VI 



HYBRIDS 183 



both of the homologous chromosomes bearing this factor have it in the 

 g form. Sometimes, however, one of these, by mutation, changes into the 

 G condition, and since G is dominant over g, all future somatic descendants 

 of this cell will be green ; thus we find green branches occasionally 

 appearing on the variegated plants. These branches are now in exactly 

 the same condition, cytologically, as if they belonged to a hybrid between 

 a green and a variegated plant, with the result that the ordinary Mendelian 

 proportions which are to be expected among their offspring are realized. 



E. STERILE AND PARTIALLY STERILE HYBRIDS 



We can provisionally distinguish two types of these hybrids : (i) 

 where the disturbance in meiosis seems to be mainly of a mechanical 

 nature, depending upon a numerical, rather than a physiological dis- 

 crepancy in the chromosomes of the two parents ; (2) where the 

 disturbance is mainly physiological. We must remember that this 

 distinction, even if justifiable, is by no means sharp, the two types 

 overlapping and grading into each other. 



As an example of the first of these two types of crosses, we may 

 take the cross between the two species of sun-dew Drosera longifolia 

 and D. rotimdifolia (Rosenberg, 1909). This cross results in a hybrid 

 intermediate in character between the parent species, and known as 

 D. ohovata. In D. rotundifolia 2n is 20 and in D. longifolia it is 40, the 

 latter species being probably a tetraploid form. In D. ohovata the somatic 

 number is 30. 



In syndesis of the hybrid, Rosenberg found ten bivalents and ten 

 univalents — that is to say, syndesis had taken place between the ten 

 rotundifolia chromosomes and one of the two sets of their homologues 

 from the tetraploid longifolia, leaving the other set of ten unpaired. 

 In anaphase the constituents of the bivalents were regularly, but the 

 univalents irregularly, distributed to the daughter nuclei, which therefore 

 sometimes received very unequal numbers of chromosomes ; for example, 

 18 and 12. Fertile pollen grains were seldom or never formed, but fertile 

 ova fairly frequently. 



Probably a closely analogous case is afforded by the crosses between 

 Oenothera gigas and other Oenotheras, which may be considered here 

 though they do not necessarily result in any notable degree of sterility. 



We have already (p. 149) seen reason to believe that 0. gigas is, like 

 D. longifolia, tetraploid ; its somatic chromosome number is 28. It 

 therefore becomes of interest to see how this form behaves in crosses 

 with the diploid forms (2w = 14). The cross most studied is 0. lata ^ xO. 



^ The fact that this Oenothera has 15 instead of 14 somatic chromosomes (p. 146) reed not 

 concern us here. 



