HYBRIDISATION OF EUPHRASIA 



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occurrence of these diploids can be accounted for by supposing that triploids have occurred 

 and have produced haploid gametes with eleven (or slightly more) chromosomes, which, 

 by back-crossing with the diploid, have produced a fertile diploid of hybrid origin, con- 

 taining germ-plasm derived from the tetraploid. This process is represented in the 

 accompanying diagram; it will now be considered in more detail. 



Diploid 

 2n = 22 



Tetraploid 

 2n = 44 



Gametes 

 n = 11 



Gametes 

 n = 22 



Occasional gametes 

 n = 11 (or 11 +) 



Diploid 



2n = 22 (or 22 +) 



The production of gametes with a viable set of eleven chromosomes is only likely to 

 happen, if, at meiosis in the triploid, eleven univalents and eleven bivalents are formed. 

 For then the twenty-two chromosomes forming the eleven bivalents will separate regularly, 

 while the others will either be scattered at random, or form a separate nucleus. In the 

 former case, haploid (or slightly more than haploid) nuclei will occasionally be formed 

 in accordance with the chance distribution of the unpaired chromosomes; in the latter, 

 the other two haploid nuclei will produce four haploid pollen grains. In the Withypool 

 triploid approximately eleven bivalents and eleven univalents were formed, and the 

 univalents appear to be distributed at random (Yeo, 1954). 



The eleven chromosomes coming from the triploid could be partly derived from the 

 diploid parent, and partly from the tetraploid. In addition they would be affected by 

 crossing over at meiosis in the triploid so that probably every one of them would contain 

 some genes from the tetraploid parent. The sorting of the chromosomes and the positions 

 of crossing-over in them would determine which genes were represented in the first 

 diploid back-crossed individual. Since these factors operate more or less at random, the 

 genetic constitution of this plant is unpredictable and could vary greatly between one 

 occurrence of the process and another. The diploid back-crossed individual is likely 

 to develop in the presence of the pure diploid, so that further back-crossing is possible. 

 Owing to the heterozygosity of the first back-crossed generation, its progeny, whether 



