Chapter 28 

 THE ORIGIN OF POLYPLOIDS 



We pointed out in previous chapters that many species of 

 plants and some animals are either autopolyploids or allopoly- 

 ploids. If we start with the assumption, and it seems reasonable 

 enough to do so, that the diploid condition is primitive and that 

 polyploids are derived from diploids, the important questions are 

 how did the polyploids arise and what mechanisms bring about 

 the polyploid state? 



Apparently only two factors can initiate polyploidy — doubling 

 of the chromosome number in somatic tissue, which results in 

 a polyploid branch, and doubling of the chromosome number 

 in the formation of germ cells, which results in germ cells with 

 a multiple of the n number of chromosomes. If the polyploid 

 branch is one that later gives rise to germinal tissue, the gametes 

 which form on it will have more than n chromosomes. These 

 two conditions can arise spontaneously in nature but can also 

 be brought about artificially by certain changes in the environ- 

 ment of the organism. If these conditions occur and polyploids 

 are produced, polyploids of still other types can be brought about 

 by a repetition of these fundamental changes or secondarily by 

 self-pollination of the polyploids or by certain crosses of which 

 polyploids are one or both of the members. These secondary 

 methods of producing polyploids may occur spontaneously or 

 may be carried out artificially. 



If a disturbance of cell division occurs during the formation 

 of megaspores or microspores (that is, at meiosis), individual 

 male or female gametes develop which are 2n. If two such unite, 

 a tetraploid is produced whereas a triploid will result from the 

 union of such a gamete with a normal haploid gamete. If the 

 disturbance arises at mitosis in a bud primordium, a 4n branch 

 will result. If a flowering branch, it will produce 2n gametes. 



Once a polyploid has arisen, others may be produced from it. 

 If doubling of the chromosome number in somatic tissue occurs 



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