CHAPTER 15 



Criteria for Judging Polyploidy 



15.1: Sterile Hybrids Made Fertile 



In the final analysis, pohploidy is determined bv connting the 

 niunber of chromosomes, and comparing this number with the dip- 

 loid or untreated plant. Some rapid and accurate methods are 

 available for judging polyploids indirectly. 



If a sterile species hybrid begins seed production after treatment 

 with colchicine, the evidence is good that polyploidy has been in- 

 duced.^i Geneticists knew that doubling the number of chromosomes 

 in a sterile species hybrid was a critical test for demonstrating the 

 effectiveness of the drug.^. 3i, 41, 14, 47 Species hybrids of Gossypium 

 were treated immediately.^ Plants that flowered, yet failed to set bolls 

 and seed, began seed production in those sections of the plant treated 

 with a proper concentration of colchicine. Therefore, without count- 

 ing the number of chromosomes, the preliminary efficiency of a treat- 

 ment could be estimated. The chance doubling that might have oc- 

 curred through unreduced gametes is of such low frequency that the 

 effects of colchicine were not obscured by natural or spontaneous 

 doubling. 



Amphiploids among Nicotlaua were made in large numbers.^- The 

 list of artificially induced polyploids increased within a few years.^^ 

 Combining the first data from Gossypium and Nicotiana proved the 

 value of colchicine beyond doubt. 



Many combinations of interspecific and intergeneric hybrids were 

 converted into amphiploids within the Triticinae.i- ^ From one pro- 

 ject, 18 amphiploids involving 10 species were created within two 

 years.^i The production of good pollen and eventually seed in the 

 sectors of treated plants that showed the effects of doubling was re- 

 liable criterion for amphiploidy. Estimates of how effective colchi- 

 cine was upon these plants could be checked on a percentage basis. 

 Some modifications were necessary because the monocotylcdonous 

 species had to be treated differently from the dicotyledonous types. 



[362] 



