POLYPLOIDY 



Species Synthesis. A very interesting allotetraploid was produced by 

 Karpechenko as long ago as 1924 while experimenting on crosses between 

 the radish, Raphanus sativus, and the cabbage, Brassica oleracea. In each 

 of these species, the haploid chromosome number is 9. In the hybrid, there 

 were 18 chromosomes (9R + 9B), but they behaved as 18 singletons at 

 meiosis, there being little or no tendency to synapse. As a result, most 

 gametes were inviable. But a few fertile hybrids were obtained. Cytologi- 

 cal examination showed that these had 18 perfectly synapsed pairs of 

 chromosomes at meiosis. Thus it was evident that allopolyploidy had 

 arisen spontaneously in a few of the hybrid plants. These showed a com- 

 bination of characters of the two genera which was completely different 

 from anything previously encountered. The allotetraploid bred true, and 

 was reproductively isolated from both parents. And so Karpechenko felt 

 justified in describing this new plant as a new, synthetically produced 

 genus, under the name Raphanobrassica. 



A similar synthesis of a new species of tobacco was performed in 1925 

 by Clausen and Goodspeed. Commercial tobacco, Nicotiana tabacum, was 

 crossed with a wild species, N. glutinosa. The hybrid was generally sterile, 

 but a single plant was fertile. This bred true, had distinctive morphologi- 

 cal traits, including the gigas habitus, and was reproductively isolated 

 from the parent species. Hence the fertile hybrid was described as a new, 

 synthetically produced species, Nicotiana digluta (Figure 108). Its syn- 

 thesis was later repeated with much greater facility by the colchicine 

 method. Nicotiana tabacum has 24 pairs of chromosomes, while N. gluti- 

 nosa has only 12 pairs. As expected, the sterile hybrids had 36 chromo- 

 somes (not pairs), but the fertile hybrids had 36 pairs of chromosomes, 

 and so were allopolyploids. Actually, these were allohexaploids, for N. 

 tabacum itself is an allotetraploid species, as will be shown below. 



Genome Analysis. There is much evidence that allopolyploids are wide- 

 spread in nature. For the most part, the evidence for this consists of series 

 of chromosome numbers in a single genus or family which are multiples 

 of a low number found in the same group. In addition, the species with 

 higher chromosome numbers frequently show combinations of the charac- 

 ters of more than one of the basic species. But the most convincing evi- 

 dence is what Clausen has called genome analysis. This means that a series 

 of experimental crosses is made in order to establish the actual source of 

 the different genomes of a suspected allopolyploid. The analysis of Nico- 

 tiana tabacum is an illustrative example. The first step in genome analysis 

 is to select probable parent species on the basis of morphological charac- 

 ters held in common by the allopolyploid and the suspected parents. Any 

 species so chosen must of course have a chromosome number such that it 



FiGtTKE 108. Nicotiana Hybrids. A, N. tabacum X N. glutinosa (36 chromosomes) is 

 sterile, while B, its colchicine-induced tetraploid with 72 chromosomes ( = N. digluta ) , 

 is fertile. C, N. glutinosa X N. sylvestris has 24 chromosomes and is sterile, while its 

 colchicine-induced tetraploid, D, with 48 chromosomes, is fertile. Note that all parts 

 of the tetraploid flowers are larger than the corresponding parts of the diploid flowers. 

 (From Warmke, H. E., and Blakeslee, A. F., /. HerecL, V. 30, 1939.) 



SOI 



