Chapter 27 

 ALLOPOLYPLOIDS 



Let us assume that a certain diploid plant has six chromo- 

 somes which we can designate AA BB CC. Since this plant has 

 two identical sets of chromosomes, the chromosomes of each set 

 would be A B C. We pointed out in the last chapter that auto- 

 polyploids have more than two genomes and that all the ge- 

 nomes of an autopolyploid are alike. In an autotriploid form 

 of this plant, the chromosome constitution would be AAA BBB 

 CCC, whereas in an autotetraploid which was derived from this 

 diploid it would be AAAA BBBB CCCC. We will use "genome" 

 here as synonymous with the more cumbersome "set of chromo- 

 somes," disregarding any differences that might be due simply 

 to different alleles. 



Let us assume now that we have two diploid plants whose 

 chromosomal constitutions are respectively AA BB CC and LL 

 MM PP and that they are able to cross together and produce a 

 hybrid. This hybrid would have a genome from each of the two 

 plants and would therefore consist of two different genomes. 

 Since its constitution would be A B C L M P, no chromosome 

 would have a mate, there would be no chromosome pairing, the 

 plant would behave like a haploid plant with six univalent chro- 

 mosomes, and the plant would be highly sterile. If, however, 

 the chromosomes of this sterile hybrid became doubled in some 

 way, a plant would be formed whose chromosomes would be 

 AA BB CC LL MM PP. This plant could be regarded as a 

 tetraploid because it had four genomes, but since two of the 

 genomes were alike and different from the other two which were 

 identical, each chromosome would be represented only twice as 

 in a diploid instead of four times as in an autotetraploid, and 

 the plant would behave as a diploid. Such a tetraploid would 

 be an allotetraploid. Because it behaves like a diploid but never- 

 theless is composed of two kinds of genomes, it is often called 

 an amphidiploid. 



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