THE CYTOGENETICS OF HYBRIDS 365 



union of gametes carrying dissimilar sets as a more frequent cause of 

 allopolyploidy. 



Although the degree of validity of this basis for the analysis of 

 chromosome complements is a subject of debate, certain cases may be 

 cited to illustrate the method. One of the best known is that of Nico- 

 tiana tabacum, which has 48 somatic chromosomes. When N. sylvestris 

 (12 in gamete) is crossed with N. tomentosa (12), the resulting hybrid 

 shows almost complete asynapsis among its 24 chromosomes. This is 

 taken to mean that the sylvestris set *S and the tomentosa set T are not 

 closely homologous. Similarly, the hybrid A'', sylvestris (12) X N. 

 rusbyi (12) shows no synapsis. When N. labacum (24) is crossed with 

 N. sylvestris (12), the 36-chromosome hybrid shows 12 bivalents and 12 

 univalents in meiosis. Also, when N. tabacum (24) is crossed with N. 

 tomentosa or N. rusbyi (12), the hybrid shows 12 bivalents and 12 uni- 

 valents. Finally, in the triple hybrid [tabacum (24) X rusbyi (12)] X syl- 

 vestris (12), there are 24 bivalents. From these data on synapsis it is 

 inferred that the gametic complement of N. tabacum actually consists of 

 two sets of 12, one of them being homologous with the tomentosa set {T) 

 and the other with the sylvestris (S) (or rusbyi) set. Hence N. tabacum 

 is an allotetraploid hybrid with the somatic composition TTSS, its 

 gametes carrying TS. It is believed to be a descendant of tomentosa 

 (or tomentosiformis) and sylvestris or closely related progenitors, the 

 cross being followed by chromosome doubling and the attainment of 

 stability and fertility." Other cases of this kind will be described in the 

 next section. 



On the basis of genetic data and synaptic behavior, Hurst has 

 advanced the theory that in the genus Rosa there are five fundamental 

 diploid species, each with its characteristic set of seven chromosomes. 

 The diploid somatic complements of these species may be represented as 

 A A, BB, CC, DD, and EE. Multiplication of any one of the sets gives 

 autopolyploid "varieties," such as AAA A or DDD. Hybridization leads 

 to the establishment of allopolyploid ''species." These may be "regu- 

 lar," with bivalents only {e.g., A A BB, or AA DD EE); or "irregular," 

 with bivalents and univalents (e.g., AA B D E,ov AA CC E). Although 

 the limits of the applicability of this theory are not agreed upon by all 

 workers, the responsibility of hybridization for the production of many 

 known rose types is unmistakable.'^ 



On similar grounds Morinaga (1928, 1929) postulates the occurrence 

 of three kinds of basic set in Brassica: A, with 10 chromosomes; B, 

 with 8; C, with 9. Synaptic behavior in hybrids and the numbers present 



«R. E. Clausen and Goodspeed (1925), Goodspeed and Clausen (19276, 1928), 

 Brieger (1930), Clausen (1932). See the criticism by East (1933). 



^ Hurst (1925 et seq.), Erlanson (1929, 1931o6), Harrison and Blackburn (1927), 

 Blackburn and Harrison (19246), Blackburn (1925). 



