370 Colchicine 



Comparisons at meiotic metaphase of diploid sterile hybrids and 

 the amphiploid are important for an understanding of the possible 

 associations that form between chromosomes of opposite genomes. 

 While this evidence is not infallible, correlations may be obtained 

 between pollen fertility, possible intergenomal exchange between 

 chromosomes, and reasons for the failure in seed setting of the poly- 

 ploid. 



15.6: Ecological Considerations 



The success of a polyploid in nature or in agriculture depends 

 upon how closely the new variety meets the requirements for each 

 situation. Productivity or adaptation are measured in terms of the 

 responses such as yield, disease resistance, drought resistance, and 

 cold tolerance. The elimination in nature occurs through competi- 

 tion and in agriculture at the hands of the agronomist. Wide dif- 

 ferences exist between diploid varieties, and considerable improve- 

 ment can be done at the diploid level without stepping up to the 

 tetraploid. Adaptation problems increase, rather than decrease, with 

 the use of tetraploids. Autotetraploid rye clearly showed that the 

 kind of plant used to make the diploid may be as important as any 

 other feature. 



Trying to measure the rates at which artificial polyploids become 

 established under natural conditions strikes at some basic problems 

 in polyploidy. Already differences have been recorded for the success 

 of the tetraploid over the diploid, or vice versa. An unusually high 

 seed production, about 75 per cent, in autoploid EJiroluita erecta 

 played some part in the establishment of the new type under natural 

 conditions. This situation held for ungrazed conditions, but where 

 grazing occurred, the low-growing habit of the diploid assured sur- 

 vival better since the flowers, being closer to the soil level, were not 

 destroyed as readily. This is one example of the critical differences 

 that determine success or failure of the tetraploid. ^^ 



Wilt diseases are devastating to watermelons in Japan. Appreci- 

 able resistance to Fusarium niveum was exhibited by the triploid and 

 tetraploid varieties. By selection, notable progress can be made for 

 insect and disease resistance if an initial advantage is provided 

 through the jjroduction of tetraploids. Autotetraploid radishes were 

 more resistant to the common club root disease, yielded more, and 

 had greater vigor than diploids. 



The succulence of water cress leaves was improved by increasing 

 the number of chromosomes, but the growth rates being slower among 

 the tetraploid reduced the yield. Fewer cuttings can be made per sea- 

 son with tetraploids. The slower growth and prolonged flowering 

 period for ornamental species is advantageous. No single trait can be 



