CHAPTER NINETEEN 



Polyploidy 



About forty years ago, Winge published a study of the numbers of 

 chromosomes in plants. Although diploid numbers ranging from 4 to well 

 over 200 were noted, the frequencies of these numbers were by no means 

 random. Twelve was the most frequent number, while 8 was the second 

 most frequent. About 50 per cent of all plants have numbers below 12. 

 Among those plants with higher chromosome numbers, the most frequent 

 numbers are multiples of the lower ones. Within a single genus, it com- 

 monly happens that there is a series of species in which the chromosome 

 numbers of some are multiples of that of another species. For example, 

 there are species of wheat with 14, 28, and 42 chromosomes. Seven chro- 

 mosomes appears to be the basic haploid number in this genus. If the 

 haploid numbers of many plants be plotted on a frequency histogram, it 

 turns out that none of the maxima fall on prime numbers. 



POLYPLOIDY. A MAJOR PHENOMENON IN 

 PLANT EVOLUTION 



Winge concluded that the most probable explanation of these facts lay in 

 the assumption that over half of the higher plants were polyploids, that 

 is, their haploid chromosome set consisted of two or more basic sets of 

 chromosomes existing side by side in the same nucleus. This could happen 

 in either of two ways. Either a single haploid set of chromosomes might 

 be present more than twice (autopolyploid), or two different sets of chro- 

 mosomes might be present, making a total of more than two genomes 

 (allopolyploidy). Winge ventured the guess that the latter type would 

 prove to be the more frequent, basing his opinion on the assumptions that 

 lack of homology would prevent pairing of the chromosomes in species 

 hybrids, and that the necessity of pairing would therefore stimulate dou- 

 bling of the whole chromosome complement in such hybrids. While his 

 reason may be doubted, this conclusion, and the rest of Winge's conclu- 

 sions, have been substantiated since his original publication. The develop- 

 ment of polyploid series seems to have been one of the major phenomena 

 of plant evolution, and one of the most thoroughly understood. 



Colchicine Induction of Polyploids. The study of polyploidy has been 

 greatly facilitated by development of experimental techniques for its artifi- 



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