CROSSING-OVER AND MUTATION 475 



parthenogenetic plant or animal are as like their parent as conditions 

 permit ; in plants they constitute a " clone " of the same uniformity 

 as one reproduced by cuttings or bulbs. Ostenfeld, however, 

 showed that two " mutations " occurred in 1260 parthenogenetic 

 seedlings of H. tridentatum. 



Similarly, male plants appear from parthenogenetic Antennaria. 

 The origin of these mutations becomes clear in the light of the 

 present interpretation of chromosome pairing in terms of crossing- 

 over. Both the general theory and the particular experiments 

 show that where either the first or the second meiotic division is 

 suppressed after certain chromosomes have paired and formed 

 chiasmata there will be a segregation of any differences between 

 these chromosomes (Ch. VII). Since, in all but the most extreme 

 and stable forms of parthenogenesis, mother-cells occur with pairing 

 of chromosomes, this pairing will account for the occasional " muta- 

 tions " observed in parthenogenetic progenies of hybrid plants and 

 animals {cf. Gustafsson, 1934 a ; Bergman, 1934). 



In short, diploid parthenogenesis appears to provide a mechanism 

 whereby an organism in which normal meiosis would give sterility 

 through excessive segregation, may show limited segregation and 

 be fertile. The mechanism is analogous in its effect to that whereby 

 segregation is restricted in the tetraploid derivatives of diploid 

 hybrids. Most segregates being more homozygous should be less 

 asexually fertile than their parent, as these were. They will 

 provide the basis of polymorphism. The mutations inferred in the 

 relatively homozygous Cladocera (Banta and Wood, 1928) cannot, 

 of course, be due to segregation of any kind. Nor is the segregation 

 found in parthenogenetic Apotettix (cf. Robertson, 193 1 a) of this 

 kind, but merely first division mendelian segregation in an organism 

 with perfect chromosome pairing. 



A second ground for expecting mutations in the progenies of 

 parthenogenetic plants is found in the imperfection of the failure 

 of meiosis when sexual reproduction is first replaced. If, occasion- 

 ally, univalents have divided before a restitution nucleus is formed 

 in the first division, progeny will be produced with varying and 

 unbalanced chromosome numbers. Such plants are produced and 

 survive in Miintzing's Poa species which on this account must be 



