!!*> Gates, Tetraploicl Mutants and Chromosome Mechanisms. 



Tetraploid Mutants and Chromosome Mechanisms. 



By R. R. Gates. 



Imperial College of Science, London. 



The discussion which has been evoked in several recent papers, 

 on the manner of origin of Ocnothera gigas, calls for additional 

 facts as well as certain comments and criticisms. Several years 

 ago (Gates, 1909 a) in a somewhat detailed study of the nuclear 

 and cell size in 0. gigas as compared with its parent, 0. Lamarck- 

 iana, I expressed the view that the doubling in the chromosome 

 number had probably occurred as the result of a suspended mitosis 

 in the fertilized egg or in an early division of the young embryo. 

 This view was based upon several facts, one of which was the 

 absence of triploid (3 X) mutants in Oenothera; and another, the 

 absence of triploid species among all the plants whose chromosome 

 numbers are known, combined with the occurrence of a considerable 

 list of tetraploid species, i. e., species having double the chromosome 

 number of their nearest relatives. 



This view placed the essential mutational change in a dif- 

 ferent part of the life cycle of 0. Lamarckiana from that which 

 the mutation theory of de Vries would suggest, namely in the 

 fertilized egg rather than during the reduction divisions in the spore 

 mother cells. 



Accordingly, Stomps (1910) stated his belief that (). gigas 

 originated from the union of two "unreduced" or diploid germ cells 

 rather than from a transformation occurring at or after the time 

 of fertilization. He cited (p. 59) as evidence for this view an 

 observation of de Vries' pupil, Geerts (1909, p. 52), who found 

 a megaspore mother cell of 0. Lamarckiana having twenty-eight 

 instead of fourteen chromosomes. Stomps expressed the opinion 

 that such a megaspore mother cell would undergo the usual chromo- 

 some reduction and would then go through the embryo sac mor- 

 phology, producing an egg with fourteen chromosomes. He further 

 assumed that such an egg was fertilized by a male cell derived 

 from a diploid pollen grain, to produce the gigas mutant with 

 twenty-eight chromosomes. Instead of this rather roundabout method, 

 involving the assumed occurrence of diploid pollen grains, I sug- 

 gested (Gates, 1911 a, p. 9H4) that it was at least equally probable 

 that the type of megaspore mother cell observed by Geerts 

 developed an embryo immediately, thus omitting both reduction 

 and fertilization. This was based upon the well-known fact that 

 higher chromosome numbers are frequently associated with some 

 form of apogamy, a list of such cases having been given in my 

 earlier paper on the subject (1909 a, p. 545). I also cited in that 

 paper several instances in which the number of chromosomes had 



