368 



INTRODUCTION TO CYTOLOGY 



parents. Moreover, only with difficulty could they be crossed with the 

 parents, and the few offspring obtained were highly sterile; hence they 

 had the sexual isolation which aids in the establishment of new independ- 

 ent types. The type was given the new generic name Raphanobrassica. 

 Other heteroploid types were obtained also; these will be discussed later 

 in the chapter. 



After crossing Triticum durum (n = 14) with /Egilops ovata {n = 14), 

 it was found that the Fe plants were fertile, genetically stable, and 



amphidiploid (56 chromosomes).^^ The 

 type was named i-Egilotricum (Tschermak 

 and Bleier). Since the F\ of such a cross 

 was later found to be diploid (28), it is 

 probable that it produced diploid spores 

 through some form of ameiosis (Kagawa). 

 Certain other Tinticum-Jilgilops hybrids 

 are more variable in later generations 

 (Taylor and Leighty). Triticum turgidum 

 {n = 14) X Triticum villosum (n = 7) gave 

 a constant and fertile hybrid with 42 

 chromosomes (Tschermak). By crossing 

 this hybrid with /Egilotricum, there was 

 obtained a plant combining the gametic 

 complements of four species (Berg, 193 1&). 

 In the notation given on page 366, the 

 somatic formula of this plant would be 

 AAABBDE. A fertile, balanced hybrid 

 between Triticum and Secale is reported 

 by Lebedeff (1932). 



Nicotiana tabacum {n = 24) crossed 

 -Diagram of mitoses with N. sylvesfris {n = 12) gave a sterile 



in microsporocytes of Fi hybrids of 

 Raphanus sativus X Brassica oleracea. 

 A, usual course of divisions, form- 

 ing monoploid spores. B, forma- 



B 



Fig 



tion of diploid spores following 

 "asynapsis." {After Karpechenko, 

 1927a.) 



Fi hybrid with 36 chromosomes. A 

 branch of this plant was treated with 

 chloroform vapor 8 days before the open- 

 ing of the flower. Self-pollination then 

 resulted in the production of a single 



seed, which grew into a fertile plant with 72 chromosomes. Probably 



the chloroform treatment induced ameiosis, leading to amphidiploidy 



(Rybin). 



The formation of unreduced gametes is also known in moth hybrids. 



Pygcera anachoreta {n = 30) X P. curtula (n = 29) gave anFi hybrid with 



^^ If, as suggested by the theory mentioned on p. 366. Triticum durum and jEgilops 

 ovata may be regarded as amphidiploids with the somatic formulae A ABB and DDEE, 

 respectively, the 56-chromosome hybrid formed by crossing them would be amphi- 

 diploid in the second degree: it would be an allo-octoploid with the formula 

 AABBDDEE. 



