although this usually results in gam- 

 etes containing only part of a genome, a 

 complete haploid gamete can sometimes be 

 produced which, upon fertilization with an- 

 other haploid gamete, forms a diploid zygote. 



By interfering with mitosis and meiosis, 

 autopolyploidy can be artificially induced by: 

 drugs like colchicine or its synthetic substi- 

 tute, colcemide (which destroys the spindle, 

 thereby preventing the anaphase movement 

 of chromosomes); environmental stresses 

 like starvation and cold; or energetic radia- 

 tions. 



Some females of Solenobia produce hap- 

 loid eggs; others produce diploid eggs. Both 

 types of eggs start development without 

 fertilization; that is, they begin developing 

 parthenogenetically. During development, 

 however, nuclei of the respective individuals 

 fuse in pairs to establish the diploid and 

 tetraploid conditions. In this case, normal 

 parthenogenesis leads to normal diploidy and 

 tetraploidy. In many other organisms, arti- 

 ficially induced parthenogenesis may begin 

 haploid development. 



In the case of an ordinarily diploid indi- 

 vidual, development as a haploid usually pro- 

 duces abnormalities. These must sometimes 

 be due to the expression of detrimental genes 

 which are not expressed in a diploid because 

 their normal alleles are present in homol- 



