CONSLQUENCES OT CIIANCr. 



that a proporiioii of unbalanced types which can survive as eggs if 

 fertilized are unable to grow dowm the style at all as pollen grains. 

 This is evidently the case, for the fertility of triploids is actually 

 lower as pollen than as egg parents (Table 14). 



TABLE 14 



THE FERTILITY OF RECIPROCAL CROSSES OF DIPLOIDS 

 AND TRIPLOIDS, SHOWING THAT IT IS LOWER WHEN 

 THE TRIPLOID IS THE POLLEN PARENT, AND ALSO 

 IN PLANTS WITH HIGHER BASIC NUMBERS OF 

 CHROMOSOMES. THE DATA ARE EXPRESSED AS PER- 

 CENTAGES OF THE INFERRED NORMAL SEED SET. 

 (UPCOTT AND PHILP, 1939) 



— indicates that no d;ita are available. 



There is another interesting sequel to the random segregation of 

 extra chromosomes and the selection of balanced types in triploids. 

 As the basic number increases, the proportion of haploid gametes 

 remains always about J\ Therefore fertility declines with increasing 

 basic number. It is high in Drosophila or Crcpis, low in Pyrus 



(Table 14). ^ u • • 1 



Tetraploids behave as we should expect on these principles 

 (Fig. 27). The chromosomes pair, that is each member of the set 

 forms two pairs; but they pair differently at different points and 

 therefore have to change partners as they do in triploids. Their 

 association at pachytene then resembles diplotene in a diploid. 

 Where they arc paired they may form chiasmata. Hence associations 

 of four appear at metaphase wherever changes of partner and 

 chiasmata permit. Failing such quadrivalents we find pairs of 

 bivalents or trivalcnts and univalents. 



The quadrivalents arrange themselves on the spindle at 

 metaphase, like the trivalents, either in a line or zig-zag, that is 



T26 



