454 Haploids and Autopolyploids 



as a diploid in a small region of the United States from central 

 Texas to northwestern Arkansas. Subsequently, apparently 

 autotetraploid strains arose and spread out over more than a 

 million square miles. T. occidentalis occupied a western area 

 reaching Arizona, Wyoming, and North Dakota, whereas T, 

 canaliculata spread through the Middle West and east of the Mis- 

 sissippi into Wisconsin, Ohio, and Virginia. 



Although autotetraploidy does not usually create new species, 

 it has produced types which can flourish under such a wider va- 

 riety of situations and can spread over such a wider region than 

 the diploids that it has been an important factor in the dissemi- 

 nation of certain species and in the establishing of certain species 

 as important members of certain areas. Autopolyploidy often 

 seems to result in strains that can cover newly exposed areas. 

 For example, in Biscutella laevigata Manton showed that the 

 diploids are restricted to small preglacial or interglacial areas in 

 central Europe but the autotetraploids are much more success- 

 ful colonizers and cover a much wider area. They appear to be 

 postglacial immigrants in many of the areas they now occupy 

 and they may still be spreading. 



QUESTIONS AND PROBLEMS 



1. What percentage of fertile gametes would be expected from hap- 

 loid plants that had respectively 6, 7, 8, and 9 chromosomes? 



2. What percentage of fertile gametes would be expected from 

 triploid plants that had respectively 18, 21, 24, and 27 chromosomes? 

 Compare results with problem 1. 



3. If you had a haploid plant and made very many self-poUinations, 

 would it be possible to obtain offspring? If so, what would be their 

 chromosome number in terms of n? What would be the result if the 

 plant was a triploid? 



4. In a given plant, red flowers (W) are dominant over white {w) 

 and long leaves (S) over short (s). If a triploid of the genotype 

 WTFiy SSs is crossed with one which is www sss and enough pollinations 

 are made to ensure an adequate number of offspring, what would be the 

 ratio of the WS, Ws, wS, and ws types in the offspring? (Assume no 

 crossing over between genes and centromeres.) 



5. In maize, wx (waxy) is recessive to Wx (nonwaxy). This gene 

 affects the endosperm. A wxwx female is crossed with a Wxwx male. 

 What is the nature of the ears that result, and what are the genotypes 



