210 FUNDAMENTALS OF CYTOLOGY 



resulting tetraploids tend to be more vigorous and highly fertile. The 

 sterility of higher autopolyploids is due in part to a tendency to form 

 multivalents (Fig. 156) which are distributed with some degree of irregu- 

 larity in meiosis. 



In general the results obtained so far with agricultural and ornamental 

 plants indicate that a limit of improvement through induced chromosomal 

 doubling is ordinarily reached at or near the tetraploid level. Further- 

 more, a deleterious effect of the chromosomal change may offset the 

 improvement. Thus doubling in tobacco plants results in an increase of 

 as much as one-third in the percentage of nicotine in the leaves, but this 

 is more than offset by a reduction of 50 per cent in the dry weight of the 

 leaves. In several cereals it has been found that although the doubled 

 varieties bear larger kernels the total yield is not larger, for an accompany- 

 ing reduction in fertility decreases the number of kernels obtained. 



Fig. 156. — Chromosomes from normal and colchicine-induced polyploid Petunia plants 

 at first meiotic metaphase: two bivalents, two trivalents, two quadrivalents, two quinque- 

 valents, one heptavalent, and one octovalent. {After A. Levan.) 



The problem of plant improvement by chromosomal doubling is there- 

 fore far from being a simple one. It is found that different kinds of 

 plants (species, varieties, inbred lines) often show widely different 

 responses to the doubling, for the type and degree of change exhibited 

 evidently depend in part upon the genie composition and physiological 

 state of the material treated. At present the results of a given experi- 

 ment cannot be predicted with any degree of certainty, nor can the limits 

 of the method's usefulness be stated. Nevertheless, investigators are 

 confident that with proper attention to the genotype and a judicious use 

 of selection and crossing much of value will be accomplished, even though 

 the ratio of error to trial may still remain high. We shall revert to this 

 topic in a discussion of hybridity in the next chapter. 



Although clear cases of polyploidy are relatively rare among animals, 

 the high numbers in some genera strongly suggest changes in number 

 in the evolution of certain natural groups. Known somatic numbers in 

 carnivores range up to 78 in the dog, and a comparable range is found in 

 rodents. Most of the investigated primates including man have 48. 

 Single individuals or strains with increased chromosome numbers are 

 occasionally encountered in the field and in the laboratory. Diploid, 

 tetraploid, and octoploid races of the brine shrimp (Artemia) exist in 



