302 Colchicine 



general outline of the amphiploids ^vill be sketched. Polyploids from 

 24 interspecific crosses involved six genera: Bromus, Agropyron, 

 EJymus, Sitanion, Melica, and Stipa. Major emphasis was given to 

 Bromus because thirteen combinations were taken from this genus. 

 Considerable cytogenetical information has already accumulated for 

 three out of five recognized sections. Representative species are na- 

 tive to the American continents; perennials and annuals and natural 

 polyploidy series exist. i"'' 



A polyploid ^vith 112 somatic chromosomes involving Bromus 

 carinatus and B. marginetus exceeds the 84-chromosome level, highest 

 known for the genus under natural conditions. The artificial poly- 

 ploid into the C4 generation was vigorous, apparently more than the 

 Fi hybrid as shown by considerable vegetative growth that occurred 

 in the garden. A successful allopolyploid wdth 112 chromosomes was 

 a remarkable new case testifying to an effective use of colchicine when 

 combined with an appropriate hybridization. ^"^-^ 



Even more notable were the polyploids B. cannatus-trinii and B. 

 maritimus-irinii, which apparently combine the genomes from seven 

 different ancestral diploid species, thereby being 14-ploid, containing 

 98 somatic chromosomes. The immediate success demonstrated by 

 these polyploids is of exceptional interest when viewed together with 

 the implications about amphiploidy mentioned in the first section of 

 this chapter. The hyl)rids were very vigorous and mciotic processes 

 were irregular after doubling; plants in the C;:. and C4 generation 

 showed seed fertility in the range from 70 to 94 per cent. In all 

 probability this is a successful polyploid. i^*^ 



As shown by this work and an increasing number of other cases, 

 sterility-fertility relationships cannot be predicted in advance. Of all 

 the problems that confront polyploidy breeders, sterility-fertility 

 status among the newly created polyploids may well be the most 

 significant.^- The lowered fertility in autoploids has been confirmed 

 again and again. A conclusion that amphiploids necessarily have 

 higher fertility can be very misleading. A breeder using artificial 

 polyploidy must face the problems of sterility. Accordingly, two fac- 

 tors stand out as deserving primary consideration: vigor and fertility. 



12.3: Gossypium 



Special methods were devised for treating interspecific, sterile 

 hybrids of Gossypium with colchicine.^- 7. 27, 34, 54, eo. 106, iis) since 

 fertile amphiploids would be produced at once upon doubling the 

 number of chromosomes, a theory of the origin of tetraploid species 

 could be tested. Skovsted proposed that the American tetrajiloids in- 

 volved genomes from an Asiatic dijjloid and an American wild di- 

 ploid species. By hybridization between the Asiatic and American 



