CHAPTER XXI 

 THE CYTOGENETICS OF AUTOHETEROPLOID PLANTS 



A somatic nucleus in a plant sporophyte or higher animal is auto- 

 heteroploid when one or more of the chromosomes of the normal set are 

 present in some number other than two.^ A gamete or gametophyte 

 nucleus is autoheteroploid when one or more of the members of the normal 

 set are present in some number other than one. In other words, only one 

 kind of set is represented, but some or all of its members are present in 

 unusual numbers. The cytological and genetical behavior shown by 

 autoheteroploid organisms may be expected to differ from that in mono- 

 ploid-diploid forms. It is the purpose of this chapter to review some of 

 the more characteristic types of such behavior. 



Tetraploids and Tetrasomics.— A plant with four similar complete 

 chromosome sets is "autotetraploid." One with a single chromosome in 

 quadruplicate but all of the others in duplicate is "simple tetrasomic." 

 It is found that the four homologous chromosomes in the latter type 

 behave in general like any one of the groups of four in the autotetraploid. 

 Hence the simple tetrasomic type, when bred, may be expected to show 

 "tetrasomic" genetic ratios in characters due to mutant factors in one 

 chromosome and ordinary "disomic" ratios for those due to factors in all 

 the other chromosomes, whereas in an autotetraploid type the ratios 

 will be tetrasomic for all characters. To illustrate such ratios, one may 

 therefore select either a tetraploid or a tetrasomic type. 



In autotetraploids and tetrasomics any four homologous chromosomes 

 ordinarily tend to form a quadrivalent group in meiosis (Figs. 199, g to i; 

 200). As synapsis begins, it is seen that any one of the four may associate 

 with any other one in a given region, or with more than one in different 

 regions. If all four form and retain such synaptic associations a quad- 

 rivalent chromosome is present at the end of the prophase, hence in a 

 tetraploid plant there may be the monoploid number of quadrivalents. 

 Often the synaptic association is such as to group the four members into 

 two bivalents, or into a trivalent and a free univalent. Thus tetraploid 

 sporocytes may sometimes exhibit the diploid number of bivalents (the 

 "double-diploid" condition) or varying numbers of quadrivalents, 

 trivalents, bivalents, and univalents. The reasons for this variation are 

 not fully known, but it seems that the manner in which the four members 



1 This does not apply to normally unpaired or multiple sex-chromosomes. Cf. 

 definitions on pp. 340 and 348. 



350 



