256 AN INTRODUCTION TO MODERN GENETICS 



have arisen from hybrids. A hybrid between species A and B may be 

 almost completely sterile if the A chromosomes cannot pair with those 

 from B. Such a hybrid may form a few diploid gametes, either by a 

 chance migration of all the chromosomes to one pole, by syndiploidy 

 or suppression of the second division or finally by a somatic doubling. 

 If diploid gametes are formed in any of these ways, triploids or tetra- 

 ploids may be formed on selfing; and the latter, being A A BB, will be 

 able to breed true, but give infertile triploids when crossed to the 

 original parents. 



Certain naturally occurring species have been synthesized in this 

 way, by the chance occurrence of chromosome doubling in hybrids ; 

 and some new forms, deserving the name of new species, have been 

 created. As examples we may take the following : 



(i) Nicotiana ''digluta.''^ — This "synthetic species" was probably 

 the first whose origin was fully understood. It arose in the Fi of a cross 

 between N. glutinosa (2« = 24) and N. Tabacum (in = 48). On 

 selfing, all the Fi was sterile except for one plant, which gave hexaploid 

 (in = ji) offspring, and must itself have had this somatic number. 

 Presumably it arose from a seed in which the chromosome number was 

 doubled soon after fertilization. 



(2) Primula kewensis.^ — This new species arose by somatic doubling 

 in a hybrid between P. florihunda and P. verticillatai both of which 

 have in = 18. The original hybrid was nearly sterile, but the tetraploid 

 branch bore flowers which were fairly fertile on selfing, though giving 

 infertile triploid hybrids with the parent species. 



(3) Raphano-Brassica.^ — The intergeneric cross between the radish 

 Raphanus sativa (in =18) and the cabbage Brassica oleracea (in =18) 

 is nearly completely sterile, since there is a complete lack of pairing 

 between the chromosomes in pachytene. Occasionally unreduced 

 gametes are formed by syndiploidy, and on selfing the hybrids can 

 therefore give a few rare tetraploids. These are quite fertile on selfing, 

 though giving unfertile hybrids with the parent species. By suitable 

 crosses with the parents, many different infertile triploid, pentapioid, 

 etc., forms have been built up. These show very clear examples of 

 balance between the radish and cabbage characters, depending on the 

 dosage of radish and cabbage genes. (Fig. 81, p. 171.) 



(4) Galeopsis tetrahit* — Muntzing crossed G. pubescens (in = 16) 

 with G. speciosa (in ^16). The chromosomes of the hybrid paired 



' Clausen and Goodspeed 1925. - Digby 1912, Newton and Feiiew 1929. 

 ^ Karpechenko 1924, 1928. * Muntzing 1932 



