The Amphiploids 303 



diploids, and dou Idling of chromosomes, a tetraploid species like G. 

 Iinsiilinn arose in natnre. Now the test could be repeated experi- 

 mentally, and those investigators who had been studying species hy- 

 brids at the time promptly ajjplied colchicine. The synthesis was 

 announced independcnth from two laboratories."' ^^ G. arboreiitii 

 (n = 13, Asiatic diploid) X ^^- tlnnhrri (n = 13, American diploid) 

 was changed from a 26-chromosome h\bricl to a 52-chr()mos()mc amphi- 

 ploid. The plants were cytologically similar to G. hivsiiiiiin. The 

 synthetic amphiploid hybridized with natural tetraploids, and sur- 

 prisingly good pairing at metaphase was obtained. A concltisive ex- 

 periment had been performed. The hypothesis of Asiatic-American 

 origin of tetraploid cotton was confirmed.'- ^•' 



A useful classification" was formulated to bring together data 

 about geographical distribution, morphology, chromosomal pairing, 

 numbers, and chromosomal structine differences. The genomes from 

 each region were gi\'en letters as follows: (1) Asiatic species, A-^ and 

 A./, (2) African diploids, B; (3) Australian species. C; (4) American 

 dijjloid species, D^ to D^r, and (5) Arabian-India diploids, E. The 

 Asiatic species represent a central position with affinities to American, 

 Australian, and Arabian-Indian sj^ecies. They are closer in relation- 

 ship to African species than the other grotips. Arabian-Indian species 

 are distant to all and jjarticularly farther front the American diploids. 

 One advantage of this system is the code that can be used for describ- 

 ing amphiploids." If the American tetraploids were derived from an 

 Asiatic and an American source, the amphij^loid should read 2 {AD) 

 with an appropriate subscript to indicate the species of tetraploid. 

 Accordingly the G. hirsutiun would be 2 (AD) ,. Table 12.2 illustrates 

 the use of genomes and some of the important species with their geo- 

 graphical distribution. 



Experimentally produced amphiploids are potentially new species 

 because the duplications made by hybridization of diploids and dou- 

 bling the chromosomes do not exactly replicate the natmal one.^'' Some 

 kind of differentiation occurred after the first amphiploids arose. A 

 spontaneously occurring amphiploid, ^^ G. davidsonii X G. anornalum, 

 showed how a new species might have arisen in nature and become 

 isolated from other types. A counterpart of tliis spontaneously oc- 

 curring cotton was made by colchicine. The data for these cases were 

 similar.'"" 



Problems in polyploidy among species of Gossypium were well 

 known before colchicine was discovered."*^ Gene systems were con- 

 cei\ed to account for the way in which diploid and tetraploid species 

 became differentiated. By the use of experimentally produced amphi- 

 ploids, relations between genomes and the problem of speciation could 

 be studied more extensively. Specialists in Gossypium began to realize 

 more specifically that problems remained unsolved. i*^" 



