140 



CHAPTER i; 



have been obtained had the individual under 

 test been fertilized by pollen from a diploid. 

 (In this case each pollen grain would con- 

 tribute one genome to the development of the 

 zygote in the seed.) The differences in size 

 of the seed capsules is due partly to the 

 number of seeds that have set or developed. 



Polyploidy is also found in animals, for 

 example, in Drosophila. Female Drosophila 

 have been found that are triploid (3X + 3 

 sets of A) and tetraploid (4X + 4 sets of A) 

 (cf. pp. 102-104). Parts of individuals have 

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

 You recall that triploid females produce 

 gametes many of which are genomically 

 abnormal. Since two tetraploid Datura can 

 be crossed, and a sufficient number of fertile 

 seeds are set to form a tetraploid race, the 

 question arises, can a tetraploid race of 

 Drosophila be produced? 



The tetraploid Drosophila female forms 

 gametes that more often contain complete 

 genomes than do the gametes of triploids. 

 (The number of genomes present must be 

 even, not odd, if each chromosome is to 

 have a partner at meiosis. So, in tetraploids, 

 where genome number is even, the four ho- 

 mologous chromosomes often segregate 2 and 

 2, but sometimes segregate 3 and 1.) So this 

 sex presents no difficulty for the continuity 

 of a tetraploid race. However, the tetraploid 

 male would have to carry 2X + 2Y + 4 sets 

 of A in order to be of normal sex. Un- 

 fortunately, during meiosis in such a male 

 the X's would synapse with each other and 

 so would the Y's, so that after meiosis each 

 sperm would carry IX and lY in addition to 

 2 A sets. Such sperm fertilizing eggs of 

 2X + 2 sets of A constitution, from tetra- 

 ploid females, would produce zygotes with 

 3X + lY + 4 sets of A which would develop 

 as sterile intersexes. Thus, a tetraploid race 

 of Drosophila, which is self-maintaining, can- 

 not be established. In fact, it becomes clear 

 that any species containing a heteromorphic 

 pair of sex chromosomes (like X and Y) 



cannot form polyploid races, since the correct 

 balance between sex chromosomes and auto- 

 somes will be upset by the meiotic divisions. 

 This factor probably explains why polyploid 

 races and species are rarer among animals 

 than among plants, where sexuality is often 

 not associated with such chromosomal differ- 

 ences (Chapter 13). 



Nevertheless, polyploid races of animals 

 are sometimes found. For example, tetra- 

 ploids of the water shrimp, Arteinia, of the 

 sea urchin. Echinus, and of the roundworm, 

 Ascaris, are known. In the moth, Solenobia, 

 some females may produce haploid eggs and 

 others diploid eggs. Both types of eggs start 

 development without fertilization, i.e., start 

 development parthenogenetically. During de- 

 velopment, however, nuclei of the respective 

 individuals fuse in pairs to establish the 

 diploid and tetraploid conditions. Poly- 

 ploid larvae of salamanders and of frogs also 

 have been obtained, although races are not 

 formed. 



One way that ploidy can increase is by the 

 addition of genomes of the same kind as are 

 present, that is, by autopoJyploidy, as was 

 the case in the Datura discussed. Autopoly- 

 ploidy can arise several different ways. An 

 organism capable of asexual reproduction 

 may fail to have a normal mitotic anaphase 

 so that the doubled number of chromosomes 

 becomes incorporated into a single nucleus, 

 which thereafter divides normally to produce 

 daughter polyploid nuclei and eventually 

 polyploid progeny. Sometimes two of the 

 haploid nuclei produced by meiosis may fuse 

 to form a diploid gamete which, after fertiliza- 

 tion with a haploid gamete, forms a triploid 

 zygote. Or haploid individuals may undergo 

 meiosis, and, while this usually results in 

 gametes containing only part of a genome, 

 a complete haploid gamete may sometimes 

 be produced which, upon fertilization with 

 another haploid gamete, forms a diploid 

 zygote. 



By interfering with mitosis and meiosis. 



