CHAPTER 14 



The Aneuploids 



14.1: Aneuploids Among the Treated Generation 



Ihe variations in numbers oi chromosomes through loss or gain 

 of extras were first appreciated for their possible value in fundamental 

 cytogenetics by Belling and Newton.-*' Since then the aneuploids have 

 been acctnnulating in large numbers for many genera. A new group of 

 ancujiloids ^vas developed when colchicine was used with large j^opu- 

 lations of treated jjlants. Certain plants were deficient for a chromo- 

 some, and among the diploid species these losses were very rare but 

 significant.^' All diploid deficient types, including the 2n — 1 Datura 

 stranioiiinin plants, failed to set seed. The origin of such types is an 

 interesting j^roblem, for the action of colchicine must be interpreted 

 somewhat differently from the usual doubling of chromosomes.^ Ap- 

 parently a mitotic disturbance, the loss of a chromosome at the time 

 of treatment, is transmitted through mitotic processes tmtil meiosis, 

 when these types are discovered. 



1 hat di])loid deficient plants are rare is emphasized w'hen we learn 

 that only 55 spontaneously occurring 2.S-chromosomal tyj^es (2)i — • 1) 

 have been recordetl from among more than 2 million Datura plants 

 recorded over a period of years." From a standard line / of Datura, the 

 frequency of a 2)i — 1 plant is 1 out of 20,879 offspring, compared 

 with 7 such types foimd among 2135 plants growing from treated 

 cultures." The frecpiencics are increased fjy colchicine more than 70 

 times over the naturally occurring rate. Since the records were made 

 from pollen mother cells, only the diploid deficiencies from the 

 sube}jidermal layer that lell in the germ line were calculated. There- 

 fore, the incidence of 2/? — 1 tissues created by colchicine was higher 

 than these figures show. 



Out of 88 plants in the deficient class, 81 were tetraj^fiMcl de- 

 ficient kinds, i.e., 1// — 1 or 4/? — 1 — 1. Similar to the dij^loid de- 

 ficient plants, the tetraploid deficient cases arose from the effects of 

 colchicine.^ 



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