BREEDING SYSTEMS 



female parent and continues to do so generation after generation. 

 Such is the undoubted origin of triploid apomictic "species" of 

 plants and animals. 



There is, however, an omission in this account which would lead 

 to an error if it were to go uncorrected. The pairing chromosomes 

 in a triploid pair because they have crossed over and formed 

 chiasmata. The sister chromatids at the second division, in a triploid 

 as in a diploid, are therefore in part derived from partner chromo- 



3x 



3x-1 



Fig. 67. — Leaves of the normal triploid Taraxacum polyodon and its eight types of 

 disomic mutants occurring as apomictic seedlings in nature. X J (after Gudjonsson, 

 1946). 



somes, i.e. they are dissimilar; their separation thus leads to the 

 segregation of differences. The triploid egg cells of the original 

 triploid apomict are not therefore genetically identical with it, or 

 with one another. As compared with sexual species variation is much 

 reduced, but it still occurs. The new apomictic species is thus often 

 subsexual (Fig. 66). 



The adaptation of apomicts for fertility in Taraxacum has evidently 

 taken the form of making the suppression of meiosis more complete 

 and therefore more regular. Efficient meiosis so desirable in a diploid 

 has become a mere nuisance in a triploid. Triploid apomicts, in fact, 

 show almost complete suppression of cliromosome pairing and of 

 the consequent reduction of number. 



266 



