HYBRIDITY AND FERTILITY 



187 



the set of nine and owing to a change in the proportionate influence 

 of the two species. Such plants, particularly those with 34 chromo- 

 somes, which have 2 chromosomes of one type and 4 of the other 8 

 types, are less fertile than the normal tetraploid (cf. Ch. VIII). 



Plants derived from hybrids between species with different 

 numbers {e.g., diploid and tetraploid Nicotiana) may show a com- 

 bination of the two extremes in the same nucleus. The Nicotiana 

 riistica-panicidata hybrid is a triploid of this kind with 12 pairs and 



Table 25 

 Fertility Showing the Origin of Polyploids after Hybridisation 

 I. Failure of Reduction {F^ more fertile than F-^ 



Raphanus X Brassica, Karpechenko, 



1927 a. 

 Digitalis purpurea X D. ainbigtta, Buxton 



and Newton, 1928. 

 Phleum pratense x P. alpinum, Gregor 



and Sansome, 1930. 

 ^gilotricum, Kihara and Katayama, 1931 

 Crepis rubra X C.futida, Poole, 1931. 



Fertility of Fj. 

 45 seeds per plant. 



200 seedlings in two plants. 



11-5 seeds perplant. 



2 7 ears gave 5 seedlings. 

 50 seeds from 42 heads. 



9 seedlings (3, 2x ; s, 



3X\ 1,4^). 



Fertility of F,. 

 30 seeds per pod. 



400 seeds per capsule. 



400 seeds per plant. 



61 per cent, fertile. 

 55 seeds from 22 heads. 



2. Somatic Doubling (F^ of equal or reduced fertility). 



Nicotiana glulinosa X A^ Tabacum, 

 Clausen and Goodspeed, 1925 ; 

 Clausen, 1928. 



Primula keweusis ; cf. Sansome, 193 1. 



Solarium nigrum x S. luteum, j0rgensen, 

 1928. 



155 seeds per fruit. 



No greater. 



30, 90 and 180 seeds per 122 seedlings per capsule. 



capsule. 

 287 germinated. 

 8-1 1 seeds per capsule {i.e., No greater. 



25 percent.). 

 (" Completely sterile " 



before doubling). 



12 univalents. Doubling, it gives rise to a hexaploid ; this has 

 two identical sets that pair with no other sets and therefore appear 

 regularly as 12 bivalents, and it has four sets which form fluctuating 

 numbers of quadrivalents and bivalents. 



The sharp difference in chromosome behaviour between the 

 allotetraploid and its hybrid parent causes a difference in fertility ; 

 this affords a means of determining the exact time and therefore 

 mode of its origin, which would not otherwise be clear in all cases. 

 The doubhng of the chromosome number occurs in two ways : by 

 failure of separation of two groups of chromosomes in a somatic 



