354 PERMANENT HYBRIDS 



(Schwemmle, 1928 ; D., 1931 d). From these arise occasional 

 tetraploid and triploid seedlings, the gigas and semi-gigas mutants 

 of De Vries. 



It was formerly assumed that the trisomic mutations of (Enothera 

 were similar to those of non-hybrid plants, and attempts were made 

 by de Vries and others to reduce them to the seven expected types. 

 This was not satisfactory, and their breeding behaviour was 

 anomalous. Some, for example, bred true. The reasons have 

 been shown by Catcheside (1933 h and 1936) and Emerson (1935, 

 1936 h). Thus in CE. Lamarckiana an extra chromosome from the 

 ring of twelve can provide a viable combination in company not 

 merely with the six gaudens chromosomes or with the six velans 

 chromosomes but also with a number of mixtures of the two sets. 

 These mixtures arise from the different possible non-disjunctional 

 arrangements. The arrangements giving rise to functional eight- 

 chromosome gametes are of two main kinds ( (i) and (2) below) ; 

 the first gives whole complexes of the parental types, the second, 

 new complexes formed by various mixtures of the old ones. One 

 example of each will suffice (neglecting the OP pair) : — 



1. A^B C^,D-DgE-E^F GJi K^,L M^N (8-gamete) 



B^C Ffi HgK LgM NgA (6-gamete) 



The 8-gamete will give with gaudens a zygote having the normal 

 Lamarckiana complement together with an extra chromosome, 

 D^E :- 



(i) A,B C^D^ J, E,,F GJI K,L M,N 



\ / \de) \ / \ / \ / \ 

 BgC ^^^^ Ffi HgK LgM NgA 



With velans the 8-gamete will give a homozygote provided the 

 D^E chromosome covers the lethals of velans : — 



\a,,b) \c^d) ^«^ Ke^f) \g^) \k^l) \M^N) 



In the second type the non-disjunction occurs at two different 

 places : — 



2. A^,B C^D-DgE Ffi HgK-K^,L M^,N (8-gamete) 



B C E,F G,H LM NgA (6-gamete) 



