MIIOSIS AND riiRTILITY IN POLYPLOIDS 



In the triploid the numbers of grains with an extra one of each kind 

 can be recorded. The average number of chromosomes should be 

 4*5, but is always less owing to loss of unpaired chromosomes at 

 meiosis (Table i2a). Samples also differ owing to the fact that the 

 balanced grains develop more quickly than the unbalanced ones and 

 some chromosomes even differ from others in their effect on balance 

 and development (Table I2b). 



TABLE 13 



PERCENTAGE DISTRIBUTION OF EXTRA CHROMOSOMES 

 AS SEEN IN THE POLLEN GRAINS (P.G.) AND IN THE 

 PROGENY (3>: X 2x AND 2;c x 3a:) OF TULIPA, WITH 

 LARGE, AND OF DATURA, WITH SMALL CHROMO- 

 SOMES. THE NUMBER OF OBSERVATIONS IS GIVEN 

 IN PARENTHESIS IN THE FIRST COLUMN. (UPCOTT 

 AND PHILP, 1939) 



Note: The basic number for both is 12, so that the mean number of extra 

 chromosomes without loss would be 6. 



What happens to the pollen and eggs with different numbers is 

 shown by the frequencies of plants with correspondingly different 

 numbers when these pollen grains and eggs are united with haploid 

 gametes, that is to say, in reciprocal crosses with diploids (Tabic 13). 

 We then fmd that there is a reduction of the types with the middle 

 numbers (which are the most unbalanced) and with the higher 

 numbers (which are most unlike the gametes of the diploid parent). 



The elimination of unfavoured types is much stronger when it is 

 the triploid parent which is providing the pollen. Evidently the 

 pollen is subjected to a stricter test than the eggs. The reason for this 

 might be that the numbers of pollen grains on the stigma are always 

 greater than the number of ovules to be fertiUzed. Or it might be 



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