1 62 EIGSTI 



polyploidy as a breeding method, rather than the actually improved varieties 

 added to agriculture (Levan, 1945). These concepts appear to be more valu- 

 able than the actual tetraploids developed from diploids. Practical men be- 

 came alarmed and disappointed because so many polyploids proved wholly 

 useless. In many cases, the results consisted in new and better knowledge 

 about polyploidy. Information can be more valuable than a direct benefit 

 from specific plants. 



Some of the principles now recognized in the breeding of polyploid crops 

 are (1) first-generation polyploids as "raw" polyploids and as such unselected 

 genotypes; (2) large populations necessary; (3) selection all important; 

 (4) choice of diploid and the need for diploids instead of tetraploids for 

 starting material; (5) optimal numbers of chromosomes for a maximum 

 efficiency; (6) use of cross-fertilizing species; (7) transfer of genes from 

 one species to another; (8) fertility and sterility barriers; (9) advantages 

 compared with the attending disadvantages; (10) procedures for testing the 

 polyploids against the diploids. 



Many plant breeders often expected the first-generation tetraploids to pro- 

 vide the superior strains in contrast to the diploid. Such improvements were 

 anticipated with the development of the polyploid. As a matter of fact, the 

 task of plant breeding only begins once the tetraploids have been created. 

 These "raw" polyploids are unselected and unbalanced genotypes even though 

 the starting diploids may have been carefully selected varieties. 



With the efficiency of colchicine in contrast to other methods, large popu- 

 lations could be handled. Instead of developing a few tetraploids from a few 

 diploids, hundreds of tetraploid individuals should be included. The largest 

 possible combinations of genotypes must be transformed from the diploid to 

 the tetraploid level. Effective selection can be done only with large popula- 

 tions. Such selection is now regarded as essential if success is expected with 

 polyploids induced from diploids. 



It should hardly be necessary to state that diploids rather than the already 

 polyploid species are prerequisites for a good polyploidy-breeding program. 

 Since there are many natural tetraploids available, numerous breeding projects 

 failed because their starting material was unsatisfactory in this respect. There 

 seems to be an optimal number of chromosomes that is tolerated for each 

 species. Numbers above or below that optimum usually lead to inferior 

 performance. Sometimes triploidy is optimal, rather than tetraploidy or 

 diploidy. 



Cross-fertilizing species as a group tend to be more useful in creating favor- 

 able tetraploids than self-fertilizing species. Also, the transfer of genes from 

 one species to another can be done at the polyploid level and not at diploid 

 levels. Higher chromosomal types act as a bridge across the species barrier. 

 Within certain groups this potential for crop improvement is exceedingly great. 



Everyone should be aware of the universally attendant characteristic of 



