POLYPLOIDY 



northerly distribution than does the diploid race. Also, the polyploids are 

 perennial, while the diploid is annual. This is also a very common, though 

 by no means universal, difference between the chromosomal races. Cross- 

 ing of the tetraploid and diploid races is more difficult than in the case of 

 Phletim. Almost all of the seed obtained from this cross was shrunken and 

 inviable. 



A particularly instructive example is that of the diploid and tetraploid 

 races of Tradescantm caniculaia, the spiderwort, in southern United States. 

 The chromosome numbers are 12 and 24. This species is widely distributed 

 over the great plains from the Rocky Mountains eastward to the Missis- 

 sippi River. Over most of this area the plants are tetraploid. But in a small 

 area in northern Texas, the diploid race is found. This area geologically 

 is the oldest part of the total range. It appears that only the tetraploid race 

 has been able to invade those territories more recently opened up to floral 

 colonization. Tliis greater aggressiveness of polyploid plants is quite char- 

 acteristic. Another species of the same genus, T. occidentalis, overlaps the 

 eastern part of the range of T. caniculata. Interestingly enough, this spe- 

 cies, which extends eastward, is also tetraploid over most of its range, 

 but the diploid form is found in the same refuge in northern Texas. In 

 both of these species, the tetraploid races are much more vigorous than 

 are the diploid races. Crosses between the races can be made, but the 

 triploid offspring are sterile. 



In most of the plants studied by Miintzing, polyploidy resulted in well- 

 marked races, but not in speciation. However, he studied sixteen pairs of 

 chromosomal races in which taxonomists had either assigned the different 

 chromosomal races to different species, or had debated the advisability of 

 doing so. These occur in such well known genera as the grasses Phleum 

 and Festuca, and the garden flowers Viola, Dianthus, and Chrysanthemum. 

 The differences between these pairs of chromosomal races or species are 

 qualitatively similar to those discussed above, but quantitatively they are 

 greater. Structural differences include some or all of the characters of the 

 gigas complex. They are usually separated geographically and ecologi- 

 cally. For example, Chrysanthemum shimotomaii is a seashore plant, while 

 C indicum inhabits mountains and inland fields. In crosses, they are highly 

 sterile. 



Experimental autotetraploids have been thoroughly studied genetically 

 and cytologically. Although the plants are frequently more hardy than 

 their diploid relatives, they have not been outstandingly successful, be- 

 cause they show markedly reduced fertility, and because they tend to 

 revert to diploidy. The cause of these characteristics is revealed by a study 

 of meiosis in the autotetraploids. For normal meiosis, the chromosomes 

 should form only pair-wise associations. But jour homologues of each kind 

 are present. Frequently, only pairs are formed, and normal gametes result. 

 But sometimes three chromosomes of a kind synapse, with the fourth one 

 behaving independently. In such a case, two of the three synapsed chro- 

 mosomes pass to one pole and one to the opposite pole. The independent 

 chromosome may now balance this situation, or it may make it even more 

 unbalanced. Again, all four chromosomes might synapse to form a tetra- 



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