194 THE BEHAVIOUR OF POLYPLOIDS 



2. PAIRING. SEGREGATION AND FERTILITY 



When we compare polyploids with the diploids from which they 

 have arisen we find a sharp change in the choice of partners and 

 consequent segregation amongst homologous chromosomes and at 

 the same time a sharp change in fertility. From this we can learn 

 something important about the physiological action of the chromo- 

 somes. At present, however, we are concerned merely with 

 inferring the constitution of polyploid species and other old poly- 

 ploid forms from our knowledge of the constitution of new ones. 



The increase in fertility in sterile diploids is in sharp contrast 



^. msticanas R. thyrstger 

 inermis 



(AA) 



(BBCC) 



RT2 RT4 





7 C->' 



/'-t: 



(ABC) (AABC) 



Fig. 66. — The origin of the tetraploid " RT4 " and triploid " RT2 " 

 from a cross between two Ruhus species, illustrated by mitotic 

 metaphas3s. A, B and C represent sets of seven chromosomes. 

 X 2000 (after Crane and D., 1927). 



with the decrease which is always found in fertile diploids that 

 double their chromosome number. In the following examples the 

 tetraploid produced is less fertile than the diploid : — 



Table 28 



Autotetraploids with Reduced Fertility 



CE. Lamarckiana . . Lutz, 1907 a ; Gates, 1909. 



Datura Stramonium . . Blakeslee et al., 1923. 



Primula sinensis . . C/. D., 1931 a. 



