308 Colchicine 



doubt only a small segment of the chromosome from A^. glutinosa was 

 transferred to a chromosome of A^ tahacxim. If more than a small 

 segment were involved, greater disturbance to the genotypical balance 

 of the tabacum genome might be expected/''^ 



Evidence that parts of chromosomes were involved was given by 

 the fact that homozygous, low-blooming, mosaic-resistant segregates^^^ 

 that were different from the Burley tobacco appeared in backcrossing 

 A^. digliita to A^. tahacinu. These segregates in one case appeared in 

 the fifth backcrossing generation. Similar segregates were obtained 

 when Gerstel's 50-chromosomc "alien additional race," which had a 

 pair of A^ glutinosa chromosomes, was backcrossed to N. tabacum. 

 The nimiber of chromosomes during crossing was reduced to 48. In 

 the process these homozygous, low-blooming, mosaic-resistant plants, 

 that diffeied from Burley tobacco, appeared much the same as when 

 A^ digluta was the starting material. ^^^ 



The assumption may be made that an interchange had occmred 

 between the two genomes. In this case a segment was transferred 

 from one chromosome of a genome to another chromosome of the 

 opposite genome. The exchange was small, and transfer was limited 

 to the disease-resistance character. When whole chromosomes of A^ 

 glutinosa were substituted for a whole chromosome of A^ tabacurji, 

 the differences were such that substitution races differed from regular 

 varieties of tobacco. ^^^ 



Morphologically and genetically distinct popidations were isolated 

 among specific amphiploids as well as diploid hybrids. If the selection 

 was directed to a j^articular character, the progress made toward a 

 certain goal was faster at the diploid level than the amphipUjid.^*'-"^ 

 Generally, the amphiploid populations were less fertile. The tre- 

 mendous power of selection that is possible among amphiploids can 

 be demonstrated if the ])articular type has some intergenomal ex- 

 change.^ 



Among species of Nicotiana the genetic systems are close enough 

 to permit hybridization, yet removed from each other and sufficiently 

 differentiated to provide sterile hybrids between species. Upon 

 doubling the number of chromosomes, the amphiploids are fertile 

 and partially sterile.^' «• i-^ -^s. 32, .ss, 3.^, 4i. ss. 102. 118 There is enough 

 pairing at the diploid level to indicate that in some combinations of 

 species, exchange between genomes can occur. Such exchange leads 

 to interspecific segregation in the Fo and subsequent generations. 



Pairing of chromosomes at the diploid level of interspecific hybrids 

 is not a true picture of pairing when the amphiploid is derived. Five 

 cases with some bivalents at the F, stage had no nudti\alcnts in the 

 polyploid. ^^ 



