POLYPLOIDY 



chromosome now has a homologous mate, and, since there is Httle tend- 

 ency of A chromosomes to synapse with B chromosomes, there is no tend- 

 ency to form complex synaptic associations. The meiotic divisions therefore 

 proceed perfectly normally, with gametes of constitution AB resulting, and 

 with no impairment of fertility. 



Another method of production of allotetraploids involves a two-step 

 utilization of nonreduction. If the hybrid AB is cross-fertilized, it is espe- 

 cially likely to be backcrossed to one of the parental species, let us say AA. 

 If AB has undergone nonreduction, the backcross progeny will then be 

 AAB. These plants may again undergo nonreduction, producing gametes 

 of formula AAB. If backcrossed to parent BB, the resulting progeny would 

 again be the allotetraploid, AABB. The allotetraploid might also be formed 

 by doubling of the chromosomes in the zygote of the original hybrid, AB, 

 in a fashion comparable to that produced by treatment with colchicine. 

 These allotetraploids show characters of both parental species, together 

 with some typical tetraploid characters, in new and distinctive combina- 

 tions. They breed true, and are effectively isolated from the parent species 

 by the sterility of the hybrid between the allotetraploid and either of the 

 parental species. This sterility results from the behavior of the chromo- 

 somes in meiosis. In the hybrid between AABB and AA, the chromosomal 

 formula will be AAB. The chromosomes of the two A genomes will synapse 

 normally and will be distributed to the daughter cells normally. But the 

 chromosomes of the B genome have no synaptic mates, and so they are 

 distributed at random. They might all go to one pole, with the result that 

 gametes of formulas A and AB would be produced in equal numbers. Or 

 they might pass in equal numbers (but not equal genetic endowment) to 

 each pole, or any intermediate result might occur. All of these latter com- 

 binations would be inviable, and, since they comprise most of the gametes 

 of an AAB plant, these triploids are highly sterile. Some viable gametes 

 would also be produced by nonreduction. Thus allotetraploids may prop- 

 erly be regarded as good species, produced in one or a few steps. It has 

 even been suggested that new genera, families, and higher categories 

 might be produced in this way. But available cases indicate that, while 

 species and genera are formed through allopolyploidy, higher groups are 

 not. Stebbins believes that this is inherent in the fact that no really new 

 genetic material (mutations) is involved, but only new combinations of 

 old and related genomes. 



The above discussion has been entirely in terms of tetraploidy. While 

 this is both the simplest and the most common type of polyploidy, many 

 higher degrees are known both in nature and in experimentally produced 

 plants. But no new principles are involved in the higher polyploids, and 

 so all degrees will be discussed without distinction. Any polyploid having 

 an even number of genomes in the somatic cells (tetraploid, hexaploid, 

 octaploid, decaploid, etc. ) should be f uUy fertile, while those with an odd 

 number of genomes (triploid, pentaploid, septaploid, etc.) should be 

 highly sterile because of abnormal meiosis, and so these can survive in 

 nature only if they have efficient means of asexual reproduction. 



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